A Magic Venn Diagram is a magic figure where regions of a Venn diagram are labeled such that the sums of the regional labels of each set are the same. We have developed a backtracking search to count the number of Magic Venn Diagrams. The algorithm could determine the number of Magic Venn Diagrams for all Venn diagrams with four sets. This paper presents the algorithm with its applied heuristics and lists the computational results.
A famous open problem in the field of rendezvous search is to ascertain the rendezvous value of the symmetric rendezvous problem on the line, wherein both agents begin two units apart. We provide a new, Bayesian framework to both create new strategies for the agents to follow and to provide a new interpretation of previously posited strategies.
Additionally, we have developed a method that modifies any strategy, even those with potentially infinite expected meeting time, into a new strategy that is guaranteed to have a finite expected meeting time. This process, combined with using our Bayesian framework to create new strategies, yields an upper bound that is within one percent of the current best upper bound for the symmetric rendezvous value.
The Astronaut Problem is an open problem in the field of rendezvous search. The premise is that two astronauts randomly land on a planet and want to find one another. Research explores what strategies accomplish this in the least expected time.
To investigate this problem, we create a discrete model which takes place on the edges of the Platonic solids. Some baseline assumptions of the model are:
The 3-dimensional nature of our model makes it different from previous work. We explore multi-step strategies, which are strategies where both agents move randomly for one step, and then follow a pre-determined sequence.
For the cube and octahedron, we are able to prove optimality of the “Left Strategy,” in which the agents move in a random direction for the first step and then turn left. In an effort to find lower expected times, we explore mixed strategies. Mixed strategies incorporate an asymmetric case which, under certain conditions, can result in lower expected times.
Most of the calculations were done using first-step decompositions for Markov chains.
Our research focuses on the winning probability of a novel problem posted on a question-and-answer website. There are \( n \) people in a line at positions \( 1, 2, \ldots, n \). For each round, we randomly select a person at position \( i \), where \( i \) is odd, to leave the line, and shift each person at a position \( j \) such that \( j > i \) to position \( j – 1 \). We continue to select people until there is only one person left, who then becomes the winner.
We are interested in which initial position has the greatest chance to survive, that is, the highest probability to be the last one remaining. Specifically, we have derived recursions to solve for exact values and the formula of the winning probabilities.
We have also considered variations of the problem, where people are grouped into triples, quadruples, etc., and the first person in each group is at the risk of being selected. We will also present various sequences we have discovered while solving for the winning probabilities of the different variations, as well as other possible extensions and related findings concerning this problem.
Our research studies the expected survival time in a novel problem posted on a question-and-answer website. There are \( n \) people in a line at positions \( 1, 2, \ldots, n \). For each round, we randomly select a person at position \( k \), where \( k \) is odd, to leave the line, and shift the person at each position \( i > k \) to position \( i-1 \). We continue to select people until there is only one person left, who then becomes the winner.
We are interested in which initial position has the largest expected number of turns to stay in the line before being selected, which we refer to as “expected survival time.” In this paper, we use a recursive approach to solve for exact values of the expected survival time. We have proved the exact formula of the expected survival time of the first and the last position as well. We will also present our work on the expected survival time of the other positions, \( 2, 3, 4, \ldots \) from an asymptotic perspective.
A set \( S \subseteq V \) is a dominating set of \( G \) if every vertex in \( V – S \) is adjacent to at least one vertex in \( S \). The domination number \( \gamma(G) \) of \( G \) equals the minimum cardinality of a dominating set \( S \) in \( G \); we say that such a set \( S \) is a \( \gamma \)-set.
A generalization of this is partial domination, which was introduced in 2017 by Case, Hedetniemi, Laskar, and Lipman. In \emph{partial domination}, a set \( S \) is a \( p \)-dominating set if it dominates a proportion \( p \) of the vertices in \( V \). The \( p \)-domination number \( \gamma_p(G) \) is the minimum cardinality of a \( p \)-dominating set in \( G \).
In this paper, we investigate further properties of partial dominating sets, particularly ones related to graph products and locating partial dominating sets. We also introduce the concept of a \( p \)-influencing set as the union of all \( p \)-dominating sets for a fixed \( p \) and investigate some of its properties.
For bipartite graphs \( F \) and \( H \) and a positive integer \( s \), the \( s \)-bipartite Ramsey number \( BR_s(F,H) \) of \( F \) and \( H \) is the smallest integer \( t \) with \( t \geq s \) such that every red-blue coloring of \( K_{s,t} \) results in a red \( F \) or a blue \( H \). We evaluate this number for all positive integers \( s \) when \( F \) and \( H \) are both stars, are both matchings, or one is a star and the other is a matching, as well as when \( F = H \) is an arbitrary double star.
In a graph \( G \), the Steiner distance \( d(S) \) of the vertex subset \( S \subseteq V(G) \) is the minimum size among all connected subgraphs whose vertex sets contain \( S \). The Steiner \( k \)-diameter of a connected graph \( G \) is the maximum \( d(S) \) among all \( k \)-element vertex subsets \( S \subseteq V(G) \).
In this paper, we examine the Steiner \( k \)-diameter for large \( k \) and then discuss the applications of the results.
A set of vertices, \( S \), in a digraph \( D \), is split dominating provided it is:
In this paper, we consider split dominating sets in strongly connected tournaments. The split domination number of a strongly connected tournament \( T \), denoted by \( \gamma_s(T) \), is the minimum cardinality of a split dominating set for that tournament.
The authors previously gave a tight lower bound for \( \gamma_s(T) \) when \( T \) is regular. In this paper, we show that when \( T \) is a nearly regular \( 2k \)-tournament, then \( \gamma_s(T) \geq \lceil \frac{2k}{3} \rceil \) and this bound is tight.
Redrawing lines for redistricting plans that represent U.S. congressional districts is a tricky business. There are many laws that dictate how lines can and cannot be drawn, such as contiguity. In fact, building all redistricting plans for a single U.S. state is an intractable problem. Researchers have turned to heuristics in order to analyze current redistricting plans. Many of these heuristics (e.g. local search heuristics and Markov chain Monte Carlo algorithms) used by researchers form new congressional districts by switching the smaller pieces (e.g. precincts or census blocks) that make up congressional districts from one congressional district to another.
In this paper, we discuss the various natural definitions involved in satisfying rules for contiguity and simply connectedness of precincts or census blocks and how these relate to contiguity and simply connectedness of congressional districts. We also propose and analyze several constructions to alleviate violations of contiguity and simply connectedness in precincts and census blocks. Finally, we develop efficient algorithms that allow practitioners to assess redistricting plans using local search heuristics or Markov chain Monte Carlo algorithms efficiently.
A set \( S \subseteq V \) is \( \alpha \)-dominating if for all \( v \in V – S \), \( |N(v) \cap S| \geq \alpha |N(v)| \). The \( \alpha \)-domination number of \( G \) equals the minimum cardinality of an \( \alpha \)-dominating set \( S \) in \( G \). Since being introduced by Dunbar et al. in 2000, \( \alpha \)-domination has been studied for various graphs and a variety of bounds have been developed.
In this paper, we propose a new parameter derived by flipping the inequality in the definition of \( \alpha \)-domination. We say a set \( S \subset V \) is a \( \beta \)-packing set of a graph \( G \) if \( S \) is a proper, maximal set having the property that for all vertices \( v \in V – S \), \( |N(v) \cap S| \leq \beta |N(v)| \) for some \( 0 < \beta \leq 1 \). The \( \beta \)-\emph{packing number} of \( G \), denoted \( \beta\text{-pack}(G) \), equals the maximum cardinality of a \( \beta \)-packing set in \( G \).
In this research, we determine \( \beta\text{-pack}(G) \) for several classes of graphs, and we explore some properties of \( \beta \)-packing sets.
A perfect matching of a graph is a subset of edges in the graph such that each vertex is contained in exactly one edge. We study the number of perfect matchings of a given graph. In particular, we are interested in the power of two that divides this number. A new type of vertex set, called a channel, is considered, whose presence is associated with powers of two in the perfect matching count. This provides a method for determining lower bounds on such powers. Algebraic and involutive proofs are given for these results, and methods for channel identification are provided. We specialize to perfect matchings on subgraphs of the square lattice, which are identified with domino tilings of the plane, and apply channels to some conjectures by Pachter.
A dominating set of a graph \( G \) is a set of vertices \( D \) such that for all \( \nu \in V(G) \), either \( \nu \in D \) or \( [\nu,d] \in E(G) \) for some \( d \in D \). The cardinality-redundance of a vertex set \( S \), \( CR(S) \), is the number of vertices \( x \in V(G) \) such that \( |N[x] \cap S| \geq 2 \). The cardinality-redundance of \( G \) is the minimum of \( CR(S) \) taken over all dominating sets \( S \). A set of vertices \( S \) such that \( CR(S) = CR(G) \) is a \( \gamma_{CR} \)-set, and the size of a minimum \( \gamma_{CR} \)-set is denoted \( \gamma_{CR}(G) \).
Here, we are concerned with extremal problems concerning cardinality-redundance. We give the maximum number of edges in a graph with a given number of vertices and given cardinality-redundance. In the cases that \( CR(G) = 0 \) or \( 1 \), we give the minimum and maximum number of edges of graphs when \( \gamma_{CR}(G) \) is fixed, and when \( CR(G) = 2 \), we give the maximum number of edges of graphs where \( \gamma_{CR}(G) \) is fixed. We give the minimum and maximum values of \( \gamma_{CR}(G) \) when the number of edges are fixed and \( CR(G) = 0, 1 \), and we give the maximum values of \( \gamma_{CR}(G) \) when the number of edges are fixed and \( CR(G) = 2 \).
It is known that for a maximal planar graph \( G \) with order \( n \geq 4 \), the independence number satisfies \( \frac{n}{4} \leq \alpha(G) \leq \frac{2n-4}{3} \). We show that the lower bound is sharp and characterize the extremal graphs for \( n \leq 12 \). For the upper bound, we characterize the extremal graphs of all orders.
The independence number \( \alpha(G) \) of a graph \( G \) is the size of the largest independent set. This parameter is difficult to determine in general, but can be bounded on various graph classes. This paper considers planar and maximal planar graphs.
Constraint Programming (CP) is a method used to model and solve complex combinatorial problems. An alternative to Integer Programming for solving large-scale industrial problems, it is, under some circumstances, more efficient than IP, but its strength lies mainly in the use of predicates to model problems. This paper presents the at-least-\( m \)-different predicate and provides a class of facet-defining inequalities of the convex hull of integer solutions. This predicate bounds the number of values that variables in a set may receive. The paper also presents a polynomial-time separation algorithm to be used in the context of a branch-and-bound optimization approach.
Let \( N_2DL(v) \) denote the set of degrees of vertices at distance \( 2 \) from \( v \). The \( 2 \)-neighborhood degree list of a graph is a listing of \( N_2DL(v) \) for every vertex \( v \). A degree restricted \( 2 \)-switch on edges \( v_1v_2 \) and \( w_1w_2 \), where \( \deg(v_1) = \deg(w_1) \) and \( \deg(v_2) = \deg(w_2) \), is the replacement of a pair of edges \( v_1v_2 \) and \( w_1w_2 \) by the edges \( v_1w_2 \) and \( v_2w_1 \), given that \( v_1w_2 \) and \( v_2w_1 \) did not appear in the graph originally. Let \( G \) and \( H \) be two graphs of diameter \( 2 \) on the same vertex set. We prove that \( G \) and \( H \) have the same \( 2 \)-neighborhood degree list if and only if \( G \) can be transformed into \( H \) by a sequence of degree restricted \( 2 \)-switches.
Let \( \Gamma \) be a finite group and let \( \Delta \) be a generating set for \( \Gamma \). A Cayley map is an orientable 2-cell imbedding of the Cayley graph \( G_\Delta(\Gamma) \) such that the rotation of arcs emanating from each vertex is determined by a unique cyclic permutation of generators and their inverses. A probability model for the set of all Cayley maps for a fixed group and generating set, where the distribution is uniform. We focus on certain finite abelian groups with generating set chosen as the standard basis. A lower bound is provided for the probability that a Cayley map for such a group and generating set is symmetrical.
A graph is asymmetric if the automorphism group of its set of vertices is trivial. A graph is called non-asymmetric if and only if it is not asymmetric. A graph \( G \) is minimally non-asymmetric if \( G \) is non-asymmetric but \( G – e \) is asymmetric for any edge \( e \) contained in \( G \).
Given a finite set \( V \) (of elements called varieties) and integers \( k \), \( r \), and \( \lambda \), a balanced incomplete block design (BIBD) is a family of \( k \)-element subsets of \( V \), called blocks, such that any element is contained in \( r \) blocks and any pair of distinct varieties \( u \) and \( w \) is contained in exactly \( \lambda \) blocks.
In this paper, we give examples of minimally non-asymmetric graphs constructed from balanced incomplete block designs.
There is a special case of a generalized Clifford algebra, known as a Clifford graph algebra, which is useful for studying a simple graph \( G_n \), with \( n \) vertices. We will discuss how this algebra \( GA(G_n) \) can represent \( G_n \), and prove that it exists in general by defining it as an appropriate sub-algebra of a classical Clifford algebra. We will then refine this process of “construction by inclusion” for the path graph \( P_n \), and the complete star graph \( K_{1,n} \), by choosing from a parent classical Clifford algebra as many bi-vectors as possible for the generators which define \( GA(P_n) \) and \( GA(K_{1,n}) \).
Elements of the Riordan group \(\mathcal{R}\) over a field \(\mathbb{F}\) of characteristic zero are infinite lower triangular matrices which are defined in terms of pairs of formal power series. We wish to bring to the forefront, as a tool in the theory of Riordan groups, the use of multiplicative roots \(a(x)^{\frac{1}{n}}\) of elements \(a(x)\) in the ring of formal power series over \(\mathbb{F}\). Using roots, we give a Normal Form for non-constant formal power series, we prove a surprisingly simple Composition-Cancellation Theorem and apply this to show that, for a major class of Riordan elements (i.e., for non-constant \(g(x)\) and appropriate \(F(x)\)), only one of the two basic conditions for checking that \((g(x), F(x))\) has order \(n\) in the group \(\mathcal{R}\) actually needs to be checked. Using all this, our main result is to generalize C. Marshall [6] and prove: Given non-constant \(g(x)\) satisfying necessary conditions, there exists a unique \(F(x)\), given by an explicit formula, such that \((g(x), F(x))\) is an involution in \(\mathcal{R}\). Finally, as examples, we apply this theorem to “aerated” series \(h(x) = g(x^r)\) to find the unique \(K(x)\) such that \((h(x), K(x))\) is an involution.
A mixed hypergraph is a triple \(\mathcal{H} = (X, \mathcal{C}, \mathcal{D})\), where \(X\) is the vertex set and each of \(\mathcal{C}\) and \(\mathcal{D}\) is a family of subsets of \(X\), the \(\mathcal{C}\)-edges and \(\mathcal{D}\)-edges, respectively. A proper \(k\)-coloring of \(\mathcal{H}\) is a mapping such that each \(\mathcal{C}\)-edge has two vertices with a common color and each \(\mathcal{D}\)-edge has two vertices with distinct colors. A mixed hypergraph \(\mathcal{H}\) is called circular if there exists a host cycle on the vertex set \(X\) such that every edge (\(\mathcal{C}\)- or \(\mathcal{D}\)-) induces a connected subgraph of this cycle. We propose an algorithm to color the \((3, 3)\)-uniform, complete, circular, mixed hypergraphs for every value in its feasible set. In doing so, we show: \(\chi(\mathcal{H}) = 2\) and \(\bar{\chi}(\mathcal{H}) = \frac{n}{2}\) when \(n\) is even and \(\bar{\chi}(\mathcal{H}) = \frac{n-1}{2}\) when \(n\) is odd.
For a positive integer \( k \), let \( \mathcal{P}^*([k]) \) be the set of nonempty subsets of the set \( [k] = \{1, 2, \ldots, k\} \). For a connected graph \( G \) of order 3 or more, let \( c: E(G) \to \mathcal{P}^*([k]) \) be an edge coloring of \( G \) where adjacent edges may be colored the same. The induced vertex coloring \( c’: V(G) \to \mathcal{P}^*([k]) \) is defined by \( c'(v) = \bigcap_{e \in E_v} c(e) \), where \( E_v \) is the set of edges incident with \( v \). If \( c’ \) is a proper vertex coloring of \( G \), then \( c \) is called a regal \( k \)-edge coloring of \( G \). The minimum positive integer \( k \) for which a graph \( G \) has a regal \( k \)-edge coloring is the regal index \( \text{reg}(G) \) of \( G \). If \( c’ \) is vertex-distinguishing, then \( c \) is a strong regal \( k \)-edge coloring of \( G \). The minimum positive integer \( k \) for which \( G \) has a strong regal \( k \)-edge coloring is the strong regal index \( \text{sreg}(G) \) of \( G \). A brief survey of known results and conjectures on strong regal indexes of graphs is presented. The relationships between the regal index \( \text{reg}(G) \) and the chromatic number \( \chi(G) \) of a connected graph \( G \) are investigated and results and problems on \( \text{reg}(G) \) are presented.
One of the routing paradigms on interconnection networks is as follows: given a source node and \(m\) destination nodes, find disjoint paths from the source to the destination nodes. If we impose the condition that these paths be the shortest ones, this problem becomes harder and more interesting because such shortest and disjoint paths do not always exist. This problem has been studied previously for \(Q_n\), the hypercube of dimension \(n\), when \(m = n\) and a necessary and sufficient condition has been found for these paths to exist. In this paper, we review the previous work for the hypercube and then consider the problem in the more general case for arbitrary \(m\), where \(1 \leq m \leq 2^n – 1\), in an \(n\)-cube by designing routing algorithms that always find disjoint and shortest paths for a maximum subset of the destination nodes for which such paths exist. The size of such a set is no more than \(n\), the degree of the \(Q_n\).
Strongly regular graphs with parameters \((q^3 + 2q^2, q^2 + q, q, q)\), \((q^3 + q^2 + q + 1, q^2 + q, q – 1, q + 1)\), and \((q^3, q^2 + q – 2, q – 2, q + 2)\) are constructed from \(k\)-arcs in affine planes of order \(q\) with \(k = q + 2, q + 1, q\). In addition, strongly regular graphs with parameters \((nq^3 – q^3 + nq^2, nq^2 – q^2 + nq – q, 2qn – 3q, qn – q)\) are constructed from maximal arcs of degree \(n\) in affine planes of order \(q\). Each of these examples generalizes previously known examples when the affine planes were assumed to be Desarguesian.
The game of Nim is at least centuries old, possibly
originating in China, but noted in the 16th century
in European countries. It consists of several stacks
of tokens, and two players alternate taking one or
more tokens from one of the stacks, and the player
who cannot make a move loses.
The formal and intense study of Nim culminated in
the celebrated Sprague-Grundy Theorem, which is now
one of the centerpieces in the theory of impartial
combinatorial games.
We study a variation on Nim, played on a graph. Graph Nim, for which the theory of
Sprague-Grundy does not provide a clear strategy.
Graph Nim was originally developed at the University
of Colorado Denver.
Graph Nim was first played on graphs of three
vertices. The winning strategy and losing position
of three-vertex Graph Nim have been discovered,
but we will expand the game to four vertices and
develop the winning strategies for four-vertex
Graph Nim.
This work was published as a chapter in the
Master’s Thesis of Trevor Williams [8].
This article discusses Kirchhoff graph uniformity—that all edge vectors in a Kirchhoff graph have the same multiplicity. For a given Kirchhoff graph, an associated digraph is constructed. Based on these graphs, the equivalence of a linear-algebraic condition and a vector graph being Kirchhoff is proven. This condition is then used to show that \( 2 \)-connected Kirchhoff graphs are uniform. Other Kirchhoff graphs need not be uniform.
Consider the following two-person game on a graph \( G \). The two players start with two color choices only, taking turns coloring any uncolored vertex with the restriction that any coloring must be a proper coloring. A third (or fourth, etc.) color can only be used when forced to maintain a proper coloring. One player, the minimizer, is trying to force the smallest number of colors possible. The other player, the maximizer, is trying to force the largest number of colors possible. This game proper chromatic number, denoted \( \chi_{(E,g)}(G) \), is the minimum number of colors used when both players play optimally.
The advantage of the game proper chromatic number is that it is comparable to other published game chromatic variants, particularly the game chromatic number II and the game Grundy number.
This paper also considers extensions of the game proper chromatic number through generalized regions of the graph. Let \( R = \{R_1, R_2, \ldots, R_t\} \) such that \( \bigcup R_i = V(G) \). It is convenient to think of these \( R_i \)’s as regions of interest in graph \( G \). In particular, extensions to closed neighborhoods and open neighborhoods maintaining the restriction that all colorings must be “proper” in the sense that no \( R_i \) is monochromatic are considered for some natural classes of graphs.
The minimum number of colors necessary provided each player plays optimally, following the rules established for the game proper chromatic number, is denoted \( \chi_{(N[v],g)}(G) \) and \( \chi_{(N(v),g)}(G) \) for the game closed neighborhood proper chromatic number and the game open neighborhood chromatic number, respectively.
A conjecture by Albertson states that if \( \chi(G) \geq n \), then \( cr(G) \geq cr(K_n) \), where \( \chi(G) \) is the chromatic number of \( G \) and \( cr(G) \) is the crossing number of \( G \). This conjecture is true for \( n \leq 16 \), but it remains open for \( n \geq 17 \).
In this paper, we consider the statements corresponding to this conjecture where the crossing number of \( G \) is replaced with:
– the genus \( \gamma(G) \) (the minimum genus of the orientable surface on which \( G \) is embeddable),
– the skewness \( \mu(G) \) (the minimum number of edges whose removal makes \( G \) planar), and
– the thickness \( \theta(G) \) (the minimum number of planar subgraphs of \( G \) whose union is \( G \)).
In 2017, Hedetniemi asked the question: “For which graphs \( G \) does the indexed family \( \{N_G(v) \mid v \in V(G)\} \) of open neighborhoods have a system of distinct representatives?” In [1], we answered that question. Now, we move on to other special set families in graphs and examine whether they do or do not have a system of distinct representatives.
We give necessary and sufficient conditions for two matroids on the same ground set to be the upper and lower matroids of a \( \Delta \)-matroid.
Let \( D \) be a digraph on \( n \) vertices. A cycle \( C \) in \( D \) is said to be 1-extendable if there exists a cycle \( C’ \) in \( D \) such that the vertex set of \( C’ \) contains the vertex set of \( C \) and \( C’ \) contains exactly one additional vertex. A digraph is 1-cycle-extendable if every non-Hamiltonian cycle is 1-extendable.
A cycle \( C \) in \( D \) is said to be 2-extendable if there exists a cycle \( C’ \) in \( D \) such that the vertex set of \( C’ \) contains the vertex set of \( C \) and \( C’ \) contains exactly two additional vertices. A digraph is 2-cycle-extendable if every cycle on at most \( n-2 \) vertices is 2-extendable.
A digraph is 1,2-cycle-extendable if every non-Hamiltonian cycle is either 1-extendable or 2-extendable. It has been previously shown that not all strong tournaments (orientations of a complete undirected graph) are 1-extendable, but are 2-extendable. The structure of all non 1-extendable tournaments is shown as a type of block Kronecker product of 1-extendable subtournaments.
For a toroidal graph \( G = (V, E) \) embedded in the torus, let \( \mathcal{F}(G) \) denote the set of faces of \( G \). Then, \( G \) is called a \( C_{n} \)-face-magic torus graph if there exists a bijection \( f: V(G) \rightarrow \{1, 2, \ldots, |V(G)|\} \) such that for any \( F \in \mathcal{F}(G) \) with \( F \cong C_{n} \), the sum of all the vertex labelings along \( C_{n} \) is a constant \( S \).
Let \( x_{v} = f(v) \) for all \( v \in V(G) \). We call \( \{x_{v} : v \in V(G)\} \) a \( C_{n} \)-face magic torus labeling on \( G \).
We say that a \( C_{4} \)-face-magic torus labeling \( \{x_{i,j} \} \) on \( C_{2n} \times C_{2n} \) is antipodal balanced if \( x_{i,j} + x_{i+n,j+n} = \frac{1}{2}S \) for all \( (i, j) \in V(C_{2n} \times C_{2n}) \).
We determine all antipodal balanced \( C_{4} \)-face-magic torus labelings on \( C_{4} \times C_{4} \) up to symmetries on a torus.
Steinhaus graphs are a small (there are \( 2^{n-1} \) of them on \( n \) vertices) but interesting family of graphs. They have been studied for over forty years, and it has been shown that almost all graphs have certain properties if and only if almost all Steinhaus graphs have these properties.
In this paper, we find and count all the complements of Steinhaus graphs that are claw-free.
Edge-Nim is a combinatorial game played on finite regular graphs with positive, integrally weighted edges. Two players alternately move from an initialized vertex to an adjacent vertex, decreasing the weight of the incident edge to a strictly non-negative integer as they travel across it. The game ends when no incident edge has a nonzero weight and a player is unable to move, in which case that player loses.
We characterize the winner of Edge-Nim on the complete bipartite graphs \( K_{2,n} \) for all positive integers \( n \), giving the solution and complete strategy for the player able to win.
Given a graph \( G \), we are interested in finding disjoint paths for a given set of distinct pairs of vertices. In 2017, we formally defined a new parameter, the pansophy of \( G \), in the context of the disjoint path problem.
In this paper, we investigate the pansophy of two classes of graphs that contain a vertex that we define as the superuser. The superuser of a graph is a vertex that is adjacent to every other vertex. We close with future research directions.
A grid on a cell of a game board attacks all neighboring cells. The domination number counts the minimum number of grids such that each cell of a board is occupied or attacked by a grid.
For square boards (chess boards), the domination number has been determined in a series of papers. Here, we start to consider grids on hexagon boards \( B_n \) as parts of the Euclidean tessellation by congruent regular hexagons, where \( B_1 \) is one hexagon, \( B_2 \) consists of the three hexagons around one vertex, and \( B_n \) for \( n \geq 3 \) consists of \( B_{n-2} \) together with all hexagons having at least one hexagon in common with \( B_{n-2} \).
An upper bound is presented for the grid domination number, and exact values are determined by computer for small \( n \).
Given a graph \( G \), we are interested in finding disjoint paths for a given set of distinct pairs of vertices. In 2017, we formally defined a new parameter, the pansophy of \( G \), in the context of the disjoint path problem.
In this paper, we construct a method to determine the pansophy of any complete bipartite graph, and then generalize the method to compute the pansophy of any complete multipartite graph. We close with future research directions.
The \emph{Reconstruction Number} of a graph \( G \), denoted \( RN(G) \), is the minimum number \( k \) such that there exist \( k \) vertex-deleted subgraphs of \( G \) which determine \( G \) up to isomorphism. More precisely, \( RN(G) = k \) if and only if there are vertex-deleted subgraphs \( G_1, G_2, \ldots, G_k \), such that if \( H \) is any graph with vertex-deleted subgraphs \( H_1, H_2, \ldots, H_k \), and \( G_i \cong H_i \) for \( i = 1, 2, \ldots, k \), then \( G \cong H \).
A \emph{unicyclic graph} is a connected graph with exactly one cycle. In this paper, we find reconstruction numbers for various types of unicyclic graphs. With one exception, all unicyclic graphs considered have \( RN(G) = 3 \).
In this work, we introduce the Interval Permutation Segment (IP-SEG) model that naturally generalizes the geometric intersection models of interval and permutation graphs.
We study properties of two graph classes that arise from the IP-SEG model and present a family of forbidden subgraphs for these classes. In addition, we present polynomial algorithms for the following problems on these classes, when the model is given as part of the input.
Let \( S_n \) be the random walk on \( (0, 1) \). The \( S_n \) have been the subject of intense study; their definition is immediately intuitive. Nevertheless, they are quite intentionally disorderly, and this disorder is mirrored by the fact that, pointwise, \( \left( \frac{S_n}{\sqrt{n}} \mid n \in \mathbb{N}^+ \right) \) behaves quite badly.
In this paper, we provide our results on the fine structure of the random walk that give insight into this behavior.
We utilize the flexible tile model presented in [13] to design self-assembling DNA structures from a graph theory perspective. These tiles represent branched junction molecules whose arms are double strands of DNA.
We consider \( 2 \times n \) triangular lattice graphs \( G_n \), where \( n \) represents the number of triangles. Given a target graph \( G_n \), we determine the minimum number of tile and bond-edge types needed in order to create \( G_n \) as a complete self-assembled complex in three different scenarios. Each scenario corresponds to a distinct level of laboratory constraint.
In the first scenario, graphs of a smaller size than \( G_n \) are allowed. In the second scenario, non-isomorphic graphs of the same size as \( G_n \) are allowed, but not graphs of smaller size. In the third scenario, only graphs isomorphic or larger in size to the target graph are allowed.
We provide optimal tile sets for all \( 2 \times n \) triangular lattice graphs \( G_n \) in Scenario 1 and Scenario 3. We also include some small examples in Scenario 2.
An upper bound on the energy of graphs is obtained using the spectral moments of the eigenvalues of the adjacency matrix associated with the graph, utilizing the method of Lagrange multipliers and properties of cubic equations
A polyhex is a set of hexagons of the Euclidean tessellation of the plane by congruent regular hexagons. Then, a polyhex graph has the vertex points of the hexagons as its vertices and the sides of the hexagons as its edges. A rectilinear drawing of a graph in the plane uses straight line segments for the edges. Partial results are given for the maximum number of crossings over all rectilinear drawings of a polyhex graph
Distinctive power of the alliance polynomial has been studied in previous works. For instance, it has been proved that the empty, path, cycle, complete, complete without one edge, and star graphs are characterized by its alliance polynomial. Moreover, it has been proved that the family of alliance polynomials of regular graphs with small degree is a very
special one, since it does not contain alliance polynomials of graphs other than regular graphs with the same degree. In this work, we prove that the alliance polynomial also
determines the wheel graphs.
An ordered tree, also known as a plane tree or a planar tree, is defined recursively as having a root and an ordered set of subtrees. A \(3\)-zebra tree is an ordered tree where all edges connected to the root (called height \( 1 \)) are tricolored, as are all edges at odd height. The edges at even height are all black as usual.
In this paper, we show that the number of \(3\)-zebra trees with \( n \) edges is equal to the number of Schröder paths with bicolored level steps.
A split graph is a graph whose vertices can be partitioned into a clique and an independent set. Most results in spectral graph theory do not address multigraph concerns. Exceptions are [2] and [4], but these papers present results involving a special class of underlying split graphs, threshold graphs, in which all pairs of nodes exhibit neighborhood nesting, and all multiple edges are confined to the clique.
We present formulas for the eigenvalues of some infinite families of regular split multigraphs in which all multiple edges occur between the clique nodes and cone nodes, with multiplicity of multiple edges \( \mu > 1 \) fixed, and which have integer eigenvalues for the adjacency, Laplacian, and signless Laplacian matrices.
A rigid vertex is a vertex with a prescribed cyclic order of its incident edges. An embedding of a rigid vertex graph preserves such a cyclic order in the surface at every vertex. A cellular embedding of a graph has the complementary regions homeomorphic to open disks.
The genus range of a \( 4 \)-regular rigid vertex graph \( \Gamma \) is the set of genera of closed surfaces that \( \Gamma \) can be cellularly embedded into. Inspired by models of DNA rearrangements, we study the change in the genus range of a graph \( \Gamma \) after the insertion of subgraph structures that correspond to intertwining two edges. We show that such insertions can increase the genus at most by \( 2 \) and decrease by at most \( 1 \), regardless of the number of new vertices inserted.
The hypercube cut number \( S(d) \) is the minimum number of hyperplanes in the \( d \)-dimensional Euclidean space \( \mathbb{R}^d \) that slice all the edges of the \( d \)-cube. The problem was originally posed by P. O’Neil in 1971. B. Grünbaum, V. Klee, M. Saks, and Z. Füredi have raised the problem in various contexts.
The identity \( S(d) = d \) has been well-known for \( d \leq 4 \) since 1986. However, it was only until the year 2000 that Sohler and Ziegler obtained a computational proof for \( S(5) = 5 \). Nevertheless, finding a short proof for the problem, independent of computer computations, remains a challenging task.
We present a short proof for the result presented by Emamy-Uribe-Tomassini in Hypercube 2002 based on Tomassini’s Thesis. The proof here is substantially shorter than the original proof of 60 pages.
Percolation models are infinite random graph models which have applications to phase transitions and critical phenomena. In the site percolation model, each vertex in an infinite graph \( G \) is retained independently with probability \( p \) and deleted otherwise. The percolation threshold is the critical probability \( p_c(G) \) such that if \( p > p_c(G) \), there is positive probability that the random subgraph induced by the retained vertices has an infinite connected component, while the probability that all of its components are finite is one if \( p < p_c(G) \).
There are few lattice graphs for which the site percolation threshold is exactly known, and rigorous bounds for unsolved lattices are very imprecise. The substitution method for computing bounds for the more common class of bond percolation models must be modified to apply to site models. Some modifications will be illustrated with an application to the \( (4,8^2) \) Archimedean lattice, which is a vertex-transitive tiling of the plane by squares and regular octagons. An improved upper bound, \( p_c^{site}(4,8^2) < 0.785661 \), is obtained.
In a finite projective plane \( \text{PG}(2, q) \), a set of \( k \) points is called a \( (k, n) \)-arc if the following two properties hold:
1. Every line intersects it in at most \( n \) points.
2. There exists a line which intersects it in exactly \( n \) points.
We are interested in determining, for each \( q \) and each \( n \), the largest value of \( k \) for which a \( (k, n) \)-arc exists in \( \text{PG}(2, q) \). If possible, we would like to classify those arcs up to isomorphism. We look at the problem for \( q = 11 \).
A cyclic triple, \( (a, b, c) \), is defined to be the set \( \{(a, b), (b, c), (c, a)\} \) of ordered pairs. A Mendelsohn triple system of order \( v \), or MTS\( (v) \), is a pair \( (M, \beta) \), where \( M \) is a set of \( v \) points and \( \beta \) is a collection of cyclic triples, each containing pairwise distinct points of \( M \) such that every ordered pair of distinct points of \( M \) exists in exactly one cyclic triple of \( \beta \). An antiautomorphism of a Mendelsohn triple system \( (M, \beta) \) is a permutation of \( M \) which maps \( \beta \) to \( \beta^{-1} \), where \( \beta^{-1} = \{(c, b, a) \mid (a, b, c) \in \beta\} \). Necessary conditions for the existence of an MTS\( (v) \) admitting an antiautomorphism consisting of two cycles of lengths \( M \) and \( N \), where \( 1 < M \leq N \), have been shown, and for the cases of \( N = M \) and \( N = 2M \), sufficiency has been shown. We show sufficiency for the cases in which \( M = 13 \) and \( N = 78, 390, \) and \( 702 \).
The study of the generalized Fermat variety
\[
\phi_j = \frac{x^j + y^j + z^j + (x+y+z)^j}{(x+y)(x+z)(y+z)}
\]
defined over a finite field \( L = \mathbb{F}_q \), where \( q = 2^n \) for some positive integer \( n \), plays an important role in the study of (APN) functions and exceptional APN functions. This study arose after a characterization by Rodier that relates these functions with the number of rational points of \( \phi_j = (x,y,z) \). The most studied cases are when \( j = 2^k + 1 \) and \( j = 2^{2k} – 2^{k} + 1 \), the Gold and Kasami-Welch numbers. In this article, we make a claim about the decomposition of \( \phi_j \) into absolutely irreducible components. If these components intersect transversally at a particular point, then the corresponding Kasami-Welch polynomial is absolutely irreducible. This implies that the function is not exceptional APN, thus helping us make progress on the stated conjecture.
For \( n \geq 1 \), let \( a_n \) count the number of strings \( s_1 s_2 s_3 \ldots s_n \), where
(i) \( s_1 = 0 \);
(ii) \( s_i \in \{0, 1, 2\} \) for \( 2 \leq i \leq n \);
(iii) \( |s_i – s_{i-1}| \leq 1 \) for \( 2 \leq i \leq n \).
Then \( a_1 = 1 \), \( a_2 = 2 \), \( a_3 = 5 \), \( a_4 = 12 \), and \( a_5 = 29 \).
In general, for \( n \geq 3 \), \( a_n = 2a_{n-1} + a_{n-2} \), and \( a_n \) equals \( P_n \), the \( n \)th \emph{Pell} number.
For these \( P_n \) strings of length \( n \), we count
(i) The number of occurrences of each symbol \( 0, 1, 2 \);
(ii) The number of times each symbol \( 0, 1, 2 \) occurs in an even or odd position;
(iii) The number of levels, rises, and descents within the strings;
(iv) The number of runs that occur within the strings;
(v) The sum of all strings considered as base \( 3 \) integers;
(vi) The number of inversions and coinversions within the strings; and
(vii) The sum of the major indices for the strings.
A family of graphs, called Generalized Johnson graphs, provides an abstraction of both Kneser and Johnson graphs.
Given the symmetric nature of Generalized Johnson graphs, we provide various decompositions of these graphs and demonstrate non-trivial instances of the impossibility of decomposing such graphs into triples.
A graphic sequence \( \pi = (d_1, d_2, \ldots, d_n) \) is said to be potentially \( K_{1^3,4} \)-graphic if there is a realization of \( \pi \) containing \( K_{1^3,4} \) as a subgraph, where \( K_{1^3,4} \) is the \( 1 \times 1 \times 1 \times 4 \) complete 4-partite graph. In this paper, we characterize the graphic sequences potentially \( K_{1^3,4} \)-graphic and the result is simple. In addition, we apply this characterization to compute the values of \( \sigma( K_{1^3,4}, n) \).
We show, using a hybrid analysis/linear algebra argument, that the diagonal vector of an infinite symmetric matrix over \(\mathbb{Z}_{2}\) is contained in the range of the matrix. We apply this result to an extension, to the countably infinite case, of the Lights Out problem.
Given a distribution of pebbles on the vertices of a connected graph \( G \), a pebbling move on \( G \) consists of taking two pebbles off one vertex and placing one on an adjacent vertex. The \( t \)-pebbling number \( \pi_t(G) \) is the smallest positive integer such that for every distribution of \( \pi_t(G) \) pebbles and every vertex \( v \), \( t \) pebbles can be moved to \( v \). For \( t = 1 \), Graham conjectured that \( \pi_1(G \Box H) \leq \pi_1(G)\pi_1(H) \) for any connected graphs \( G \) and \( H \), where \( G \Box H \) denotes the Cartesian product of \( G \) and \( H \). Herscovici further conjectured that \( \pi_{st}(G \Box H) \leq \pi_s(G)\pi_t(H) \) for any positive integers \( s \) and \( t \). Lourdusamy [A. Lourdusamy, “\(t\)-pebbling the product of graphs”, Acta Ciencia Indica, XXXII(1)(2006), 171-176] also conjectured that \( \pi_t(C_m \Box C_n) \leq \pi_1(C_m)\pi_t(C_n) \) for cycles \( C_m \) and \( C_n \). In this paper, we show that \( \pi_{st}(C_m \Box C_n) \leq \pi_s(C_m)\pi_t(C_n) \), which confirms this conjecture due to Lourdusamy.
The enhanced hypercube is basically a hypercube with additional edges augmented, where the additional edges connect all pairs of complementary nodes in the hypercube. Taking into account the minimal routing function and the structural properties of the enhanced hypercube, \( n+1 \) internal disjoint paths from one node to other distinct \( n+1 \) nodes have been constructed in an \( n \)-dimensional enhanced hypercube. The results can be used to provide an efficient and reliable routing to avoid congestion, accelerate transmission rate, and provide alternative transmission routes in enhanced hypercube networks, thus remarkably improving the performance of the interconnect networks.
The newly introduced neighborhood matrix extends the power of adjacency and distance matrices to describe the topology of graphs. The adjacency matrix enumerates which pairs of vertices share an edge and it may be summarized by the degree sequence, a list of the adjacency matrix row sums. The distance matrix shows more information, namely the length of shortest paths between vertex pairs. We introduce and explore the neighborhood matrix, which we have found to be an analog to the distance matrix what the degree sequence is to the adjacency matrix. The neighbor matrix includes the degree sequence as its first column and the sequence of all other distances in the graph up to the graph’s diameter, enumerating the number of neighbors each vertex has at every distance present in the graph. We prove this matrix to contain eleven oft-used graph statistics and topological descriptors. We also provide insight into two applications that show potential utility of the neighbor matrix in comparing graphs and identifying topologically significant vertices in a graph.
In this paper, for the Catalan-Larcombe-French sequence \( \{P_n\}_{n\geq0} \) and the Fennessey-Larcombe-French sequence \( \{V_n\}_{n\geq0} \), we mainly discuss the log-behavior of some sequences related to \( \{P_n\}_{n\geq0} \) and \( \{V_n\}_{n\geq0} \). For example, we study the log-behavior of some sequences such as \( \{P_n^2\}_{n\geq0} \), \( \{n!nV_n\}_{n\geq1} \), \( \{n!V_n\}_{n\geq0} \), and \( \{V_n-P_n\}_{n\geq2} \). In addition, we discuss the monotonicity of some sequences involving \( \{P_n\}_{n\geq0} \) and \( \{V_n\}_{n\geq0} \).
A graph \( G \) of order \( |V(G)| \) and size \( |E(G)| \) is called edge-magic if there exists a bijection \( f : V(G) \cup E(G) \to \{1, 2, 3, \dots, |V(G)| + |E(G)|\} \) such that \( f(x) + f(xy) + f(y) \) is a constant for every edge \( xy \in E(G) \). An edge-magic graph \( G \) is said to be super if \( f(V(G)) = \{1, 2, 3, \dots, |V(G)|\} \). Furthermore, the edge-magic deficiency of a graph \( G \), denoted \( \mu(G) \), is defined as the minimum nonnegative integer \( n \) such that \( G \cup nK_1 \) is edge-magic. Similarly, the \emph{super edge-magic deficiency} of a graph \( G \), denoted \( \mu_s(G) \), is either the minimum nonnegative integer \( n \) such that \( G \cup nK_1 \) is super edge-magic or \( +\infty \) if there exists no such integer \( n \). In this paper, we investigate the (super) edge-magic deficiency of chain graphs. Based on these, we propose some open problems.
Given a large finite point set, \( P \subset \mathbb{R}^2 \), we obtain upper bounds on the number of triples of points that determine a given pair of dot products. That is, for any pair of nonzero real numbers, \( (\alpha, \beta) \), we bound the size of the set \[ \{(p, q, r) \in P \times P \times P : p \cdot q = \alpha, p \cdot r = \beta\}. \]
An explicit formula for the number of spanning trees of the lexi- cographic product GLH] of two arbitrary graphs G and H is deduced in terms of structure parameters of G and H. Some properties on the number of spanning trees of G[H] are revealed. Sharp lower and upper bounds for the number of spanning trees of lexicographic product of graphs are established. In particular, simple formulae for the number of spanning trees of the lexicographic product of some special graphs are derived, which extend some previously known results
in the literature.
For a graph \( G \), the expression \( G \overset{v}{\rightarrow} (a_1,\ldots,a_s) \) means that for any \( s \)-coloring of the vertices of \( G \), there exists \( i \in \{1,\ldots,s\} \) such that there is a monochromatic \( a_i \)-clique of color \( i \). The vertex Folkman numbers
\[ F_v(a_1,\ldots,a_s;m-1) = \min\{|V(G)|: G \overset{v}{\rightarrow} (a_1,\ldots,a_s) \text{ and } K_{m-1} \nsubseteq G\} \]
are considered, where \( m = \sum_{i=1}^s (a_i – 1) + 1 \).
With the help of a computer, we show that \( F_v(2,2,5;6) = 16 \), and then we prove
\[ F_v(a_1,\ldots,a_s;m-1) = m+9, \]
if \( \max\{a_1,\ldots,a_s\} = 5 \).
We also obtain the bounds
\[ m+9 \leq F_v(a_1,\ldots,a_s;m-1) \leq m+10, \]
if \( \max\{a_1,\ldots,a_s\} = 6 \).
The diameter of a graph can be affected by the addition or the deletion of some edges. In [3], we have studied the diameter variability of the Cartesian product of graphs. In this paper, we discuss about two fundamental products, strong and lexicographic products of graphs, whose diameter increases (decreases) by the deletion (addition) of a single edge. The problems of minimality and maximality of the product graphs with respect to its diameter are also solved. These problems are motivated by the fact that these graph products are good interconnection networks.
The concept of the skew energy of a digraph was introduced by Adiga, Balakrishnan and So in 2010. An oriented graph \( G^{\sigma} \) is a simple undirected graph \( G \) with an orientation, which assigns to each edge a direction so that \( G^{\sigma} \) becomes a directed graph. Then \( G \) is called the underlying graph of \( G^{\sigma} \). Let \( S(G^{\sigma}) \) be the skew-adjacency matrix of \( G^{\sigma} \) and \( \lambda_1, \lambda_2, \ldots, \lambda_n \) denote all the eigenvalues of \( S(G^{\sigma}) \). The skew energy of \( G^{\sigma} \) is defined as the sum of the absolute values of all eigenvalues of \( S(G^{\sigma}) \). Recently, Gong, Li and Xu determined all oriented graphs with minimal skew energy among all connected oriented graphs on \( n \) vertices with \( m \) (\( n \leq m \leq 2(n-2) \)) arcs. In this paper, we determine all oriented graphs with the second and the third minimal skew energy among all connected oriented graphs with \( n \) vertices and \( m \) (\( n \leq m < 2(n-2) \)) arcs. In particular, when the oriented graphs are unicyclic digraphs or bicyclic digraphs, the second and the third minimal skew energy is determined.
In this paper, according to the symmetric Lanczos algorithm and general Gauss-type quadrature rule, we give some lower bounds on the Resolvent Estrada index \( EE_r(G) \) and the Resolvent energy \( ER(G) \).
The chordal-(\(k,l\)) sandwich and strongly chordal-(\(k,l\)) sandwich problems were considered in recent work [8, 9] where classification of the complexities of the problems for all possible nonnegative integer values for \(k, l\) was considered. We extend the classification in [8, 9] by presenting polynomial time algorithms for some cases that remained open; currently, very few graph sandwich problems are known to be solvable in polynomial time.
An odd open dominating set of a graph is a subset of the graph’s vertices with the property that the open neighborhood of each vertex in the graph contains an odd number of vertices in the subset. An odd closed \( r \)-dominating set is a subset of the graph’s vertices with the property that the closed \( r \)-ball centered at each vertex in the graph contains an odd number of vertices in the subset.
We show that the \( n \)-fold direct product of simple graphs has an odd open dominating set if and only if each factor has an odd open dominating set. Secondly, we show that the \( n \)-fold strong product of simple graphs has an odd closed \( r \)-dominating set if and only if each factor has an odd closed \( r \)-dominating set.
Multireceiver authentication codes allow one sender to construct an authenticated message for a group of receivers such that each receiver can verify the authenticity of the received message. In this paper, we construct one multireceiver authentication code from pseudo-symplectic geometry over finite fields. The parameters and the probabilities of deceptions of the codes are also computed. The smaller the probability of successful attack, the higher the security of the authentication codes.
Ryser’s Conjecture states that for any \( r \)-partite \( r \)-uniform hypergraph, the vertex cover number is at most \( r-1 \) times the matching number. This conjecture is only known to be true for \( r \leq 3 \). For intersecting hypergraphs, Ryser’s Conjecture reduces to saying that the edges of every \( r \)-partite intersecting hypergraph can be covered by \( r-1 \) vertices. This special case of the conjecture has only been proven for \( r \leq 5 \).
It is interesting to study hypergraphs which are extremal in Ryser’s Conjecture, i.e., those hypergraphs for which the vertex cover number is exactly \( r-1 \) times the matching number. There are very few known constructions of such graphs. For large \( r \), the only known constructions come from projective planes and exist only when \( r-1 \) is a prime power. Mansour, Song, and Yuster studied how few edges a hypergraph which is extremal for Ryser’s Conjecture can have. They defined \( f(r) \) as the minimum integer so that there exists an \( r \)-partite intersecting hypergraph \( \mathcal{H} \) with \( \tau(\mathcal{H}) = r-1 \) and with \( f(r) \) edges. They showed that \( f(3) = 3 \), \( f(4) = 6 \), \( f(5) = 9 \), and \( 12 \leq f(6) \leq 15 \).
In this paper, we focus on the cases when \( r = 6, 7 \), and \( 11 \). We show that \( f(6) = 13 \), improving previous bounds. Also, by providing the first known extremal hypergraphs for the \( r = 7 \) and \( r = 11 \) case of Ryser’s Conjecture, we show that \( f(7) \leq 22 \) and \( f(11) \leq 51 \). Our results for \( f(6) \) and \( f(7) \) have been obtained independently by Aharoni, Barat, and Wanless.
Let \( T = S \setminus \left( \cup \left\{ A : A \, \text{ in } \, \mathcal{A} \right\} \right) \), where \( S \) is an orthogonal polytope in \( \mathbb{R}^d \) for \( d \geq 2 \) and where \( \mathcal{A} \) is a collection of \( n \) pairwise disjoint open boxes contained in \( S \). Point \( x \) belongs to \(\text{Ker } T \) if and only if \( x \) belongs to \(\text{Ker } S \) and no coordinate line at \( x \) meets any \( A \) in \( \mathcal{A} \). In turn, this relationship between the staircase kernels of \( S \) and \( T \) produces a Krasnosel’skii-type result for \( T \) in terms of \( n \), extending the class of orthogonal polytopes for which such a theorem exists.
A graph is said to be claw-free if it does not contain an induced subgraph isomorphic to \(K_{1,3}\). Let \(k\) be an integer with \(k \geq 2\). We prove that if \(G\) is a claw-free graph of order at least \(14k – 13\) and with minimum degree at least four, then \(G\) contains \(k\) vertex-disjoint copies of \(K_{1,4}\). This partially supports a conjecture proposed by Jiang, Chiba, Fujita and Yan.
A graph \( G \) is said to be \( k \)-\(\gamma\)-edge critical if the domination number \(\gamma(G) = k\) and \(\gamma(G + uv) < k\) for every \( uv \notin E(G) \). For the connected domination number \(\gamma_c(G) = k\), the total domination number \(\gamma_t(G) = k\) and the independent domination number \( i(G) = k \), a \( k \)-\(\gamma_c\)-edge critical graph, a \( k \)-\(\gamma_t\)-edge critical graph and a \( k \)-\(i\)-edge critical graph are similarly defined. In our previous work, we proved that every \( 2 \)-connected \( k \)-\(\gamma_c\)-edge critical graph is hamiltonian for \( 1 \leq k \leq 3 \) and we provided a class of \( l \)-connected \( k \)-\(\gamma_c\)-edge critical non-hamiltonian graphs for \( k \geq 4 \) and \( 2 \leq l \leq \frac{n-3}{k-1} \). The problem of interest is to determine a sufficient condition for \( k \)-\(\gamma_c\)-edge critical graphs to be hamiltonian for \( k \geq 4 \). In this paper, we prove that every \( 2 \)-connected \( 4 \)-\(\gamma_c\)-edge critical claw-free graph is hamiltonian. For \( k \geq 5 \), we provide a class of \( k \)-\(\gamma_c\)-edge critical claw-free non-hamiltonian graphs of connectivity two. We further show that all \( 3 \)-connected \( k \)-\(\gamma_c\)-edge critical claw-free graphs are hamiltonian for \( 1 \leq k \leq 6 \). Our methodology also establishes some results on the hamiltonian properties of \( 3 \)-connected \( k \)-\(\mathcal{D} \)-edge critical claw-free graphs where \( \mathcal{D} \in \{ \gamma, \gamma_t, i \} \).
A star forest is a forest each of whose components is a star. The star arboricity of a graph \(G\), denoted by \(\textrm{st}(G)\), is the minimum number of star forests whose union covers all the edges of \(G\). A nonzero element of a commutative ring \(R\) with unity is said to be a \({zero-divisor}\) of \(R\) if there exists a nonzero element \(y \in R\) such that \(xy = 0\). Given a ring \(R\) with unity, the \({zero-divisor\; graph}\) of \(R\), denoted by \(\Gamma(R)\), is the graph whose vertex set consists of the zero divisors of \(R\) and two vertices \(x, y \in V(\Gamma(R))\) are adjacent if and only if \(xy = 0\) in \(R\). This paper investigates the star arboricities of the zero divisor graphs \(\Gamma(\mathbb{Z}_{p^n})\), where \(n, p \in \mathbb{N}\) and \(p\) is a prime. In particular, we give bounds for \(\textrm{st}(\Gamma(\mathbb{Z}_{p^n}))\) when \(n\) is odd and determine the values of \(\textrm{st}(\Gamma(\mathbb{Z}_{p^n}))\) when \(n\) is even.
An adjacent vertex distinguishing total coloring of a graph \(G\) is a proper total \(k\)-coloring of \(G\) such that any two adjacent vertices have different color sets, where the color set of a vertex \(v\) contains the color of \(v\) and the colors of its incident edges. Let \(\chi_{a}^{”}(G)\) denote the smallest value \(k\) in such a coloring of \(G\). In this paper, by using the Combinatorial Nullstellensatz and the discharging method, we prove that if a planar graph \(G\) with maximum degree \(\Delta \geq 9\) contains no \(5\)-cycles with more than one chord, then \(\chi_{a}^{”}(G) \leq \Delta + 3\).
The concept of the skew energy of a digraph was introduced by Adiga, Balakrishnan and \(S_0\) in \(2010\). Let \(\overrightarrow{G}\) be an oriented graph of order \(n\) and \(\lambda_1, \lambda_2, \dots, \lambda_n\) denote all the eigenvalues of the skew-adjacency matrix of \(\overrightarrow{G}\). The skew energy \(\varepsilon_s(\overrightarrow{G}) = \sum\limits_{i=1}^{n} |\lambda_i|\). Hou, Shen and Zhang determined the minimal and the second minimal skew energy of the oriented unicyclic graphs. In this paper, the oriented unicyclic graphs with the third, fourth and fifth minimal skew energy are characterized, respectively.
Two graphs are defined to be adjointly equivalent if their complements are chromatically equivalent. Recently, we introduced a new invariant of a graph \(G\), denoted as \(R_5(G)\). Using this invariant and the properties of the adjoint polynomials, we completely determine the adjoint equivalence class of \(\psi_n^3({n-3,1})\). According to the relations between adjoint polynomial and chromatic polynomial, we also simultaneously determine the chromatic equivalence class of \(\psi_n^3({n-3,1})\).
In this article, we prove a conjecture about the equality of two generating functions described in “From Parking Functions to Gelfand Pairs” (Aker, Can, 2012) attached to two sets whose cardinalities are given by Catalan numbers. We establish a combinatorial bijection between the two sets on which the two generating functions were based.
Let \(G\) be a finite cyclic group. Every sequence \(S\) of length \(l\) over \(G\) can be written in the form \(S = (x_1g) + \cdots + (x_lg)\), where \(g \in G\) and \(x_1, \ldots, x_l \in [1, ord(g)]\), and the index \(ind(S)\) of \(S\) is defined to be the minimum of \((x_1 + \cdots + x_l)/ord(g)\) over all possible \(g \in G\) such that \(\langle g \rangle = G\). Recently, the second and third authors determined the index of any minimal zero-sum sequence \(S\) of length \(5\) over a cyclic group of a prime order where \(S =g^2 \cdot (x_2g)\cdot (x_3g)\cdot (x_4g)\). In this paper, we determine the index of any minimal zero-sum sequence \(S\) of length \(5\) over a cyclic group of a prime power order. It is shown that if \(G = \langle g \rangle\) is a cyclic group of prime power order \(n = p^{\mu}\) with \(p \geq 7\) and \(\mu \geq 2\), and \(S = (x_1g) \cdot (x_2g) \cdot (x_3g) \cdot (x_4g) \cdot (x_5g)\) with \(x_1 = x_2\) is a minimal zero-sum sequence with \(\gcd(n, x_1, x_2, x_3, x_4, x_5) = 1\), then \(ind(S) = 2\) if and only if \(S = (mg) \cdot (mg) \cdot (m\frac{n-1}{2}g) \cdot (m\frac{n+3}{2}g) \cdot (m(n-3)g)\) where \(m\) is a positive integer such that \(\gcd(m,n) = 1\).
Let \(G\) be a graph with vertex set \(V(G)\). For any integer \(k \geq 1\), a signed \(k\)-dominating function is a function \(f: V(G) \rightarrow \{-1, 1\}\) satisfying \(\sum_{x \in N[v]} f(t) \geq k\) for every \(v \in V(G)\), where \(N[v]\) is the closed neighborhood of \(v\). The minimum of the values \(\sum_{v \in V(G)} f(v)\), taken over all signed \(k\)-dominating functions \(f\), is called the signed \(k\)-domination number. In this note, we present some new lower bounds on the signed \(k\)-domination number of a graph. Some of our results improve known bounds.
In this paper, we define and study the \(k\)-order Gaussian Fibonacci and Lucas numbers with boundary conditions. We identify and prove the generating functions, the Binet formulas, the summation formulas, matrix representation of \(k\)-order Gaussian Fibonacci numbers, and some significant relationships between \(k\)-order Gaussian Fibonacci and \(k\)-order Lucas numbers, connecting them with usual \(k\)-order Fibonacci numbers.
A vertex-colored path is vertex-rainbow if its internal vertices have distinct colors. For a connected graph \(G\) with connectivity \(\kappa(G)\) and an integer \(k\) with \(1 \leq k \leq \kappa(G)\), the rainbow vertex \(k\)-connectivity of \(G\) is the minimum number of colors required to color the vertices of \(G\) such that any two vertices of \(G\) are connected by \(k\) internally vertex-disjoint vertex-rainbow paths. In this paper, we determine the rainbow vertex \(k\)-connectivities of all small cubic graphs of order \(8\) or less.
For a simple graph \(G = (V, E)\), a vertex labeling \(\alpha: V \rightarrow \{1, 2, \ldots, k\}\) is called a \(k\)-labeling. The weight of an edge \(xy\) in \(G\), denoted by \(w_\phi(xy)\), is the sum of the labels of end vertices \(x\) and \(y\), i.e., \(w_\phi(xy) = \phi(x) + \phi(y)\). A vertex \(k\)-labeling is defined to be an edge irregular \(k\)-labeling of the graph \(G\) if for every two different edges \(e\) and \(f\) there is \(w_\phi(e) \neq w_\phi(f)\). The minimum \(k\) for which the graph \(G\) has an edge irregular \(k\)-labeling is called the edge irregularity strength of \(G\), denoted by \(\mathrm{es}(G)\). In this paper, we determine the exact value for certain families of graphs with path \(P_2\).
We give a \(q\)-analogue of some Dixon-like summation formulas obtained by Gould and Quaintance [Fibonacci Quart. 48 (2010), 56-61] and Chu [Integral Transforms Spec. Funct. 23 (2012), 251-261], respectively. For example, we prove that
\(\sum\limits_{k=0}^{2m} (-1)^{m-k} q^{\binom{m-k}{2}} \binom{2m} {k} \binom{x+k} {2m+r}\binom{x+2m-k} {2m+r}\) = \(\frac{q^{m(x-m-r)}\binom{2m}{m}}{\binom{2m+r}{m}}\binom{x}{m+r}\binom{x+m}{m+r}\) where \(\binom{x}{k}\) denotes the \(q\)-binomial coefficient.
A pentangulation is a simple plane graph such that each face is bounded by a cycle of length \(5\). We consider two diagonal transformations in pentangulations, called \(\mathcal{A}\) and \(\mathcal{B}\). In this paper, we shall prove that any two pentangulations with the same number of vertices can be transformed into each other by \(\mathcal{A}\) and \(\mathcal{B}\). In particular, if they are not isomorphic to a special pentangulation, then we do not need \(\mathcal{B}\).
The harmonic index \(H(G)\) of a graph \(G\) is defined as the sum of the weights of all edges \(uv\) of \(G\), where the weight of \(uv\) is \(\frac{2}{d(u) + d(v)}\), with \(d(u)\) denoting the degree of the vertex \(u\) in \(G\). In this work, we compute the harmonic index of a graph with a cut-vertex and with more than one cut-vertex. As an application, this topological index is computed for Bethe trees and dendrimer trees. Also, the harmonic indices of Fasciagraph and a special type of trees, namely, polytree, are computed.
Let \(G^{\sigma}\) be an oriented graph obtained by assigning an orientation \(\sigma\) to the edge set of a simple undirected graph \(G\). Let \(S(G^{\sigma})\) be the skew adjacency matrix of \(G^{\sigma}\). The skew energy of \(G^{\sigma}\) is defined as the sum of the absolute values of all eigenvalues of \(S(G^{\sigma})\). In this paper, we give the skew energy order of a family of digraphs and determine the oriented bicyclic graphs of order \(n \geq 13\) with the first five largest skew energies, which extends the results of the paper [X. Shen, Y. Hou, C. Zhang, Bicyclic digraphs with extremal skew energy, Electron. J. Linear Algebra 23 (2012) 340-355].
Let \(P_n\) denote the \(n\)-th Catalan-Larcombe-French number. Recently, the \(2\)-log-convexity of the Catalan-Larcombe-French sequence was proved by Sun and Wu. Moreover, they also conjectured that the quotient sequence \(\{\frac{P_{n}}{P_{n-1}}\}_{n= 0}^\infty\) of the Catalan-Larcombe-French sequence is log-concave. In this paper, this conjecture is confirmed by utilizing the upper and lower bounds for \(\frac{P_{n}}{P_{n-1}}\) and finding a middle function \(f(n)\).
It is claimed in [13] that the metric dimension of the Möbius ladder \(M_n\) is \(3\) when \(n \not\equiv 2 \pmod{8}\), but it is wrong; we give a counterexample when \(n \equiv 6 \pmod{8}\). In this paper, we not only give the correct metric dimension in this case but also solve the open problem regarding the metric dimension of \(M_n\) when \(n \equiv 2 \pmod{8}\). Moreover, we conclude that \(M_n\) has two subfamilies with constant metric dimensions.
An edge-colored graph \(G\) is (strong) rainbow connected if any two vertices are connected by a (geodesic) path whose edges have distinct colors. The (strong) rainbow connection number of a connected graph \(G\), denoted by \(\mathrm{src}(G)\) (resp. \(\mathrm{rc}(G)\)), is the smallest number of colors that are needed in order to make \(G\) (strong) rainbow connected. The join \(P_m \vee P_n\) of \(P_m\) and \(P_n\) is the graph consisting of \(P_m\cup P_n\), and all edges between every vertex of \(P_m\) and every vertex of \(P_n\), where \(P_m\) (resp. \(P_n\)) is a path of \(m\) (resp. \(n\)) vertices. In this paper, the precise values of \(\mathrm{rc}(P_m \vee P_n)\) and \(\mathrm{src}(P_m \vee P_n)\) are given for any positive integers \(m\) and \(n\).
Let \(MG(i,n)\) be a connected molecular graph without multiple edges on \(n\)vertices whose minimum degree of vertices is \(i\), where \(i \leq i \leq 4\). One of the newest topological indices is the first Geometric-Arithmetic index. In this paper, we determine the graph with the minimum and the maximum value of the first Geometric-Arithmetic index in the family of graphs \(M{G}(i,n)\),\(l\leq i \leq 3\).
Two graphs are said to be Tutte-equivalent if their Tutte polynomials are equal. In this paper, we provide several different constructions for Tutte-equivalent graphs, including some that are not self-complementary but Tutte-equivalent to their complements (the Akiyama-Harary problem) and some “large” Tutte-equivalent graphs obtained from “small” Tutte-equivalent graphs by \(2\)-sum operations.
Let \(s(n, k) = \binom{6k}{3k} \binom{3k}{k} (\binom{3(n-k)}{n-k} / (2n-1) \binom{3n}{n})\). Recently, Guo confirmed a conjecture of \(Z.-W\). Sun by showing that \(s(n, k)\) is an integer for \(k = 0, 1, \ldots, n\). Let \(d = (3n + 2) / \gcd(3n + 2, 2n – 1)\). In this paper, we prove that \(s(n, k)\) is a multiple of the odd part of \(d\) for \(k = 0, 1, \ldots, n\). Furthermore, if \(\gcd(k, n) = 1\), then \(s(n, k)\) is also a multiple of \(n\). We also show that the \(2\)-adic order of \(s(n, k)\) is at least the sum of the digits in the binary expansion of \(3n\).
For any non-trivial abelian group \(A\) under addition, a graph \(G\) is said to be strong \(A\)-magic if there exists a labeling \(f\) of the edges of \(G\) with non-zero elements of \(A\) such that the vertex labeling \(f^+\) defined as \(f^+(v) = \sum f(uv)\) taken over all edges \(uv\) incident at \(v\) is a constant, and the constant is same for all possible values of \(|V(G)|\). A graph is said to be strong \(A\)-magic if it admits strong \(A\)-magic labeling. In this paper, we consider \((\mathbb{Z}_4, +)\) as an abelian group and we prove strong \(\mathbb{Z}_4\)-magic labeling for various graphs and generalize strong \(\mathbb{Z}_{4p}\)-magic labeling for those graphs. The graphs which admit strong \(\mathbb{Z}_{4p}\)-magic labeling are called as strong \(\mathbb{Z}_{4p}\)-magic graphs.
The well-known Mantel’s Theorem states that a graph on \(n\) vertices and \(m\) edges contains a triangle if \(m > \frac{n^2}{4}\). Nosal proved that every graph on \(m\) edges contains a triangle if the spectral radius \(\lambda_1 > \sqrt{m}\), which is a spectral analog of Mantel’s Theorem. Furthermore, by using Motzkin-Straus Inequality, Nikiforov sharpened Nosal’s result and characterized the extremal graphs when the equality holds. Our first contribution in this note is to give two new proofs of the spectral concise Mantel’s Theorem due to Nikiforov (without help of Motzkin-Straus Inequality). Nikiforov also obtained some results concerning the existence of consecutive cycles and spectral radius. Second, we prove a theorem concerning the existence of consecutive even cycles and spectral radius, which slightly improves a result of Nikiforov. At last, we focus on spectral radius inequalities. Hong proved his famous bound for spectral radius. Later, Hong, Shu, and Fang generalized Hong’s bound to connected graphs with given minimum degree. By using quite different techniques, Nikiforov proved Hong et al.’s bound for general graphs independently. In this note, we prove a new spectral inequality by applying the technique of Nikiforov. Our result extends Stanley’s spectral inequality.
The alliance polynomial of a graph with order \(n\) and maximum degree \(\Delta\) is the polynomial \(A(\Gamma; x) = \sum_{k=-\delta_1}^{\delta_1}A_k(\Gamma) x^{n+k}\), where \(A_k(G)\) is the number of exact defensive \(k\)-alliances in \(G\). We provide an algorithm for computing the alliance polynomial. Furthermore, we obtain some properties of \(A(\Gamma; x)\) and its coefficients. In particular, we prove that the path, cycle, complete, and star graphs are characterized by their alliance polynomials. We also show that the alliance polynomial characterizes many graphs that are not distinguished by other usual polynomials of graphs.
Let \(a\), \(b\), and \(k\) be three nonnegative integers with \(a \geq 2\) and \(b \geq a(k+1)+2\). A graph \(G\) is called a \(k\)-Hamiltonian graph if \(G – U\) contains a Hamiltonian cycle for every subset \(U \subseteq V(G)\) with \(|U| = k\). An \([a, b]\)-factor \(F\) of \(G\) is called a Hamiltonian \([a, b]\)-factor if \(F\) contains a Hamiltonian cycle. If \(G – U\) has a Hamiltonian \([a, b]\)-factor for every subset \(U \subseteq V(G)\) with \(|U| = k\), then we say that \(G\) admits a \(k\)-Hamiltonian \([a, b]\)-factor. Suppose that \(G\) is a \(k\)-Hamiltonian graph of order \(n\) with \(n \geq a+k+2\). In this paper, it is proved that \(G\) includes a \(k\)-Hamiltonian \([a, b]\)-factor if \(\delta(G) \geq a+k\) and \(t(G) \leq a-1+\frac{(a-1)(k+1)}{b-2}\).
Graph embedding has been known as a powerful tool for implementation of parallel algorithms or simulation of different interconnection networks. An embedding \(f\) of a guest graph \(G\) into a host graph \(H\) is a bijection on the vertices such that each edge of \(G\) is mapped into a path of \(H\). In this paper, we introduce a graph called the generalized book and the main results obtained are: (1) For \(r \geq 3\), the minimum wirelength of embedding \(r\)-dimensional hypercube \(Q_r\) into the generalized book \(\mathrm{GB}[2^{r_1}, 2^{r_2}, 2^{r_3}]\), where \(r_1 + r_2 + r_3 = r\). (2) A linear time algorithm to compute the exact wirelength of embedding hypercube into generalized book. (3) An algorithm for embedding hypercube into generalized book with dilation 3, proving that the lower bound obtained by Manuel et al. [28] is sharp.
In this paper, we present a new approach to the convolved Fibonacci numbers arising from the generating function of them and give some new and explicit identities for the convolved Fibonacci numbers.
The generalized Fibonacci cube \(Q_d(f)\) is the graph obtained from the hypercube \(Q_d\) by removing all vertices that contain a given binary word \(f\). A binary word \(f\) is called good if \(Q_d(f)\) is an isometric subgraph of \(Q_d\) for all \(d \geq 1\), and bad otherwise. A non-extendable sequence of contiguous equal digits in a word \(f\) is called a block of \(f\). The question to determine the good (bad) words consisting of at most three blocks was solved by Ilié, Klavžar, and Rho. This question is further studied in the present paper. All the good (bad) words consisting of four blocks are determined completely, and all bad \(2\)-isometric words among consisting of at most four blocks words are found to be \(1100\) and \(0011\).
In this paper, we provide a construction of \(\mathrm{PG}(2,4)\) by a collage of \(\mathrm{AG}(2,3)\) and its dual \(\mathrm{DAG}(2,3)\). Moreover, we prove that the construction is unique.
In this paper, we first present a combinatorial proof of the recurrence relation about the number of the inverse-conjugate compositions of \(2n+1\), \(n > 1\). And then we get some counting results about the inverse-conjugate compositions for special compositions. In particular, we show that the number of the inverse-conjugate compositions of \(4k+1\), \(k > 0\) with odd parts is \(2^k\), and provide an elegant combinatorial proof. Lastly, we give a relation between the number of the inverse-conjugate odd compositions of \(4k+1\) and the number of the self-inverse odd compositions of \(4k+1\).
In this study, by using Jacobsthal and Jacobsthal Lucas matrix sequences, we define \(k\)-Jacobsthal and \(k\)-Jacobsthal Lucas matrix sequences depending on one parameter \(k\). After that, by using two parameters \((s,t)\), we define \((s,t)\)-Jacobsthal and \((s,t)\)-Jacobsthal Lucas matrix sequences. And then, we establish combinatoric representations of all of these matrices.
A graph \(G\) is \(1\)-planar if it can be embedded in the plane \(\mathbb{R}^2\) so that each edge of \(G\) is crossed by at most one other edge. In this paper, we show that each \(1\)-planar graph of maximum degree \(\Delta\) at least \(7\) with neither intersecting triangles nor chordal \(5\)-cycles admits a proper edge coloring with \(\Delta\) colors.
Dirac showed that in a \((k-1)\)-connected graph there is a path through each \(k\) vertices. The path \(k\)-connectivity \(\pi_k(G)\) of a graph \(G\), which is a generalization of Dirac’s notion, was introduced by Hager in 1986. Recently, Mao introduced the concept of path \(k\)-edge-connectivity \(\omega_k(G)\) of a graph \(G\). Denote by \(G \circ H\) the lexicographic product of two graphs \(G\) and \(H\). In this paper, we prove that \(\omega_4(G \circ H) \geq \omega_4(G) |V(H)|\) for any two graphs \(G\) and \(H\). Moreover, the bound is sharp.
A graph \(G = (V(G), E(G))\) is even graceful and equivalently graceful, if there exists an injection \(f\) from the set of vertices \(V(G)\) to \(\{0, 1, 2, 3, 4, \ldots, 2|E(G)|\}\) such that when each edge \(uv\) is assigned the label \(|f(u) – f(v)|\), the resulting edge labels are \(2, 4, 6, \ldots, 2|E(G)|\). In this work, we use even graceful labeling to give a new proof for necessary and sufficient conditions for the gracefulness of the cycle graph. We extend this technique to odd graceful and super Fibonacci graceful labelings of cycle graphs via some number theoretic concept, called a balanced set of natural numbers.
A graph is \(1\)-planar if it can be drawn on the plane so that each edge is crossed by at most one other edge. In this paper, we prove that every \(1\)-planar graph without \(4\)-cycles or adjacent \(5\)-vertices is \(5\)-colorable.
In previous researches on classification problems, there are some similar results obtained between \(f\)-coloring and \(g_c\)-coloring. In this article, the author shows that there always are coincident classification results for a regular simple graph \(G\) when the \(f\)-core and the \(g_c\)-core of \(G\) are same and \(f(v) = g(v)\) for each vertex \(v\) in the \(f\)-core (the \(g_c\)-core) of \(G\). However, it is not always coincident for nonregular simple graphs under the same conditions. In addition, the author obtains some new results on the classification problem of \(f\)-colorings for regular graphs. Based on the coincident correlation mentioned above, new results on the classification problem of \(g_c\)-colorings for regular graphs are deduced.
The integrity of a graph \(G = (V, E)\) is defined as \(I(G) = \min\{|S| + m(G-S): S \subseteq V(G)\}\), where \(m(G-S)\) denotes the order of the largest component in the graph \(G-S\). This is a better parameter to measure the stability of a network, as it takes into account both the amount of work done to damage the network and how badly the network is damaged. Computationally, it belongs to the class of intractable problems known as NP-hard. In this paper, we develop a heuristic algorithm to determine the integrity of a graph. Extensive computational experience on \(88\) randomly generated graphs ranging from \(20\%\) to \(90\%\) densities and from \(100\) to \(200\) vertices has shown that the proposed algorithm is very effective.
For a finite group \( G \), a bijection \( \theta: G \to G \) is a \({strong \;complete \;mapping}\) if the mappings \( g \mapsto g\theta(g) \) and \( g \mapsto g^{-1}\theta(g) \) are both bijections. A group is \({strongly \;admissible}\) if it admits strong complete mappings. Strong complete mappings have several combinatorial applications. There exists a Latin square orthogonal to both the multiplication table of a finite group \( G \) and its normal multiplication table if and only if \( G \) is strongly admissible. The problem of characterizing strongly admissible groups is far from settled. In this paper, we will update progress towards its resolution. In particular, we will present several infinite classes of strongly admissible dihedral and quaternion groups and determine all strongly admissible groups of order at most \(31\).
The complete directed graph of order \(n\), denoted \({K}_n^*\), is the directed graph on \(n\) vertices that contains the arcs \((u,v)\) and \((v,u)\) for every pair of distinct vertices \(u\) and \(v\). For a given directed graph \(D\), the set of all \(n\) for which \({K}_n^*\) admits a \(D\)-decomposition is called the spectrum of \(D\). In this paper, we find the spectrum for each bipartite subgraph of \({K}_4^*\) with 5 or fewer arcs.
A bipartite graph on \(n\) vertices, with \(n\) even, is called uniquely bi-pancyclic (UBPC) if it contains precisely one cycle of length \(2m\) for every \(2 \leq m \leq \frac{n}{2}\). In this note, using computer programs, we show that if \(32 \leq n \leq 56\), and \(n \neq 44\), then there are no UBPC graphs of order \(n\). We also present the six non-isomorphic UBPC graphs of order 44. This improves the recent results on UBPC graphs of order at most 30.
A graph \(G\) with vertex set \(V\) and edge set \(E\) is called super edge-graceful if there is a bijection \(f\) from \(E\) to \(\{0, \pm 1, \pm 2, \dots, \pm (|E| – 1)/2\}\) when \(|E|\) is odd and from \(E\) to \(\{\pm1, \pm2, \dots, \pm|E|/2\}\) when \(|E|\) is even, such that the induced vertex labeling \(f^*\) defined by \(f^*(u) = \sum f(uv)\) over all edges \(uv\) is a bijection from \(V\) to \(\{0, \pm 1, \pm 2, \dots, \pm (|V| – 1)/2\}\) when \(|V|\) is odd and from \(V\) to \(\{\pm1, \pm2, \dots, \pm|V|/2\}\) when \(|V|\) is even. A kite is a graph formed by merging a cycle and a path at an endpoint of the path. In this paper, we prove that all kites with \(n \geq 5\) vertices, \(n \neq 6\), are super edge-graceful.
A graph with \(v\) vertices is \((r)\)-pancyclic if it contains precisely \(r\) cycles of every length from \(3\) to \(v\). A bipartite graph with an even number of vertices \(v\) is said to be \((r)\)-bipancyclic if it contains precisely \(r\) cycles of each even length from \(4\) to \(v\). A bipartite graph with an odd number of vertices \(v\) and minimum degree at least \(2\) is said to be oddly \((r)\)-bipancyclic if it contains precisely \(r\) cycles of each even length from \(4\) to \(v-1\). In this paper, using a computer search, we classify all \((r)\)-pancyclic and \((r)\)-bipancyclic graphs, \(r \geq 2\), with \(v\) vertices and at most \(v+5\) edges. We also classify all oddly \((r)\)-bipancyclic graphs, \(r \geq 1\), with \(v\) vertices and at most \(v+4\) edges.
A handicap distance antimagic labeling of a graph \(G = (V,E)\) with \(n\) vertices is a bijection \(f : V \to \{1,2,\dots,n\}\) with the property that \(f(x_i) = i\) and the sequence of the weights \(w(x_1), w(x_2), \dots, w(x_n)\) (where \(w(x_i) = \sum_{x_j \in N(x_i)}{f(x_j)}\)) forms an increasing arithmetic progression. A graph \(G\) is a handicap distance antimagic graph if it allows a handicap distance antimagic labeling.
We construct regular handicap distance antimagic graphs for every feasible odd order.
A well-known subclass of chordal graphs is formed by proper interval graphs. Due to their very special structural properties, several problems proved hard to solve for interval graphs can have better solutions for this subclass. In this paper, we address the recognition problem, proposing an update of one of the first existing linear algorithms. The outcome is a simple and efficient algorithm. In addition, we present a certifying algorithm for the recognition of proper interval graphs
A bipartite graph on \(2n\) vertices is called bipancyclic if it contains cycles of every length from \(4\) to \(2n\). In this paper we address the question: what is the minimum number of edges in a bipancyclic graph? We present a simple analysis of some small orders using chord patterns.
A vertex set \(U \subset V\) in a connected graph \(G = (V, E)\) is a cutset if \(G – U\) is disconnected. If no proper subset of \(U\) is also a cutset of \(G\), then \(U\) is a minimal cutset. An \(\mathcal{MVC}\)-partition \(\pi = \{V_1, V_2, \dots, V_k\}\) of the vertex set \(V(G)\) of a connected graph \(G\) is a partition of \(V(G)\) such that every \(V_i \in \pi\) is a minimal cutset of \(G\). For an \(\mathcal{MVC}\)-partition \(\pi\) of \(G\), the \(\pi\)-graph \(G_{\pi}\), of \(G\) has vertex set \(\pi\) such that \(V’,V” \in \pi\) are adjacent in \(G_{\pi}\), if and only if there exist \(v’ \in V’\) and \(v” \in V”\) such that \(v’v” \in E(G)\). Graphs that are a \(\pi\)- graphs of cycles are characterized. A homomorphic image \(H\) of a graph \(G\) can be obtained from a partition \(\mathcal{P} = {V_1, V_2,…, V_k}\) of \(V(G)\) into independent sets such that \(V(H) = {v_1,v_2,…, v_k}\), where \(v_i\) is adjacent to \(v_j\); if and only if some vertex of \(V_j\) is adjacent to some vertex of \(V_j\) in \(G\). By investigating graphs \(H\) that are homomorphic images of the Cartesian product \(H\Box K_2\), it is shown that for every nontrivial connected graph \(H\) and every integer \(r \geq 2\), there exists an \(r\)-regular graph \(G\) such that \(H\) is a homomorphic image of \(G\). It is also shown that every nontrivial tree \(T\) is a homomorphic image of \(T\Box K_2\) but that not all graphs \(H\) are homomorphic images of \(H\Box K_2\).
A Hamiltonian graph \(G\) is said to be \(\ell\)-path-Hamiltonian, where \(\ell\) is a positive integer less than or equal to the order of \(G\), if every path of order \(\ell\) in \(G\) is a subpath of some Hamiltonian cycle in \(G\). The Hamiltonian cycle extension number of \(G\) is the maximum positive integer \(L\) for which \(G\) is \(\ell\)-path-Hamiltonian for every integer \(\ell\) with \(1 \leq \ell \leq L\). Hamiltonian cycle extension numbers are determined for several well-known cubic Hamiltonian graphs. It is shown that if \(G\) is a cubic Hamiltonian graph with girth \(g\), where \(3 \leq g \leq 7\), then \(G\) is \(\ell\)-path-Hamiltonian only if \(1 \leq \ell \leq g\).
In a red-blue coloring of a graph \(G\), every edge of \(G\) is colored red or blue. For two graphs \(F\) and \(H\), the Ramsey number \(R(F, H)\) of \(F\) and \(H\) is the smallest positive integer \(n\) such that every red-blue coloring of the complete graph \(K_n\) of order \(n\) results in either a subgraph isomorphic to \(F\) all of whose edges are colored red or a subgraph isomorphic to \(H\) all of whose edges are colored blue. While the study of Ramsey numbers has been a popular area of research in graph theory, over the years a number of variations of Ramsey numbers have been introduced. We look at several of these, with special emphasis on some of those introduced more recently.
For a graph \(G\) of size \(m\), a graceful labeling of \(G\) is an injective function \(f : V(G) \to \{0, 1, \dots, m\}\) that gives rise to a bijective function \(f’ : E(G) \to \{1, 2, \dots, m\}\) defined by \(f'(uv) = |f(u) – f(v)|\). A graph \(G\) is graceful if \(G\) has a graceful labeling. Over the years, a number of variations of graceful labelings have been introduced, some of which have been described in terms of colorings. We look at several of these, with special emphasis on some of those introduced more recently.
For a connected graph \(G\) of order at least \(3\), let \(c : E(G) \to \{1, 2, \dots, k\}\) be an edge coloring of \(G\) where adjacent edges may be colored the same. Then \(c\) induces a vertex coloring \(c’\) of \(G\) by assigning to each vertex \(v\) of \(G\) the set of colors of the edges incident with \(v\). The edge coloring \(c\) is called a majestic \(k\)-edge coloring of \(G\) if the induced vertex coloring \(c’\) is a proper vertex coloring of \(G\). The minimum positive integer \(k\) for which a graph \(G\) has a majestic \(k\)-edge coloring is the majestic chromatic index of \(G\) and denoted by \(\chi_{m}^{‘} (G)\). For a graph \(G\) with \(\chi_{m}^{‘}(G) = k\), the minimum number of distinct vertex colors induced by a majestic \(k\)-edge coloring is called the majestic chromatic number of \(G\) and denoted by \(\psi(G)\). Thus, \(\psi(G)\) is at least as large as the chromatic number \(\chi(G)\) of a graph \(G\). Majestic chromatic indexes and numbers are determined for several well-known classes of graphs. Furthermore, relationships among the three chromatic parameters \(\chi_m(G)\), \(\psi(G)\), and \(\chi(G)\) of a graph \(G\) are investigated.
This paper investigates vertex colorings of graphs such that some rainbow subgraph \(R\) and some monochromatic subgraph \(M\) are forbidden. Previous work focused on the case that \(R = M\). Here we consider the more general case, especially the case that \(M = K_2\).
Let \(R\) be a commutative ring with identity. For any integer \(k > 1\), an element is a \(k\)-zero divisor if there are distinct \(k\) elements including the given one, such that the product of all is zero but the product of fewer than all is nonzero. Let \(Z(R,k)\) denote the set of the \(k\)-zero divisors of \(R\). In this paper we consider rings which are not \(k\)-integral domains (i.e. \(Z(R,k)\) is nontrivial) with finite \(Z(R,k)\). We show that a uniform \(n\) exists such that \(a^n = 0\) for all elements \(a\) of the nil-radical \(N\) and deduce that a ring \(R\) which is not a \(k\)-integral domain with more than \(k\) minimal prime ideals and whose nil-radical is finitely generated is finite, if \(Z(R,k)\) is finite.
Recording actual user interactions with a system is often useful for testing software applications. User-session based test suites that contain records of such interactions often find a complementary set of faults compared to test suites created by testers. This work utilizes such test suites and presents a new prioritization method that extends the existing combinatorial two-way inter-window prioritization by introducing weights on the distance between windows. We examine how a window distance between a pair of the parameter-value tuples influences the fault detection effectiveness. We evaluate several approaches used to calculate weights. Results show improvement over the original two-way inter-window prioritization technique, while the comparison of different weighting approaches reveals that a negative linear weighting calculation generally performs better in our experiments. The study demonstrates that the distance between windows in a pair is an important factor to consider in test suite prioritization, and that distinguishing windows by their order in a test case also improves the fault detection rate compared to using window labels that were utilized in previous methods. This work provides motivation for future work to develop general n-way combinatorial distance-based prioritization methods that take into account space and processing time requirements to address potential issues with large test suites.
In this paper, we identify \(LW\) and \(OW\) graphs, find the minimum \(\lambda\) for decomposition of \(\lambda K_n\) into these graphs, and show that for all viable values of \(\lambda\), the necessary conditions are sufficient for \(LW\)- and \(OW\)-decompositions using cyclic decompositions from base graphs.
For a connected graph \(G = (V, E)\), its inverse degree is defined as
\(\sum_{v\in V} \frac{1}{d(v)}\). Using an upper bound for the inverse degree of a graph obtained by Cioaba in [6], in this note, we present new sufficient conditions for some Hamiltonian properties of graphs.
A cancellable number (CN) is a fraction in which a decimal digit can be removed (“cancelled”) in the numerator and denominator without changing the value of the number; examples include \(\frac{16}{64}\) where the \(6\) can be cancelled and \(\frac{49}{98}\) where the \(9\) can be cancelled. We provide explicit infinite families of CNs with certain properties, subject to the restriction that the numerator and denominator have equal decimal length and the cancellable digits “line up”, i.e., all cancellation lines are vertical. The properties in question are that the CNs contain one of the following: a cancellable nine, a cancellable zero, a sequence of adjacent cancellable zeros, sequences of adjacent cancellable zeros and nines of the same length, and sequences of adjacent cancellations for certain other digits.
A \(\mathrm{GDD}(n_1+n_2,3; \lambda_1,\lambda_2)\) is a group divisible design with two groups of sizes \(n_1\) and \(n_2\), where \(n_1 < n_2\), with block size \(3\) such that each pair of distinct elements from the same group occurs in \(\lambda_1\) blocks and each pair of elements from different groups occurs in \(\lambda_2\) blocks. We prove that the necessary conditions are sufficient for the existence of group divisible designs \(\mathrm{GDD}(n_1+n_2, 3; \lambda_1, \lambda_2)\) with equal number of blocks of configuration \((1, 2)\) and \((0, 3)\) for \(n_1 + n_2 \leq 20\), \(n_1 \neq 2\) and in general for \(n_1 = 1, 3, 4, n_2 – 1\), and \(n_2 – 2\).
Given nonnegative integers \(a\), \(b\), \(c\), and \(d\), the transition function \(\nabla\) is defined by \(\nabla(a, b, c, d) = (|a-b|, |b-c|, |c-d|, |d-a|)\). The Diffy problem asks if it can reach \((0, 0, 0, 0)\) after some iterations of \(\nabla\) on the four numbers. If \((a, b, c, d)\) can transfer to \((0, 0, 0, 0)\) iterated by \(\nabla\) operations, the smallest \(N\) such that \(\nabla^N(a, b, c, d) = (0, 0, 0, 0)\) is called the stopping steps of the Diffy problem. In this paper, we will show that there exists \(N\) such that \(\nabla^N(a, b, c, d) = (0, 0, 0, 0)\) and the loose upper bound and exact upper bound of \(N\). In addition, we will also show that we can find a starting vector \((a, b, c, d)\) so that it reaches the zero vector \((0, 0, 0, 0)\) after exact \(k\) steps for any given positive integer \(k\).
The half of an infinite lower triangular matrix \(G = (g_{n,k})_{n,k\geq 0}\) is defined to be the infinite lower triangular matrix \(G^{(1)} = (g^{(1)}_{n,k \geq 0})\) such that \(g^{(1)}_{n,k} = g_{2n-k,n}\) for all \(n \geq k \geq 0\). In this paper, we will show that if \(G\) is a Riordan array, then its half \(G^{(1)}\) is also a Riordan array. We use Lagrange inversion theorem to characterize the generating functions of \(G^{(1)}\) in terms of the generating functions of \(G\). Consequently, a tight relation between \(G^{(1)}\) and the initial array \(G\) is given, hence it is possible to invert the process and rebuild the original Riordan array \(G\) from the array \(G^{(1)}\). If the process of taking half of a Riordan array \(G\) is iterated \(r\) times, then we obtain a Riordan array \(G^{(r)}\). The further relation between the result array \(G^{(r)}\) and the initial array \(G\) is also considered. Some examples and applications are presented.
A graph \(G\) is list \(k\)-arborable if for any sets \(L(v)\) of cardinality at least \(k\) at its vertices, one can choose an element (color) for each vertex \(v\) from its list \(L(v)\) so that the subgraph induced by every color class is an acyclic graph (a forest). In the paper, it is proved that every planar graph with \(5\)-cycles not adjacent to \(3\)-cycles and \(4\)-cycles is list \(2\)-arborable.
For two vertices \(u\) and \(v\) in a strong digraph \(D\), the strong distance between \(u\) and \(v\) is the minimum number of arcs of a strong subdigraph of \(D\) containing \(u\) and \(v\). The strong eccentricity of a vertex \(v\) of \(D\) is the strong distance between \(v\) and a vertex farthest from \(v\). The strong diameter (strong radius) of \(D\) is the maximum (minimum) strong eccentricity among all vertices of \(D\). The lower orientable strong diameter (lower orientable strong radius), \(\mathrm{sdiam}(G)\) (\(\mathrm{srad}(G)\)), of a 2-edge-connected graph \(G\) is the minimum strong diameter (minimum strong radius) over all strong orientations of \(G\). In this paper, a conjecture of Chen and Guo is disproved by proving \(\mathrm{sdiam}(K_{3} \square K_{3}) = \mathrm{sdiam}(K_{3} \square K_{4}) = 5\), \(\mathrm{sdiam}(K_{m} \square P_{n})\) is determined, \(\mathrm{sdiam}(G)\) and \(\mathrm{srad}(G)\) for cycle vertex multiplications are computed, and some results concerning \(\mathrm{sdiam}(G)\) are described.
The aim of this note is to present a short proof of a result of Alaeiyan et al. [Bull. Austral. Math. Soc.\( 77 (2008) 315-323;\)
Proc. Indian Acad. Sci., Math. Sci. \(119 (2009) 647-653\)] concerning the non-existence of cubic semisymmetric graphs of order \(8p\) or \(8p^2\), where \(p\) is a prime. In those two papers, the authors choose the heavy weaponry of covering techniques. Our proof relies on the analysis of the subgroup structure of the full automorphism group of the graph and the normal quotient graph theory.
Let \(G\) be a graph of order \(n\) with adjacency matrix \(A(G)\) and diagonal degree matrix \(D(G)\). The generalized characteristic polynomial of \(G\) is defined to be \(f_G(x,t) = \det (xI_n – (A(G) – tD(G)))\). The \(R\)-graph of \(G\), denoted by \(R(G)\), is obtained by adding a new vertex for each edge of \(G\) and joining each new vertex to both end vertices of the corresponding edge. The generalized \(R\)-vertex corona, denoted by \(R(G) \boxdot \wedge _i^n H\), is the graph obtained from \(R(G)\) and \(H\) by joining the \(i\)-th vertex of \(V(G)\) to every vertex of \(H\). In this paper, we determine the generalized characteristic polynomial of \(R(G) \boxdot \wedge _i^n H\). As applications, we get infinitely many pairs of generalized cospectral graphs, the number of spanning trees and Kirchhoff index of \(R(G) \boxdot\wedge _i^n H\).
In this paper, we introduce an \(O(n^2)\) time algorithm to determine the cyclic edge connectivity of a planar graph, where \(n\) is the order of the planar graph. This is the first correct square time algorithm for cyclic edge connectivity of planar graphs.
Let \(G = (V, E)\), with \(|V| = n\), be a simple connected graph. An edge-colored graph \(G\) is rainbow edge-connected if any two vertices are connected by a path whose edges are colored by distinct colors. The rainbow connection number of a connected graph \(G\), denoted by \(rc(G)\), is the smallest number of colors that are needed in order to make \(G\) rainbow edge-connected. In this paper, we obtain tight bounds for \(rc(G)\). We use our results to generalize previous results for graphs with \(\delta(G) \geq 3\).
In this paper, we provide the 4-way combinatorial interpretations of some Rogers–Ramanujan type identities using partitions with “\(n + t\) copies of \(n\)”, lattice paths, \(k\)-partitions, and ordinary partitions.
In this paper, we study the Fibonacci polynomials modulo \(m\) such that \(x^2 = x + 1\) and then we obtain miscellaneous properties of these sequences. Also, we extend the Fibonacci polynomials to the ring of complex numbers. We define the Fibonacci Polynomial-type orbits \(F^R_{(a,b)}(x) = \{x_i\}\), where \(R\) is a 2-generator ring and \((a,b)\) is a generating pair of the ring \(R\). Furthermore, we obtain the periods of the Fibonacci Polynomial-type orbits \(F^R_{(a,b)}(x)\) in finite 2-generator rings of order \(p^2\).
A theta graph is the union of three internally disjoint paths that have the same two distinct end vertices. We show that every graph of order \(n \geq 12\) and size at least\(\max\left\{\left\lceil\frac{3n+79}{2}\right\rceil,\left\lfloor\frac{11n-33}{2}\right\rfloor\right\}\) contains three disjoint theta graphs. As a corollary, every graph of order \(n \geq 12\) and size at least \(\max\left\{\left\lceil\frac{3n+79}{2}\right\rceil ,\left\lfloor\frac{11n-33}{2}\right\rfloor\right\}\) contains three disjoint cycles of even length. The lower bound on the size is sharp in general.
A simple undirected graph is said to be semisymmetric if it is regular and edge-transitive but not vertex-transitive. A semisymmetric graph must be bipartite whose automorphism group has two orbits of the same size on the vertices. One of our long-term goals is to determine all the semisymmetric graphs of order \(2p^3\), for any prime \(p\). All these graphs \(\Gamma\) with the automorphism group \(Aut(\Gamma)\), are divided into two subclasses: (I) \(Aut(\Gamma)\) acts unfaithfully on at least one bipart; and (II) \(Aut(\Gamma)\) acts faithfully on both biparts. In [9],[19] and [20], a complete classification was given for Subclass (I). In this paper, a partial classification is given for Subclass (II), when \(Aut(\Gamma)\) acts primitively on one bipart.
The notions of \(L\)-tree-coloring and list vertex arboricity of graphs are introduced in the paper, while a sufficient condition for a plane graph admitting an \(L\)-tree-coloring is given. Further, it is proved that every graph without \(K_{5}\)-minors or \(K_{3,3}\)-minors has list vertex arboricity at most \(3\), and this upper bound is sharp.
A model for cleaning a graph with brushes was first introduced by Messinger, Nowakowski, and Pralat in 2008. Later, they focused on the problem of determining the maximum number of brushes needed to clean a graph. This maximum number of brushes needed to clean a graph in the model is called the broom number of the graph. In this paper, we show that the broom number of a graph is equal to the size of a maximum edge-cut of the graph, and prove the \(\mathcal{NP}\)-completeness of the problem of determining the broom number of a graph. As an application, we determine the broom number exactly for the Cartesian product of two graphs.
We give more results in mean cordial and harmonic mean labelings, such as: upper bounds for the number of edges of graphs of given orders for both labelings with direct results, labeling all trees of order \(\leq 9\) to be harmonic mean with the restriction of using the floor function of the definition, and labeling all graphs of order \(\leq 5\) that are harmonic mean graphs without using the label \(q + 1\) in labeling the vertices. Also, we give mean cordial labelings for some families of graphs.
Linkage is very important in Very Large Scale Integration (VLSI) physical design. In this paper, we mainly study the relationship between minors and linkages. Thomassen conjectured that every \((2k + 2)\)-connected graph is \(k\)-linked. For \(k \geq 4\), \(K_{3k-1}\) with \(k\) disjoint edges deleted is a counterexample to this conjecture, however, it is still open for \(k = 3\). Thomas and Wollan proved that every \(6\)-connected graph on \(n\) vertices with \(5n – 14\) edges is \(3\)-linked. Hence they obtain that every \(10\)-connected graph is \(3\)-linked. Chen et al. showed that every \(6\)-connected graph with \(K_{9}^-\) as a minor is \(3\)-linked, and every \(7\)-connected graph with \(K_{9}^-\) as a minor is \((2,2k-1)\)-linked. Using a similar method, we prove that every \(8\)-connected graph with \(K_{2k+3}^-\) as a minor is \(4\)-linked, and every \((2k + 1)\)-connected graph with \(K_{2k+3}^-\) as a minor is \((2,2k – 1)\)-linked. Our results extend Chen et al.’s conclusions, improve Thomas and Wollan’s results, and moreover, they give a class of graphs that satisfy Thomassen’s conjecture for \(k = 4\).
Consider any undirected and simple graph \(G = (V, E)\), where \(V\) and \(E\) denote the vertex set and the edge set of \(G\), respectively. Let \(|G| = |V| = n \geq 3\). The well-known Ore’s theorem states that if \(\deg_G(u) + \deg_G(v) \geq n + k\) holds for each pair of nonadjacent vertices \(u\) and \(v\) of \(G\), then \(G\) is traceable for \(k = -1\), Hamiltonian for \(k = 0\), and Hamiltonian-connected for \(k = 1\). In this paper, we investigate any graph \(G\) with \(\deg_G(u) + \deg_G(v) \geq n – 1\) for any nonadjacent vertex pair \(\{u,v\}\) of \(G\), in particular. We call it the \((*)\) condition. We derive four graph families, \(\mathcal{H}_i\), \(1 \leq i \leq 4\), and prove that all graphs satisfying \((*)\) are Hamiltonian-connected unless \(G \in \bigcup_{i=1}^{4} \mathcal{H}_i\). We also establish a comprehensive theorem for \(G\) satisfying \((*)\), which shows that \(G\) is traceable, Hamiltonian, pancyclic, or Hamiltonian-connected unless \(G\) belongs to different subsets of \(\{\mathcal{H}_i | 1 \leq i \leq 4\}\), respectively.
Given a group \(G\), we define the power graph \(P(G)\) as follows: the vertices are the elements of \(G\) and two vertices \(x\) and \(y\) are joined by an edge if \(\langle x\rangle \subseteq \langle y \rangle\) or \(\langle y\rangle \subseteq \langle x \rangle\). Obviously, the power graph of any group is always connected, because the identity element of the group is adjacent to all other vertices. In the present paper, among other results, we will find the number of spanning trees of the power graph associated with specific finite groups. We also determine, up to isomorphism, the structure of a finite group \(G\) whose power graph has exactly \(n\) spanning trees, for \(n < 5^{3}\). Finally, we show that the alternating group \(A_5\) is uniquely determined by the tree-number of its power graph among all finite simple groups.
Let \(G\) be a graph of order \(n\). The number of positive eigenvalues of \(G\) is called the positive inertia index of \(G\) and denoted by \(p(G)\). The minimum number of complete multipartite subgraphs in any complete multipartite graph edge decomposition of graph \(G\), in which the edge-induced subgraph of each edge subset of the decomposition is a complete multipartite graph, is denoted by \(\epsilon(G)\). In this paper, we prove \(\epsilon(G) \geq p(G)\) for any graph \(G\). Especially, if \(\epsilon(G) = 2\), then \(p(G) = 1\). We also characterize the graph \(G\) with \(p(G) = n – 2\).
The distance spectral gap of a connected graph is defined as the difference between its first and second distance eigenvalues. In this note, the unique \(n\)-vertex trees with minimal and maximal distance spectral gaps, and the unique \(n\)-vertex unicyclic graph with minimal distance spectral gap are determined.
A simple graph \(G = (V, E)\) admits an \(H\)-covering if every edge in \(E\) belongs to at least one subgraph of \(G\) isomorphic to a given graph \(H\). An \((a, d)\)-\(H\)-antimagic labeling of \(G\) admitting an \(H\)-covering is a bijective function \(f : V \cup E \rightarrow \{1, 2, \ldots, |V| + |E|\}\) such that, for all subgraphs \(H’\) of \(G\) isomorphic to \(H\), the \(H’\)-weights, \(wt(H’) = \sum_{v \in V(H’)} f(v) + \sum_{e \in E(H’)} f(e)\), constitute an arithmetic progression with the initial term \(a\) and the common difference \(d\). Such a labeling is called super if \(f(V) = \{1, 2, \ldots, |V|\}\). In this paper, we study the existence of super \((a, d)\)-\(H\)-antimagic labelings for graph operation \(G ^ H\), where \(G\) is a (super) \((b, d^*)\)-edge-antimagic total graph and \(H\) is a connected graph of order at least \(3\).
This article proves that the square of a Halin graph \(G\) with \(\Delta(G) = 5\) has the chromatic number \(6\). This gives a positive answer to an open problem in [Y. Wang, Distance two labelling of Halin graphs, Ars Combin. 114 (2014), 331–343].
There are many rectangular arrays whose \(n^{th}\) column is the \(n\)-fold convolution of the \(0^{th}\) column in combinatorics. For this type of rectangular arrays, we prove a formula for evaluating the determinant of certain submatrices, which was conjectured by Hoggatt and Bicknell. Our result unifies the determinant evaluation of submatrices of the rectangular arrays consisting of binomial coefficients, multinomial coefficients, Fibonacci numbers, Catalan numbers, generalized Catalan and Motzkin numbers.
In this paper, we obtain the following upper bounds for the largest Laplacian graph eigenvalue: \[\mu \leq \max\limits_{i} \left\{\sqrt{ 2d_i (m_i + d_i) + n – 2d_i – 2 \sum\limits_{j:j\sim i}{ |N_i \cap N_j|}} \right\}\] where \(d_i\) and \(m_i\) are the degree of vertex \(i\) and the average degree of vertex \(i\), respectively; \(|N_i \cap N_j|\) is the number of common neighbors of vertices \(i\) and \(j\). We also compare this bound with some known upper bounds.
A three-colored digraph \(D\) is primitive if and only if there exist nonnegative integers \(h\), \(k\), and \(v\) with \(h+k+v > 0\) such that for each pair \((i, j)\) of vertices there is an \((h, k, v)\)-walk in \(D\) from \(i\) to \(j\). The exponent of the primitive three-colored digraph \(D\) is defined to be the smallest value of \(h + k + v\) over all such \(h\), \(k\), and \(v\). In this paper, a class of special primitive three-colored digraphs with \(n\) vertices, consisting of one \(n\)-cycle and two \((n-1)\)-cycles, are considered. For the case \(a = c – 1\), some primitive conditions, the tight upper bound on the exponents, and the characterization of extremal three-colored digraphs are given.
Skew-quasi-cyclic codes over a finite field are viewed as skew-cyclic codes on a noncommutative ring of matrices over a finite field. This point of view gives a new construction of skew-quasi-cyclic codes. Let \(\mathbb{F}_q\) be the Galois field with \(q\) elements and \(\theta\) be an automorphism of \(\mathbb{F}_q\). We propose an approach to consider the relationship between left ideals in \(M_l(\mathbb{F}_q)[X, \theta]/(X^s – 1)\) and skew-quasi-cyclic codes of length \(ls\) and index \(l\) over \(\mathbb{F}_q\), under \(\theta\), which we denote by \(\theta\)-SQC codes (or SQC codes for short when there is no ambiguity). We introduce the construction of SQC codes from the reversible divisors of \(X^s – 1\) in \(M_l(\mathbb{F}_q)[X, \theta]\). In addition, we give an algorithm to search for the generator polynomials of general SQC codes.
In this paper, we investigate the concepts of \(k\)-limited packing and \(k\)-tuple domination in graphs and give several bounds on the size of them. These bounds involve many well-known parameters of graphs. Also, we establish a connection between these concepts that implies some new results in this area. Finally, we improve many bounds in the literature.
Let \(G = (V, E)\) be a simple graph. A paired-dominating set of a graph \(G\) is a dominating set whose induced subgraph contains a perfect matching. The paired domination number of a graph \(G\), denoted by \(\gamma_p(G)\), is the minimum cardinality of a paired-dominating set in \(G\). In this paper, we study the paired domination number of generalized Petersen graphs \(P(n,2)\) and prove that for any integer \(n \geq 6\), \(\gamma_p(P(n, 2)) = 2 \left\lfloor \frac{n}{3} \right\rfloor + n \pmod{3}\).
The Estrada index of a simple connected graph \(G\) of order \(n\) is defined as \(EE(G) = \sum_{i=1}^{n} e^{\lambda_i}\), where \(\lambda_1, \lambda_2, \ldots, \lambda_n\) are the eigenvalues of the adjacency matrix of \(G\). In this paper, we characterize all pentacyclic graphs of order \(n\) with maximal Estrada index.
Let \(\Pi\) be a finite polar space of rank \(n \geq 2\) fully embedded into a projective space \(\Sigma\). In this note, we determine all tight sets of \(\Pi\) of the form \((\Sigma_1 \cap \mathcal{P}) \cup (\Sigma_2 \cap \mathcal{P})\), where \(\mathcal{P}\) denotes the point set of \(\Pi\) and \(\Sigma_1, \Sigma_2\) are two mutually disjoint subspaces of \(\Sigma\). In this way, we find two families of \(2\)-tight sets of elliptic polar spaces that were not described before in the literature.
In this paper, we define a new matrix identity for bi-periodic Fibonacci and Lucas numbers. By using the matrix method, we give simple proofs of several properties of these numbers. Moreover, we obtain a new binomial sum formula for bi-periodic Fibonacci and Lucas numbers, which generalize the former results.
Hein and Sarvate show how to decompose \(\lambda\) copies of a complete graph \(K_n\), for some minimal value of \(\lambda\), into so-called LOE and OLE graphs. In this paper, we will show that for all possible values of \(\lambda\), the necessary conditions are sufficient for the LOE and OLE decompositions.
Let \(R\) be a commutative ring. The regular digraph of ideals of \(R\), denoted by \(\mathcal{R}(R)\), is a digraph whose vertex-set is the set of all non-trivial ideals of \(R\) and, for every two distinct vertices \(I\) and \(J\), there is an arc from \(I\) to \(J\), whenever \(I\) contains a non-zero divisor of \(J\). In this paper, we investigate the planarity of \(\mathcal{R}(R)\). We also completely characterize the rings \(R\) such that \(\mathcal{R}(R)\) is a ring graph, and the situations under which the genus of \(\mathcal{R}(R)\) is finite. Moreover, we study the independence number and the girth of \(\mathcal{R}(R)\), and also we find all cases that \(\mathcal{R}(R)\) is bipartite.
In this paper, the existence of Yang Hui type magic squares of order \(n\) with \(t\)-powered sum (YMS(\(n\), \(t\))) for general \(t\) is investigated. Some constructions of YMS(\(n\), \(t\)) are obtained by using strongly symmetric self-orthogonal diagonal Latin squares and magic rectangles. Applying these constructions, it is proved that for an integer \(t > 1\) there exist both a symmetric elementary YMS(\(2^t\), \(2t – 2\)) and a symmetric elementary YMS(\(2^t – k\), \(2t\)) for odd \(k > 1\), which improves the known result on YMSs.
To gain a better understanding of clean rings and their relatives, the clean graph of a commutative ring with identity is introduced and its various properties are established. Further investigation of clean graphs leads to additional results concerning other classes of rings.
For a connected graph, the distance spectral radius is the largest eigenvalue of its distance matrix. In this paper, of all trees with both given order and fixed diameter, the trees with the minimal distance spectral radius are completely characterized.
In this paper, a domination-type parameter, called dynamical \(2\)-domination number, will be introduced. Let \(G = (V(G), E(G))\) be a graph. A subset \(D \subseteq V(G)\) is called a \(2\)-dominating set in \(G\) if every vertex in \(V(G) \setminus D\) is adjacent to at least two vertices in \(D\), and in this paper \(D\) is called a dynamical \(2\)-dominating set if there exists a sequence of sets \(D = V_0 \subseteq V_1 \subseteq V_2 \subseteq \cdots \subseteq V_k = V(G)\) such that, for each \(i\), \(V_{i-1}\) is a \(2\)-dominating set in \(\langle V_i \rangle\), the induced subgraph generated by \(V_i\). Also, for a given graph \(G\), the size of its dynamical \(2\)-dominating sets of minimum cardinality will be called the dynamical \(2\)-domination number of \(G\) and will be denoted by \(\bar{\gamma}_{2}(G)\). We study some basic properties of dynamical \(2\)-dominating sets and compute \(\bar{\gamma}_{2}(G)\) for some graph classes. Also, some results about \(\bar{\gamma}_{2}\) of a number of binary operations on graphs are proved. A characterization of graphs with extreme values of \(\bar{\gamma}_{2}\) is presented. Finally, we study this concept for trees and give an upper bound and a lower bound for the dynamical \(2\)-domination number of trees.
A graph \(G\) is said to be equitably \(k\)-colorable if the vertex set of \(G\) can be divided into \(k\) independent sets for which any two sets differ in size at most one. The equitable chromatic number of \(G\), \(\chi_{=}(G)\), is the minimum \(k\) for which \(G\) is equitably \(k\)-colorable. The equitable chromatic threshold of \(G\), \(\chi_m^*(G)\), is the minimum \(k\) for which \(G\) is equitably \(k’\)-colorable for all \(k’ \geq k\). In this paper, the exact values of \(\chi_m^*(P_{n’,2} \square K_{m,n})\) and \(\chi_{=}(P_{n’,m} \square K_{m,n})\) are obtained except that \(3 \leq \xi_m^*(P_{5,2} \square K_{m,n}) = \chi_{=}(P_{s,m} \square K_{m,n}) \leq 4\) when \(m+n \geq 3\min\{m,n\} + 2\) or \(m+n < 3\min\{m,n\} – 2\).
The sum-connectivity energy of a graph is defined as the sum of the absolute value of all the eigenvalues of its sum-connectivity matrix. In this paper, we give further lower and upper bounds for the sum-connectivity energy in terms of the number of vertices, number of edges, the harmonic index, and determinant of the sum-connectivity matrix. We also show that among connected graphs with \(n\) vertices, the star graph \(K_{1,n-1}\) has the minimum sum-connectivity energy.
A graph is one-regular if its automorphism group acts regularly on the set of its arcs. In this paper, \(4\)-valent one-regular graphs of order \(5p^2\), where \(p\) is a prime, are classified.
In this paper, we obtain that the characteristic polynomials of the signless Laplacian matrix of \(Q(G)\), \(R(G)\), \(T(G)\) can be expressed in terms of the characteristic polynomial of \(G\) when \(G\) is a regular or semiregular graph, from which upper bounds for the incidence energy of \(Q(G)\), \(R(G)\), \(T(G)\) are deduced.
Let \(R\) be a commutative ring with unity. The co-maximal ideal graph of \(R\), denoted by \(\Gamma(R)\), is a graph whose vertices are the proper ideals of \(R\) which are not contained in the Jacobson radical of \(R\), and two vertices \(I_1\) and \(I_2\) are adjacent if and only if \(I_1 + I_2 = R\). We classify all commutative rings whose co-maximal ideal graphs are planar. In 2012, the following question was posed: If \(\Gamma(R)\) is an infinite star graph, can \(R\) be isomorphic to the direct product of a field and a local ring? In this paper, we give an affirmative answer to this question.
In this paper, a generalization of the Stirling numbers of the first and second kind, called \(m\) -Stirling numbers of the first and second kind, are derived. Based on the colored base-\(m\) number system, we give a combinatorial interpretation of \(m\) -Stirling numbers of the second kind. Some basic properties of the two kinds of \(m\) -Stirling numbers, including generating functions, explicit expressions, and recurrence relations, are also obtained.
A proper \(k\)-total coloring of a simple graph \(G\) is called \(k\)-vertex-distinguishing proper total coloring (\(k\)-VDTC) if for any two distinct vertices \(u\) and \(v\) of \(G\), the set of colors assigned to \(u\) and its incident edges differs from the set of colors assigned to \(v\) and its incident edges. The minimum number of colors required for a vertex-distinguishing proper total coloring of \(G\), denoted by \(\chi_{vt}(G)\), is called the vertex-distinguishing proper total chromatic number. For \(p\) even, \(p \geq 4\) and \(q \geq 3\), we will obtain vertex-distinguishing proper total chromatic numbers of complete \(p\)-partite graphs with each part of cardinality \(q\).
Let \(G\) be a simple graph of order \(n\). The domination polynomial of \(G\) is the polynomial \(D(G, x) = \sum_{i=0}^{n} d(G, i)\lambda^i\), where \(d(G, i)\) is the number of dominating sets of \(G\) of size \(i\). Every root of \(D(G, \lambda)\) is called a domination root of \(G\). It is clear that \((0, \infty)\) is a zero-free interval for the domination polynomial of a graph. It is interesting to investigate graphs that have complex domination roots with positive real parts. In this paper, we first investigate the complexity of the domination polynomial at specific points. Then, we present and investigate some families of graphs whose complex domination roots have positive real parts.
A quasi-tree is a graph for which the deletion of some vertex results in a tree. We determine the unique graph with minimum distance spectral radius among quasi-trees with fixed order and the unique graph with maximum distance spectral radius among cycle-containing quasi-trees with fixed order.
We initiate the study of double outer-independent domination in graphs. A vertex of a graph is said to dominate itself and all of its neighbors. A double outer-independent dominating set of a graph \(G\) is a set \(D\) of vertices of \(G\) such that every vertex of \(G\) is dominated by at least two vertices of \(D\), and the set \(V(G) \setminus D\) is independent. The double outer-independent domination number of a graph \(G\) is the minimum cardinality of a double outer-independent dominating set of \(G\). First, we discuss the basic properties of double outer-independent domination in graphs. We find the double outer-independent domination numbers for several classes of graphs. Next, we prove lower and upper bounds on the double outer-independent domination number of a graph, and we characterize the extremal graphs. Then, we study the influence of removing or adding vertices and edges. We also give Nordhaus-Gaddum type inequalities.
For any two graphs \(F_1\) and \(F_2\), the graph Ramsey number \(r(F_1, F_2)\) is the smallest positive integer \(N\) with the property that every graph of at least \(N\) vertices contains \(F_1\) or its complement contains \(F_2\) as a subgraph. In this paper, we consider the Ramsey numbers for theta-complete graphs. In fact, we prove that \(r(\theta_n, K_5) = 4n-3\) for \(n \geq 6\) and \(n \geq 10\).
The Randić index \(R\) is an important topological index in chemistry. In order to attack some conjectures concerning the Randić index, a modification \(R’\) of this index was introduced by Dvorak et al. [6]. The \(R’\) index of a graph \(G\) is defined as the sum of the weights \(\frac{1}{\max\{{d(u)d(v)}\}}\) of all edges \(uv\) of \(G\), where \(d(u)\) denotes the degree of a vertex \(u\) in \(G\). We first give a best possible lower bound of \(R’\) for a graph with minimum degree at least two and characterize the corresponding extremal graphs, and then we establish some relations between \(R’\) and the chromatic number, the girth of a graph.
There are operations that transform a map \(\mathcal{M}\) (an embedding of a graph on a surface) into another map on the same surface, modifying its structure and consequently its set of flags \(\mathcal{F(M)}\). For instance, by truncating all the vertices of a map \(\mathcal{M}\), each flag in \(\mathcal{F(M)}\) is divided into three flags of the truncated map. Orbanić, Pellicer, and Weiss studied the truncation of \(k\)-orbit maps for \(k \leq 3\). They introduced the notion of \(T\)-compatible maps in order to give a necessary condition for a truncation of a \(k\)-orbit map to be either \(k\)-, \(\frac{3k}{2}\)-, or \(3k\)-orbit map. Using a similar notion, by introducing an appropriate partition on the set of flags of the maps, we extend the results on truncation of \(k\)-orbit maps for \(k \leq 7\) and \(k = 9\).
Let \(\Re_\beta\) denote the set of trees on \(n = kG + 1\) (\(k \geq 2\)) vertices with matching number \(\beta\). In this paper, the trees with minimal spectral radius among \(\Re_\beta\) (\(2 \leq \delta \leq 4\)) are determined, respectively.
Generalized whist tournament designs and ordered whist tournament designs are relatively new specializations of whist tournament designs, having first appeared in \(2003\) and \(1996\), respectively. In this paper, we extend the concept of an ordered whist tournament to a generalized whist tournament and introduce an entirely new combinatorial design, which we call a generalized ordered whist tournament. We focus specifically on generalized whist tournaments for games of size \(6\) and teams of size \(3\), where the number of players is a prime of the form \(6n+1\), and prove that these tournaments exist for all primes \(p\) of the form \(p=6n+1\), with the possible exception of \(p \in \{7, 13, 19, 37, 61, 67\}\).
A regular graph \(\Gamma\) is said to be semisymmetric if its full automorphism group acts transitively on its edge set but not on its vertex set. Some authors classified semisymmetric cubic graphs of orders \(10p\) and \(10p^2\). Also, it is proved that there is no connected semisymmetric cubic graph of order \(10p^3\). In this paper, we continue this work and prove that there is no connected semisymmetric cubic graph of order \(10p^n\), where \(n \geq 4\), \(p \geq 7\), and \(p \neq 11\).
In this.paper, by joint tree model, we obtain the genera of two types of graphs, which are suspensions of cartesian products of two types of bipartite graphs from a vertex.
Let \(G\) be a connected graph with a perfect matching on \(2n\) vertices (\(n \geq 2\)). A graph \(G’\) is a contraction of \(G\) if it can be obtained from \(G\) by a sequence of edge contractions. Then \(G\) is said to be edge contractible if for any contraction \(G’\) of \(G\) with \(|V(G’)|\) even, \(G’\) has a perfect matching. In this note, we obtain a sufficient and necessary condition for a graph to be an edge contractible graph.
All finite Jacobson graphs with a Hamiltonian cycle or path, or Eulerian tour or trail are determined, and it is shown that a finite Jacobson graph is Hamiltonian if and only if it is pancyclic. Also, the length of the longest induced cycles and paths in finite Jacobson graphs are obtained.
A vertex subset \(S\) of a digraph \(D\) is called a dominating set of \(D\) if every vertex not in \(S\) is adjacent from at least one vertex in \(S\). The domination number of \(D\), denoted by \(\gamma(D)\), is the minimum cardinality of a dominating set of \(D\). We characterize the rooted trees and connected contrafunctional digraphs \(D\) of order \(n\) satisfying \(\gamma(D) = \left\lceil \frac{n}{2}\right\rceil\). Moreover, we show that for every digraph \(D\) of order \(n\) with minimum in-degree at least one, \(\gamma(D) \leq \frac{(k+1)n}{2k+1}\), where \(2k+1\) is the length of a shortest odd directed cycle in \(D\), and we characterize the corresponding digraphs achieving this upper bound. In particular, if \(D\) contains no odd directed cycles, then \(\gamma(D) \leq \frac{n}{2}\).
A graph is called degree-magic if it admits a labelling of the edges by integers \(\{1, 2, \ldots, |E(G)|\}\) such that the sum of the labels of the edges incident with any vertex \(v\) is equal to \(\left(1 + |E(G)|\right)/2 \deg(v)\). In this paper, we show that a class of join graphs are degree-magic.
A vertex-deleted unlabeled subgraph of a graph \(G\) is called a card of \(G\). A card of \(G\) with which the degree of the deleted vertex is also given is called a degree-associated card or dacard of \(G\). The degree-associated reconstruction number, \(\mathrm{drn}(G)\), of a graph \(G\) is the size of the smallest collection of dacards of \(G\) that uniquely determines \(G\). The maximal subgraph without end vertices of a graph \(G\) that is not a tree is called the pruned graph of \(G\). It is shown that \(\mathrm{drn}\) of some connected graphs with regular pruned graph is \(2\) or \(3\).
The Wiener index of a connected graph is the sum of distances between all pairs of vertices in the graph. Feng et al. in [The hyper-Wiener index of bicyclic graphs, Utilitas Math., \(84(2011) 97-104\)] determined the bicyclic graphs having the largest Wiener index. In this article, we determine the graphs having the second up to seventh largest Wiener indices among all bicyclic graphs with \(n\) vertices.
This paper obtains new combinatorial batch codes (CBCs) from old ones, studies properties of uniform CBCs, and constructs uniform CBCs using semilattices.
For graphs \(F\) and \(H\), where \(H\) has chromatic index \(t\), the proper Ramsey number \(PR(F, H)\) is the smallest positive integer \(n\) such that every \(t\)-edge coloring of \(K_n\) results in a monochromatic \(F\) or a properly colored \(H\). The proper Ramsey number \(PR(F, H)\) is investigated for certain pairs \(F, H\) of connected graphs when \(t = 2\), namely when \(F\) is a complete graph, star, or path and when \(H\) is a path or even cycle of small order. In particular, \(PR(F, H)\) is determined when (1) \(F\) is a complete graph and \(H\) is a path of order 6 or less, (2) \(F\) is a complete graph and \(H\) is a 4-cycle, (3) \(F\) is a star and \(H\) is a 4-cycle or a 6-cycle, and (4) \(F\) is a star and \(H\) is a path of order 8 or less.
The \(k\)-rainbow index \(rx_k(G)\) of a connected graph \(G\) was introduced by Chartrand, Okamoto, and Zhang in 2010. Let \(G\) be a nontrivial connected graph with an edge-coloring \(c: E(G) \to \{1, 2, \ldots, q\}\), \(q \in \mathbb{N}\), where adjacent edges may be colored the same. A tree \(T\) in \(G\) is called a rainbow tree if no two edges of \(T\) receive the same color. For a graph \(G = (V, E)\) and a set \(S \subseteq V\) of at least two vertices, an \(S\)-Steiner tree or a Steiner tree connecting \(S\) (or simply, an \(S\)-tree) is a subgraph \(T = (V’, E’)\) of \(G\) that is a tree with \(S \subseteq V’\). For \(S \subseteq V(G)\) and \(|S| \geq 2\), an \(S\)-Steiner tree \(T\) is said to be a rainbow \(S\)-tree if no two edges of \(T\) receive the same color. The minimum number of colors that are needed in an edge-coloring of \(G\) such that there is a rainbow \(S\)-tree for every \(k\)-set \(S\) of \(V(G)\) is called the \(k\)-rainbow index of \(G\), denoted by \(rx_k(G)\). In this paper, we consider when \(|S| = 3\). An upper bound of complete multipartite graphs is obtained. By this upper bound, for a connected graph \(G\) with \(\text{diam}(G) \geq 3\), we give an upper bound of its complementary graph.
A tree \( T \), in an edge-colored graph \( G \), is called a rainbow tree if no two edges of \( T \) are assigned the same color. For a vertex subset \( S \subseteq V(G) \), a tree that connects \( S \) in \( G \) is called an \( S \)-tree. A \( k \)-rainbow coloring of \( G \) is an edge coloring of \( G \) having the property that for every set \( S \) of \( k \) vertices of \( G \), there exists a rainbow \( S \)-tree \( T \) in \( G \). The minimum number of colors needed in a \( k \)-rainbow coloring of \( G \) is the \( k \)-rainbow index of \( G \), denoted by \( rx_k(G) \). It is NP-hard to compute the \( rx_k(G) \) for the general graphs \( G \). We consider the \( 3 \)-rainbow index of complete bipartite graphs \( K_{s,t} \). For \( 3 \leq s \leq t \), we have determined the tight bounds of \( rx_3(K_{s,t}) \). In this paper, we continue the study. For \( 2 \leq s \leq t \), we develop a converse idea and apply it with the model of chessboard to study the problem. Finally, we obtain the exact value of \( rx_3(K_{s,t}) \) with \( 2 \leq s \leq t \).
The domination polynomials of binary graph operations, aside from union, join and corona, have not been widely studied. We compute and prove recurrence formulae and properties of the domination polynomials of families of graphs obtained by various products, including both explicit formulae and recurrences for specific families.
A graph \( G \) is a homomorphic preimage of another graph \( H \), or equivalently \( G \) is \( H \)-colorable, if there exists a graph homomorphism \( f: G \to H \). A classic problem is to characterize the family of homomorphic preimages of a given graph \( H \). A geometric graph \(\overline{G}\) is a simple graph \( G \) together with a straight line drawing of \( G \) in the plane with the vertices in general position. A geometric homomorphism (resp. isomorphism) \(\overline{G} \to \overline{H}\) is a graph homomorphism (resp. isomorphism) that preserves edge crossings (resp. and non-crossings). The homomorphism poset \(\mathcal{G}\) of a graph \( G \) is the set of isomorphism classes of geometric realizations of \( G \) partially ordered by the existence of injective geometric homomorphisms. A geometric graph \(\overline{G}\) is \(\mathcal{H}\)-colorable if \(\overline{G} \to \overline{H}\) for some \(\overline{H} \in \mathcal{H}\). In this paper, we provide necessary and sufficient conditions for \(\overline{G}\) to be \(C_n\)-colorable for \(3 \leq n \leq 5\).
The mixed discriminant of an \(n\)-tuple of \(n \times n\) matrices \(A_1, \ldots, A_n\) is defined as $$\mathcal{D}(A_1, A_2, \ldots, A_n) = \frac{1}{n!} \sum_{\sigma \in S(n)} \det(A_{\sigma(1)}^{(1)}, A_{\sigma(2)}^{(2)}, \ldots, A_{\sigma(n)}^{(n)}),$$
where \(A^{(i)}\) denotes the \(i\)th column of the matrix \(A\) and \(S(n)\) denotes the group of permutations of \(1, 2, \ldots, n\). For \(n\) matrices \(A_1, \ldots, A_n\) and indeterminates \(\lambda_1, \ldots, \lambda_n\), set $$\Phi_{\lambda_1, \ldots, \lambda_n}(A_1, \ldots, A_n) = \mathcal{D}(\lambda_1 I – A_1, \ldots, \lambda_n I – A_n).$$
It is shown that \(\Phi_{A_1, \ldots, A_n}(A_1, \ldots, A_n) = 0\).
For a graph \(H\) and a positive integer \(\lambda\), let \( ^{\lambda}{H} \) denote the multigraph obtained by replacing each edge of \(H\) with \(\lambda\) parallel edges. Let \(G\) be a multigraph with edge multiplicity \(2\) and with \(C_4\) as its underlying simple graph. We find necessary and sufficient conditions for the existence of a \(G\)-decomposition of \( ^{\lambda}{K_n} \) for all positive integers \(\lambda\) and \(n\).
The size of a minimum total dominating set in the \(m \times n\) grid graph is denoted by \(\gamma_t(P_m \square P_n)\). Here a dynamic programming algorithm that computes \(\gamma_t(P_m \square P_n)\) for any \(m\) and \(n\) is presented, and it is shown how properties of the algorithm can be used to derive formulae for a fixed, small value of \(m\). Using this method, formulae for \(\gamma_t(P_m \square P_n)\) for \(m \leq 28\) are obtained. Formulae for larger \(m\) are further conjectured, and a new general upper bound on \(\gamma_t(P_m \square P_n)\) is proved.
The 2-cell embeddings of graphs on closed surfaces have been widely studied. It is well known that (2-cell) embedding a given graph \(G\) on a closed orientable surface is equivalent to cyclically ordering the darts incident to each vertex of \(G\). In this paper, we study the following problem: given a genus \(g\) embedding \(\in\) of the graph \(G\) and a vertex of \(G\), how many different ways of reembedding the vertex such that the resulting embedding \(\in’\) is of genus \(g + \Delta g\)? We give formulas to compute this quantity and the local minimal genus achieved by reembedding. In the process, we obtain miscellaneous results. In particular, if there exists a one-face embedding of \(G\), then the probability of a random embedding of \(G\) to be one-face is at least \(\prod_{v \in V(G)} \frac{2}{\deg(v) + 2}\) where \(\deg(v)\) denotes the vertex degree of \(v\). Furthermore, we obtain an easy-to-check necessary condition for a given embedding of \(G\) to be an embedding of minimum genus.
We show that all but \(4489\) integers \(n\) with \(4 < n \leq 4 \cdot 10^{30}\) cannot occur as the order of a circulant Hadamard matrix. Our algorithm allows us to search \(10000\) times farther than prior efforts, while substantially reducing memory requirements. The principal improvement over prior methods involves the incorporation of a separate search for double Wieferich prime pairs \(\{p, q\}\), which have the property that \(p^{q-1} \equiv 1 \pmod{q^2}\) and \(q^{p-1} \equiv 1 \pmod{p^2}\).
A graph \( G = (V, E) \) is word-representable if there exists a word \( w \) over the alphabet \( V \) such that letters \( x \) and \( y \) alternate in \( w \) if and only if \( (x, y) \) is an edge in \( E \).
A recent elegant result of Akrobotu \( et \, al. \) \([1]\) states that a triangulation of any convex polyomino is word-representable if and only if it is 3-colourable. In this paper, we generalize a particular case of this result by showing that the result of Akrobotu \( et \, al. \) \([1]\) is true even if we allow a domino tile, instead of having just \(1 \times 1\) tiles on a rectangular polyomino.
The eternal domination number of a split graph is shown to equal either its domination number, or its domination number plus one. A characterization of the split graphs which achieve equality in either instance is given. It is shown that the problem of deciding whether the domination number of a Hamiltonian split graph is at most a given integer \(k\) is NP-complete, as is the problem of deciding whether the eternal domination number of a Hamiltonian split graph is at most a given integer \(k\). Finally, the problem of computing the eternal domination number is shown to be polynomial for any subclass of split graphs for which the domination number can be computed in polynomial time, in particular for strongly chordal split graphs.
A graceful labeling of a graph \( G \) of order \( n \) and size \( m \) is a one-to-one function \( f : V(G) \rightarrow \{0, 1, \ldots, m\} \) that induces a one-to-one function \( f’ : E(G) \rightarrow \{1, 2, \ldots, m\} \) defined by \( f'(uv) = |f(u) – f(v)| \). A graph that admits a graceful labeling is a graceful graph. A proper coloring \( c : V(G) \rightarrow \{1, 2, \ldots, k\} \) is called a graceful \( k \)-coloring if the induced edge coloring \( c’ \) defined by \( c'(uv) = |c(u) – c(v)| \) is proper. The minimum positive integer \( k \) for which \( G \) has a graceful \( k \)-coloring is its graceful chromatic number \( \chi_g(G) \). The graceful chromatic numbers of cycles, wheels, and caterpillars are determined. An upper bound for the graceful chromatic number of trees is determined in terms of its maximum degree.
For a graph \( G = (V, E) \) with a coloring \( f : V(G) \rightarrow \mathbb{Z}_2 \), let \( v_f(i) = |f^{-1}(i)| \). We say \( f \) is friendly if \( |v_f(1) – v_f(0)| \leq 1 \). The coloring \( f \) induces an edge labeling \( f_+ : E \rightarrow \mathbb{Z}_2 \) defined by \( f_+(uv) = f(u) + f(v) \mod 2 \), for each \( uv \in E \). Let \( e_f = |f_+^{-1}(i)| \). The friendly index set of the graph \( G \), denoted by \( FI(G) \), is defined by \(\{|e_f(1) – e_f(0)| : f \text{ is a friendly coloring of } G \}\). We say \( G \) is fully cordial if \( FI(G) = \{|E|, |E| – 2, |E| – 4, \ldots, |E| – 2[\binom{|E|}{2}]\} \). In this paper, we develop a new technique to calculate friendly index sets without labeling vertices, and we develop a technique to create fully cordial graphs from smaller fully cordial graphs. In particular, we show the first examples of fully cordial graphs that are not trees, as well as new infinite classes of fully cordial graphs.
This paper studies the unitary Cayley graph associated with the ring of dual numbers, \(\mathbb{Z}_n[\alpha]\). It determines the exact diameter, vertex chromatic number, and edge chromatic number. In addition, it classifies all perfect graphs within this class.
Stanton-type graphs were introduced recently. In this paper, we define generalized Stanton-type graphs. We also identify LO and OE graphs, find the minimum \(\lambda\) for decomposition of \(\lambda K_n\) into these graphs, and show that for all viable values of \(\lambda\), the necessary conditions are sufficient for LO- and OE-decompositions using cyclic decompositions from base graphs.
For a finite graph \(G\) with vertices \(\{v_1, \ldots, v_r\}\), a representation of \(G\) modulo \(n\) is a set \(\{a_1, \ldots, a_r\}\) of distinct, nonnegative integers with \(0 \leq a_i < n\), satisfying \(\gcd(a_i – a_j, n) = 1\) if and only if \(v_i\) is adjacent to \(v_j\). The representation number, \(Rep(G)\), is the smallest \(n\) such that \(G\) has a representation modulo \(n\). Evans \(et \, al.\) obtained the representation number of paths. They also obtained the representation number of a cycle except for cycles of length \(2^k + 1\), \(k \geq 3\). In the present paper, we obtain upper and lower bounds for the representation number of a caterpillar, and get its exact value in some cases.
By applying the method of generating functions, the purpose of this paper is to give several summations of reciprocals related to the quintuple product of the general second-order recurrence \(\{W_{rn}\}\) for arbitrary positive integers \(r\). As applications, some identities involving Fibonacci and Lucas numbers are obtained.
A collection \(\mathcal{S}\) of proper subgroups of a group \(G\) is said to be a cover (or covering) for \(G\) if the union of the members of \(\mathcal{S}\) is all of \(G\). A cover \(\mathcal{C}\) of minimal cardinality is called a minimal cover for \(G\) and \(|\mathcal{C}|\) is called the covering number of \(G\), denoted by \(\sigma(G)\). In this paper we determine the covering numbers of the alternating groups \(A_9\) and \(A_{11}\).
Given a (not necessarily proper) coloring of a digraph \( c:V(D)\rightarrow {N}\), let \( OC(v)\) denote the set of colors assigned to the out-neighbors of \(v\). Similarly, let \( IC(v)\) denote the set of colors assigned to the in-neighbors of \(v\). Then \(c\) is a set coloring of \(D\) provided \((u,v) \in A(D)\) implies \( OC(u) \neq OC(v)\). Analogous to the set chromatic number of a graph given by Chartrand, \(et\) \(al.\) \([3]\), we define \( \chi_s(D) \) as the minimum number of colors required to produce a set coloring of \(D\). We find bounds for \(\chi_s(D)\) where \(D\) is a digraph and where \(D\) is a tournament. In addition we consider a second set coloring, where \((u,v) \in A(D)\) implies \( OC(u) \neq IC(v)\).
Let \(\mathcal{C}\) be a finite family of distinct boxes in \(\mathbb{R}^d\), with \(G\) the intersection graph of \(\mathcal{C}\), and let \(S = \cup\{C : C \in \mathcal{C}\}\). For each block of \(G\), assume that the corresponding members of \(\mathcal{C}\) have a staircase convex union. Then when \(S\) is staircase starshaped, its staircase kernel will be a staircase convex set. Moreover, this result (and others) will hold for more general families \(\mathcal{C}\) as well.
The domination chain \(\iota_r(G) \leq \gamma(G) \leq \iota(G) \leq \beta_o(G) \leq \Gamma(G) \leq IR(G)\), which holds for any graph \(G\), is the subject of much research. In this paper, we consider the maximum number of edges in a graph having one of these domination chain parameters equal to \(2\) through a unique realization. We show that a specialization of the domination chain still holds in this setting.
The matching energy of a graph was introduced by Gutman and Wagner in \(2012\) and defined as the sum of the absolute values of zeros of its matching polynomial. In this paper, we completely determine the graph with minimum matching energy in tricyclic graphs with given girth and without \(K_4\)-subdivision.
In this paper, we define and study the Gaussian Fibonacci and Gaussian Lucas \(p\)-numbers. We give generating functions, Binet formulas, explicit formulas, matrix representations, and sums of Gaussian Fibonacci \(p\)-numbers by matrix methods. For \(p = 1\), these Gaussian Fibonacci and Gaussian Lucas \(p\)-numbers reduce to the Gaussian Fibonacci and the Gaussian Lucas numbers.
Let \(G\) be a graph of order \(n\) and let \(Q(G, x) = \det(xI – Q(G)) = \sum_{i=0}^{n}(-1)^i\zeta_i(G)x^{n-i}\) be the characteristic polynomial of the signless Laplacian matrix of \(G\). We show that the Lollipop graph, \(L_{n,3}\), has the maximal \(Q\)-coefficients, among all unicyclic graphs of order \(n\) except \(C_n\). Moreover, we determine graphs with minimal \(Q\)-coefficients, among all unicyclic graphs of order \(n\).
Let \(G\) be a graph with \(n\) vertices, \(\mathcal{G}(G)\) the subdivision graph of \(G\). \(V(G)\) denotes the set of original vertices of \(G\). The generalized subdivision corona vertex graph of \(G\) and \(H_1, H_2, \ldots, H_n\) is the graph obtained from \(\mathcal{G}(G)\) and \(H_1, H_2, \ldots, H_n\) by joining the \(i\)th vertex of \(V(G)\) to every vertex of \(H_i\). In this paper, we determine the Laplacian (respectively, the signless Laplacian) characteristic polynomial of the generalized subdivision corona vertex graph. As an application, we construct infinitely many pairs of cospectral graphs.
In the paper, we show that the orientable genus of the generalized Petersen graph \(P(km, m)\) is at least \( \frac{km}{4} – \frac{m}{2}-\frac{km}{4m-4}+1\) if \(m\geq 4\) and \(k \geq 3\). We determine the orientable genera of \(P(3m, m)\), \(P(4k, 4)\), \(P(4m, m)\) if \(m \geq 4\), \(P(6m, m)\) if \(m \equiv 0 \pmod{2}\) and \(m \geq 6\), and so on.
Assume that \(\mu_1, \mu_2, \ldots, \mu_n\) are the eigenvalues of the Laplacian matrix of a graph \(G\). The Laplacian Estrada index of \(G\), denoted by \(LEE(G)\), is defined as \(LEE(G) = \sum_{i=1}^{n} e^{\mu_i}\). In this note, we give an upper bound on \(LEE(G)\) in terms of chromatic number and characterize the corresponding extremal graph.
In this note, we provide a combinatorial proof of a recent formula for the total number of peaks and valleys (either strict or weak) within the set of all compositions of a positive integer into a fixed number of parts.
The adjacent vertex distinguishing total chromatic number \(\chi_{at}(G)\) of a graph \(G\) is the smallest integer \(k\) for which \(G\) admits a proper \(k\)-total coloring such that no pair of adjacent vertices are incident to the same set of colors. Snarks are connected bridgeless cubic graphs with chromatic index \(4\). In this paper, we show that \(\chi_{at}(G) = 5\) for two infinite subfamilies of snarks, i.e., the Loupekhine snark and Blanusa snark of first and second kind. In addition, we give an adjacent vertex distinguishing total coloring using \(5\) colors for Watkins snark and Szekeres snark, respectively.
Let \(G\) be a tricyclic graph. Tricyclic graphs are connected graphs in which the number of edges equals the number of vertices plus two. In this paper, we determine graphs with the largest signless Laplacian spectral radius among all the tricyclic graphs with \(n\) vertices and diameter \(d\).
A pebbling move on a graph \(G\) consists of taking two pebbles off one vertex and placing one on an adjacent vertex. The pebbling number of a graph \(G\), denoted by \(f(G)\), is the least integer \(n\) such that, however \(n\) pebbles are located on the vertices of \(G\), we can move one pebble to any vertex by a sequence of pebbling moves. For any connected graphs \(G\) and \(H\), Graham conjectured that \(f(G \times H) \leq f(G)f(H)\). In this paper, we give the pebbling number of some graphs and prove that Graham’s conjecture holds for the middle graphs of some even cycles.
Graph embedding is an important factor to evaluate the quality of an interconnection network. It is also a powerful tool for implementation of parallel algorithms and simulation of different interconnection networks. In this paper, we compute the exact wirelength of embedding circulant networks into cycle-of-ladders.
In this paper, we characterize the extremal digraph with the maximal signless Laplacian spectral radius and the minimal distance signless Laplacian spectral radius among all simple connected digraphs with a given dichromatic number, respectively.
Given a graph \(G = (V, E)\) with no isolated vertex, a subset \(S \subseteq V\) is a total dominating set of \(G\) if every vertex in \(V\) is adjacent to a vertex in \(S\). A total dominating set \(S\) of \(G\) is a locating-total dominating set if for every pair of distinct vertices \(u\) and \(v\) in \(V – S\), we have \(N(u) \cap S \neq N(v) \cap S\), and \(S\) is a differentiating-total dominating set if for every pair of distinct vertices \(u\) and \(v\) in \(V\), we have \(N(u) \cap S \neq N(v) \cap S\). The locating-total domination number (or the differentiating-total domination number) of \(G\), denoted by \(\gamma_t^L(G)\) (or \(\gamma_t^D(G)\)), is the minimum cardinality of a locating-total dominating set (or a differentiating-total dominating set) of \(G\). In this paper, we investigate the bounds of locating and differentiating-total domination numbers of unicyclic graphs.
Motzkin posed the problem of finding the maximal density \(\mu(M)\) of sets of integers in which the differences given by a set \(M\) do not occur. The problem is already settled when \(|M| \leq 2\) or \(M\) is a finite arithmetic progression. In this paper, we determine \(\mu(M)\) when \(M\) has some other structure. For example, we determine \(\mu(M)\) when \(M\) is a finite geometric progression.
For vertices \(u, v\) in a connected graph \(G\), a \(u-v\) chordless path in \(G\) is a \(u-v\) monophonic path. The monophonic interval \(J_G[u, v]\) consists of all vertices lying on some \(u-v\) monophonic path in \(G\). For \(S \subseteq V(G)\), the set \(J_G[S]\) is the union of all sets \(J_G[u, v]\) for \(u, v \in S\). A set \(S \subseteq V(G)\) is a monophonic set of \(G\) if \(J_G[S] = V(G)\). The cardinality of a minimum monophonic set of \(G\) is the monophonic number of \(G\), denoted by \(mn(G)\). In this paper, bounds for the monophonic number of the strong product graphs are obtained, and for several classes, improved bounds and exact values are obtained.
A hypergraph is a useful tool to model complex systems and can be considered a natural generalization of graphs. In this paper, we define some operations of fuzzy hypergraphs and strong fuzzy \(r\)-uniform hypergraphs, such as Cartesian product, strong product, normal product, lexicographic product, union, and join. We prove that if a hypergraph \(H\) is formed by one of these operations, then this hypergraph is a fuzzy hypergraph or a strong fuzzy \(r\)-uniform hypergraph. Finally, we discuss an application of fuzzy hypergraphs.
Let \(p_e(n)\) be the number of ways to make change for \(n\) cents using pennies, nickels, dimes, and quarters. By manipulating the generating function for \(p_e(n)\), we prove that the sequence \(\{p_e(n) \pmod{\ell^j}\}\) is periodic for every prime power \(\ell\).
In 1972, Chvatal and Erdős showed that the graph \(G\) with independence number \(\alpha(G)\) no more than its connectivity \(\kappa(G)\) (i.e., \(\kappa(G) \geq \alpha(G)\)) is hamiltonian. In this paper, we consider a kind of Chvatal and Erdős type condition on edge-connectivity \(\lambda(G)\) and matching number (edge independence number). We show that if \(\lambda(G) \geq \alpha'(G) – 1\), then \(G\) is either supereulerian or in a well-defined family of graphs. Moreover, we weaken the condition \(\kappa(G) \geq \alpha(G) – 1\) in [11] to \(\lambda(G) \geq \alpha(G) – 1\) and obtain a similar characterization on non-supereulerian graphs. We also characterize the graph which contains a dominating closed trail under the assumption \(\lambda(G) \geq \alpha'(G) – 2\).
The coloring number \(col(G)\) of a graph \(G\) is the smallest number \(k\) for which there exists a linear ordering of the vertices of \(G\) such that each vertex is preceded by fewer than \(k\) of its neighbors. It is well known that \(\chi(G) \leq col(G)\) for any graph \(G\), where \(\chi(G)\) denotes the chromatic number of \(G\). The Randić index \(R(G)\) of a graph \(G\) is defined as the sum of the weights \(\frac{1}{\sqrt{d(u)d(v)}}\) of all edges \(uv\) of \(G\), where \(d(u)\) denotes the degree of a vertex \(u\) in \(G\). We show that \(\chi(G) \leq col(G) \leq 2R'(G) \leq R(G)\) for any connected graph \(G\) with at least one edge, and \(col(G) = 2R'(G)\) if and only if \(G\) is a complete graph with some pendent edges attaching to its same vertex, where \(R'(G)\) is a modification of Randić index, defined as the sum of the weights \(\frac{1}{\max\{d(u), d(v)\}}\) of all edges \(uv\) of \(G\). This strengthens a relation between Randić index and chromatic number by Hansen et al. [7], a relation between Randić index and coloring number by Wu et al. [17] and extends a theorem of Deng et al. [2].
For any vertex \(x\) in a connected graph \(G\) of order \(n \geq 2\), a set \(S \subseteq V(G)\) is a \(z\)-detour monophonic set of \(G\) if each vertex \(v \in V(G)\) lies on a \(x-y\) detour monophonic path for some element \(y \in S\). The minimum cardinality of a \(x\)-detour monophonic set of \(G\) is the \(x\)-detour monophonic number of \(G\), denoted by \(dm_z(G)\). An \(x\)-detour monophonic set \(S_x\) of \(G\) is called a minimal \(x\)-detour monophonic set if no proper subset of \(S_x\) is an \(x\)-detour monophonic set. The upper \(x\)-detour monophonic number of \(G\), denoted by \(dm^+_x(G)\), is defined as the maximum cardinality of a minimal \(x\)-detour monophonic set of \(G\). We determine bounds for it and find the same for some special classes of graphs. For positive integers \(r, d,\) and \(k\) with \(2 \leq r \leq d\) and \(k \geq 2\), there exists a connected graph \(G\) with monophonic radius \(r\), monophonic diameter \(d\), and upper \(z\)-detour monophonic number \(k\) for some vertex \(x\) in \(G\). Also, it is shown that for positive integers \(j, k, l,\) and \(n\) with \(2 \leq j \leq k \leq l \leq n – 7\), there exists a connected graph \(G\) of order \(n\) with \(dm_x(G) = j\), \(dm^+_x(G) = l\), and a minimal \(x\)-detour monophonic set of cardinality \(k\).
Many authors define certain generalizations of the usual Fibonacci, Pell, and Lucas numbers by matrix methods and then obtain the Binet formulas and combinatorial representations of the generalizations of these number sequences. In this article, we firstly define and study the generalized Gaussian Fibonacci numbers and then find the matrix representation of the generalized Gaussian Fibonacci numbers and prove some theorems by these matrix representations.
Given two sets \(A, B \subset \mathbb{F}_q\), of elements of the finite field \(\mathbb{F}_q\), of \(q\) elements, Shparlinski (2008) showed that the product set \(\mathcal{AB} = \{ab \mid a \in \mathcal{A}, b \in \mathcal{B}\}\) contains an arithmetic progression of length \(k \geq 3\) provided that \(k
3\) is the characteristic of \(\mathbb{F}\), and \(|\mathcal{A}||\mathcal{B}| \geq 2q^{2-1/(k-1)}\). In this paper, we recover Shparlinski’s result for the case of 3-term arithmetic progressions via spectra of product graphs over finite fields. We also illustrate our method in the setting of residue rings. Let \(m\) be a large integer and \(\mathbb{Z}/m\mathbb{Z}\) be the ring of residues mod \(m\). For any two sets \(\mathcal{A}, \mathcal{B} \subset \mathbb{Z}/m\mathbb{Z}\) of cardinality \[|\mathcal{A}||\mathcal{B}| > m(\frac{r(m)m}{r(m)^{\frac{1}{2}} + 1})\], the product set \(\mathcal{AB}\) contains a \(3\)-term arithmetic progression, where \(r(m)\) is the smallest prime divisor of \(m\) and \(r(m)\) is the number of divisors of \(m\). The spectral proofs presented in this paper avoid the use of character and exponential sums, the usual tool to deal with problems of this kind.
A proper edge-coloring of a graph \(G\) with colors \(1, \ldots, t\) is called an interval \(t\)-coloring if the colors of edges incident to any vertex of \(G\) form an interval of integers. A graph \(G\) is interval colorable if it has an interval \(t\)-coloring for some positive integer \(t\). For an interval colorable graph \(G\), the least value of \(t\) for which \(G\) has an interval \(t\)-coloring is denoted by \(w(G)\). A graph \(G\) is outerplanar if it can be embedded in the plane so that all its vertices lie on the same (unbounded) face. In this paper, we show that if \(G\) is a 2-connected outerplanar graph with \(\Delta(G) = 3\), then \(G\) is interval colorable and \[ w(G) = \begin{cases} 3, & \text{if } |V(G)| \text{ is even}, \\ 4, & \text{if } |V(G)| \text{ is odd}. \end{cases} \]
We also give a negative answer to the question of Axenovich on the outerplanar triangulations.
In this paper, we characterize all finite abelian groups with isomorphic intersection graphs. This solves a conjecture proposed by \(B\).Zelinka.
This paper devotes to solving the following conjecture proposed by Gvozdjak: “An \((a, b; n)\)-graceful labeling of \(P_n\) exists if and only if the integers \(a, b, n\) satisfy (1) \(b – a\) has the same parity as \(n(n + 1)/2\); (2) \(0 < |b – a| \leq (n + 1)/2\) and (3) \(n/2 \leq a + b \leq 3n/2\).'' Its solving can shed some new light on solving the famous Oberwolfach problem. It is shown that the conjecture is true for every \(n\) if the conjecture is true when \(n \leq 4a + 1\) and \(a\) is a fixed value. Moreover, we prove that the conjecture is true for \(a = 0, 1, 2, 3, 4, 5, 6\).
The aim of this paper is to show that the corona \(P_n \bigodot P_m\) between two paths \(P_n\) and \(P_m\) is cordial for all \(n \geq 1\) and \(m \geq 1\). Also, we prove that except for \(n\) and \(m\) being congruent to \(2 \pmod{4}\), the corona \(C_n \bigodot C_m\) between two cycles \(C_n\) and \(C_m\) is cordial. Furthermore, we show that if \(n \equiv 2 \pmod{4}\) and \(m\) is odd, then \(C_n \bigodot C_m\) is not cordial.
In this paper, we establish some general identities involving the weighted row sums of a Riordan array and hyperharmonic numbers. From these general identities, we deduce some particular identities involving other special combinatorial sequences, such as the Stirling numbers, the ordered Bell numbers, the Fibonacci numbers, the Lucas numbers, and the binomial coefficients.
In this paper, we consider the relationship between toughness and the existence of \([a, b]\)-factors with inclusion/exclusion properties. We obtain that if \(t(G) \geq a – 1 + \frac{a – 1}{b}\) with \(b > a > 2\), where \(a, b\) are two integers, then for any two given edges \(e_1\) and \(e_2\), there exist an \([a, b]\)-factor including \(e_1, e_2\); and an \([a, b]\)-factor including \(e_1\) and excluding \(e_2\); as well as an \((a, b)\)-factor excluding \(e_1, e_2\). Furthermore, it is shown that the results are best possible in some sense.
In this paper, we will determine the NBB bases with respect to a certain standard ordering of atoms of lattices of \(321\)-\(312\)-\(231\)-avoiding permutations and of \(321\)-avoiding permutations with the weak Bruhat order. Using our expressions of NBB bases, we will calculate the Möbius numbers of these lattices. These values are shown to be related to Fibonacci polynomials.
Let \(D(G)\) denote the signless Dirichlet spectral radius of the graph \(G\) with at least a pendant vertex, and \(\pi_1\) and \(\pi_2\) be two nonincreasing unicyclic graphic degree sequences with the same frequency of number \(1\). In this paper, the signless Dirichlet spectral radius of connected graphs with a given degree sequence is studied. The results are used to prove a majorization theorem of unicyclic graphs. We prove that if \(\pi_1 \unrhd \pi_2\), then \(D(G_1) \leq D(G_2)\) with equality if and only if \(\pi_1 = \pi_2\), where \(G_1\) and \(G_2\) are the graphs with the largest signless Dirichlet spectral radius among all unicyclic graphs with degree sequences \(\pi_1\) and \(\pi_2\), respectively. Moreover, the graphs with the largest signless Dirichlet spectral radius among all unicyclic graphs with \(k\) pendant vertices are characterized.
A function \(f\) is called a graceful labeling of a graph \(G\) with \(m\) edges, if \(f\) is an injective function from \(V(G)\) to \(\{0, 1, 2, \ldots, m\}\) such that when every edge \(uv\) is assigned the edge label \(|f(u) – f(v)|\), then the resulting edge labels are distinct. A graph which admits a graceful labeling is called a graceful graph. A graceful labeling of a graph \(G\) with \(m\) edges is called an \(\alpha\)-labeling if there exists a number \(\alpha\) such that for any edge \(uv\), \(\min\{f(u), f(v)\} \leq \lambda < \max\{f(u), f(v)\}\). The characterization of graceful graphs appears to be a very difficult problem in Graph Theory. In this paper, we prove a basic structural property of graceful graphs, that every tree is a subtree of a graceful graph, an \(\alpha\)-labeled graph, and a graceful tree, and we discuss a related open problem towards settling the popular Graceful Tree Conjecture.
We use rook placements to prove Spivey’s Bell number formula and other identities related to it, in particular, some convolution identities involving Stirling numbers and relations involving Bell numbers. To cover as many special cases as possible, we work on the generalized Stirling numbers that arise from the rook model of Goldman and Haglund. An alternative combinatorial interpretation for the Type II generalized \(q\)-Stirling numbers of Remmel and Wachs is also introduced, in which the method used to obtain the earlier identities can be adapted easily.
An \(H\)-triangle is a triangle with corners in the set of vertices of a tiling of \(\mathbb{R}^2\) by regular hexagons of unit edge. Let \(b(\Delta)\) be the number of the boundary \(H\)-points of an \(H\)-triangle \(\Delta\). In [3] we made a conjecture that for any \(H\)-triangle with \(k\) interior \(H\)-points, we have \(b(\Delta) \in \{3, 4, \ldots, 3k+4, 3k+5, 3k+7\}\). In this note, we prove the conjecture is true for \(k = 4\), but not true for \(k = 5\) because \(b(\Delta)\) cannot equal \(15\).
In this paper, we study further bounds of constant dimension codes in Grassmannian space \(\mathcal{G}_q(n,k)\). There is increasing interest in subspace codes since they are essential for error-correction in networks. Additionally, there is a connection to the theory over finite fields. By revising the specific construction methods of the constant dimension codes in [1], [2], we improve some bounds on \(q\)-ary constant dimension codes in certain cases.
In this paper, we use a recent result of Bryant, Horsley, and Pettersson in [1] to provide an alternate and more straightforward proof of results concerning neighborhood graphs in maximum packings of \(2K_n\) with triples, some of which were only recently obtained.
To set the stage, consider any partial triple system \((V,B)\) of \(2K_n\). In this system, the neighborhood of a vertex \(v\) is defined as the subgraph induced by the set \(\{\{x,y\} \mid \{v,x,y\} \in B\}\). This concept plays a crucial role in the results initially obtained by Colbourn and Rosa for \(n \equiv 0,1 \pmod{3}\) and by Chaffee and Rodger for \(n \equiv 2 \pmod{3}\). These results offer a complete characterization of the possible neighborhoods in a maximum packing of \(2K_n\).
In both of these original papers, the authors employed difference methods—a combinatorial technique that often involves selecting pairs of elements from a group and studying their differences—and a pull-up technique, which is used to modify the neighborhood of a vertex. However, despite the effectiveness of these methods, neither approach seems to lend itself easily to deriving the results of the other.
In our paper, we present a more unified and simplified proof that brings both of these results together. By leveraging the recent findings of Bryant, Horsley, and Pettersson, we can bypass the need for the more complex difference methods and pull-up techniques, instead relying on the underlying principles elucidated in their work. This approach not only simplifies the proof process but also provides a clearer and more direct route to understanding the structure of neighborhood graphs in these maximum packings.
Compressed sensing (CS) has broken through the traditional Nyquist sampling theory as it is a new technique in signal processing. According to CS theory, compressed sensing makes full use of sparsity so that a sparse signal can be reconstructed from very few measurements. It is well known that the construction of CS matrices is the central problem. In this paper, we provide one kind of deterministic sensing matrix by describing a combinatorial design. Then, we obtain two cases by instantiating the RIP framework with the obtained design, with the latter case being the majorization of the former. Finally, we show that our construction has better properties than DeVore’s construction using polynomials over finite fields.
In this paper, with the help of the residue method, we find some interesting formulas relating residue and ordinary Bell polynomials, \(\hat{B}_{n,k}(x_1,x_2,\ldots)\). Further, we prove identities involving some combinatorial numbers to demonstrate the application of the formulas.
We expand the theory of pebbling to graphs with weighted edges. In a weighted pebbling game, one player distributes a set amount of weight on the edges of a graph and his opponent chooses a target vertex and places a configuration of pebbles on the vertices. Player one wins if, through a series of pebbling moves, he can move at least one pebble to the target. A pebbling move of \(p\) pebbles across an edge with weight \(w\) leaves \(\lfloor pw \rfloor\) pebbles on the next vertex. We find the weighted pebbling numbers of stars, graphs with at least \(2|V|-1\) edges, and trees with given targets. We give an explicit formula for the minimum total weight required on the edges of a length-2 path, solvable with \(p\) pebbles, and exhibit a graph that requires an edge with weight \(1/3\) in order to achieve its weighted pebbling number.
We examine two particular constructions of Costas arrays known as the Taylor variant of the Lempel construction, or the \(T_4\) construction, and the variant of the Golomb construction, or the \(G_4\) construction. We connect these with Fibonacci primitive roots, and show that under the Extended Riemann Hypothesis, the \(T_4\) and \(G_4\) constructions are valid infinitely often.
The authentication codes with arbitration are said to be \(A^2\)-codes. Two constructions of \(A^2\)-codes with secrecy from polynomials over finite fields are constructed to prevent communication systems from attacks which come from the opponent, the transmitter and the receiver. Parameters of the codes and probabilities of successful attacks are also computed. At last, two constructions are compared with a known one. It is important that a source state can’t be recovered from the message without the knowledge of the transmitter’s encoding rule or the receiver’s decoding rule. It must be decoded before verification.
For \(n \geq 1\), we let \(a_n\) count the number of nonempty subsets \(S\) of \(\{1,2,3,\ldots,n\} = [n]\), where the size of \(S\) equals the minimal element of \(S\). Such a subset is called an extraordinary subset of \([n]\), and we find that \(a_n = F_n\), the \(n\)th Fibonacci number. Then, for \(n \geq k \geq 1\), we let \(a(n, k)\) count the number of times the integer \(k\) appears among these \(a_n\) extraordinary subsets of \(n\). Here we have \(a(n, k) = a(n-1, k) + a(n-2, k-1)\), for \(n \geq 3\) and \(n > k \geq 2\). Formulas and properties for \(t_n = \sum_{k=1}^n a(n, k)\) and \(s_n = \sum_{k=1}^n ka(n, k)\) are given for \(n \geq 1\). Finally, for fixed \(n \geq 1\), we find that the sequence \(a(n, k)\) is unimodal and examine the maximum element for the sequence. In this context, the Catalan numbers make an entrance.
The cycle length distribution (CLD) of a graph of order \(n\) is \((c_1, c_2, \ldots, c_n)\), where \(c_i\) is the number of cycles of length \(i\), for \(i = 1, 2, \ldots, n\). For an integer sequence \((a_1, a_2, \ldots, a_n)\), we consider the problem of characterizing those graphs \(G\) with the minimum possible edge number and with \(\text{CLD}(G) = (c_1, c_2, \ldots, c_n)\) such that \(c_i \geq a_i\) for \(i = 1, 2, \ldots, n\). The number of edges in such a graph is denoted by \(g(a_1, a_2, \ldots, a_n)\). In this paper, we give the lower and upper bounds of \(g(0, 0, k, \ldots, k)\) for \(k = 2, 3, 4\).
Two-fold automorphisms (or “TF-isomorphisms”) of graphs are a generalisation of automorphisms. Suppose \(\alpha, \beta\) are two permutations of \(V = V(G)\) such that for any pair \((u,v)\), \(u, v \in V\), \((u,v)\) is an arc of \(G\) if and only if \((\alpha(u), \beta(v))\) is an arc of \(G\). Such a pair of permutations is called a two-fold automorphism of \(G\). These pairs form a group that is called the two-fold automorphism group. Clearly, it contains all the pairs \((\alpha, \alpha)\) where \(\alpha\) is an automorphism of \(G\). The two-fold automorphism group of \(G\) can be larger than \(\text{Aut}(G)\) since it may contain pairs \((\alpha, \beta)\) with \(\alpha \neq \beta\). It is known that when this happens, \(\text{Aut}(G) \times \mathbb{Z}_2\) is strictly contained in \(\text{Aut}(G \times K_2)\). In the literature, when this inclusion is strict, the graph \(G\) is called unstable.
Now let \(\Gamma \leq S_V \times S_V\). A two-fold orbital (or “TF-orbital”) of \(F\) is an orbit of the action \((\alpha, \beta) : (u,v) \mapsto (\alpha(u), \beta(v))\) for \((\alpha, \beta) \in \Gamma\) and \(u,v \in V\). Clearly, \(\Gamma\) is a subgroup of the TF-automorphism group of any of its TF-orbitals. We give a short proof of a characterization of TF-orbitals which are disconnected graphs and prove that a similar characterization of TF-orbitals which are digraphs might not be possible. We shall also show that the TF-rank of \(\Gamma\), that is the number of its TF-orbitals, can be equal to \(1\) and we shall obtain necessary and sufficient conditions on I for this to happen.
We define a new fairness notion on edge-colorings, requiring that the number of vertices in the subgraphs induced by the edges of each color are within one of each other. Given a (not necessarily proper) \( k \)-edge-coloring of a graph \( G \), for each color \( i \in \mathbb{Z}_k \), let \( G[i] \) denote the (not necessarily spanning) subgraph of \( G \) induced by the edges colored \( i \). Let \( \nu_{i}(G) = |V(G[i])| \). Formally, a \( k \)-edge-coloring of a graph \( G \) is said to be vertex-equalized if for each pair of colors \( i, j \in \mathbb{Z}_k \), \( |\nu_{i}(G) – \nu_{j}(G)| \leq 1 \). In this paper, a characterization is found for connected graphs that have vertex-equalized \( k \)-edge-colorings for each \( k \in \{2, 3\} \) (see Corollary 4.1 and Corollary 4.2).
Let \( G = (V, E) \) be a graph. The open neighborhood of a vertex \( v \in V \) is the set \( N(v) = \{u \mid uv \in E\} \) and the closed neighborhood of \( v \) is the set \( N[v] = N(v) \cup \{v\} \). The open neighborhood of a set \( S \) of vertices is the set \( N(S) = \bigcup_{v \in S} N(v) \), while the closed neighborhood of a set \( S \) is the set \( N[S] = \bigcup_{v \in S} N[v] \). A set \( S \subset V \) dominates a set \( T \subset V \) if \( T \subseteq N[S] \), written \( S \rightarrow T \). A set \( S \subset V \) is a dominating set if \( N[S] = V \); and is a minimal dominating set if it is a dominating set, but no proper subset of \( S \) is also a dominating set; and is a \( \gamma \)-set if it is a dominating set of minimum cardinality. In this paper, we consider the family \( \mathcal{D} \) of all dominating sets of a graph \( G \), the family \( \mathcal{MD} \) of all minimal dominating sets of a graph \( G \), and the family \( \Gamma \) of all \( \gamma \)-sets of a graph \( G \). The study of these three families of sets provides new characterizations of the distance-2 domination number, the upper domination number, and the upper irredundance number in graphs.
Irregular-spotty-byte error control codes were devised by the author in [2] and their properties were further studied in [3] and [4]. These codes are suitable for semi-conductor memories where an I/O word is divided into irregular bytes not necessarily of the same length. The \(i\)-spotty-byte errors are defined as \(t_i\) or fewer bit errors in an \(i\)-byte of length \(n_i\), where \(1 \leq t_i \leq n_i\) and \(1 \leq i \leq s\). However, an important and practical situation is when \(i\)-spotty-byte errors caused by the hit of high energetic particles are confined to \(i\)-bytes of the same size only which are aligned together or in words errors occur usually in adjacent RAM chips at a particular time. Keeping this view, in this paper, we propose a new model of \(i\)-spotty-byte errors, viz. uniform \(i\)-spotty-byte errors and present a new class of codes, viz. uniform \(i\)-spotty-byte error control codes which are capable of correcting all uniform \(i\)-spotty-byte errors of \(i\)-spotty measure \( \mu \) (or less). The study made in this paper will be helpful in designing modified semi-conductor memories consisting of irregular RAM chips with those of equal length aligned together.
Let \( \mathcal{M} \) denote the set of matrices over some semiring. An upper ideal of matrices in \( \mathcal{M} \) is a set \( \mathcal{U} \) such that if \( A \in \mathcal{U} \) and \( B \) is any matrix in \( \mathcal{M} \), then \( A + B \in \mathcal{U} \). We investigate linear operators that strongly preserve certain upper ideals (that is, linear operators on \( \mathcal{M} \) with the property that \( X \in \mathcal{U} \) if and only if \( T(X) \in \mathcal{U} \)). We then characterize linear operators that strongly preserve sets of tournament matrices and sets of primitive matrices. Specifically, we show that if \( T \) strongly preserves the set of regular tournaments when \( n \) is odd or nearly regular tournaments when \( n \) is even, then for some permutation matrix \( P \), \( T(X) = P^{t}XP \) for all matrices \( X \) with zero main diagonal, or \( T(X) = P^{t}X^{t}P \) for all matrices \( X \) with zero main diagonal. Similar results are shown for linear operators that strongly preserve the set of primitive matrices whose exponent is \( k \) for some values of \( k \), and for those that strongly preserve the set of nearly reducible primitive matrices.
For a poset \( P = (V(P), \leq_P) \), the strict semibound graph of \( P \) is the graph \( ssb(P) \) on \( V(ssb(P)) = V(P) \) for which vertices \( u \) and \( v \) of \( ssb(P) \) are adjacent if and only if \( u \neq v \) and there exists an element \( x \in V(P) \) distinct from \( u \) and \( v \) such that \( x \leq_P u,v \) or \( u,v \leq_P x \). We prove that a poset \( P \) is connected if and only if the induced subgraph \(\langle max(P)\rangle_{ssb(P)}\) is connected. We also characterize posets whose strict semibound graphs are triangle-free.
In this paper, we revisit LE graphs, find the minimum \( \lambda \) for decomposition of \( \lambda K_n \) into these graphs, and show that for all viable values of \( \lambda \), the necessary conditions are sufficient for LE-decompositions using cyclic decompositions from base graphs.
We determine the signless Laplacian spectrum for the \( H \)-join of regular graphs \( G_1, \ldots, G_p \). We also find an expression and upper bounds for the signless Laplacian spread of the \( H \)-join of regular graphs \( G_1, \ldots, G_p \).
Placing degree constraints on the vertices of a path yields the definitions of uphill and downhill paths. Specifically, we say that a path \( \pi = v_1, v_2, \ldots, v_{k+1} \) is a downhill path if for every \( i \), \( 1 \leq i \leq k \), \( \deg(v_i) \geq \deg(v_{i+1}) \). Conversely, a path \( \pi = u_1, u_2, \ldots, u_{k+1} \) is an uphill path if for every \( i \), \( 1 \leq i \leq k \), \( \deg(u_i) \leq \deg(u_{i+1}) \). The downhill domination number of a graph \( G \) is defined to be the minimum cardinality of a set \( S \) of vertices such that every vertex in \( V \) lies on a downhill path from some vertex in \( S \). The uphill domination number is defined as expected. We explore the properties of these invariants and their relationships with other invariants. We also determine a Vizing-like result for the downhill (respectively, uphill) domination numbers of Cartesian products.
Set-to-Set Broadcasting is an information distribution problem in a connected graph, \( G = (V, E) \), in which a set of vertices \( A \), called originators, distributes messages to a set of vertices \( B \) called receivers, such that by the end of the broadcasting process each receiver has received the messages of all the originators. This is done by placing a series of calls among the communication lines of the graph. Each call takes place between two adjacent vertices, which share all the messages they have. Gossiping is a special case of set-to-set broadcasting, where \( A = B = V \). We use \( F(A, B, G) \) to denote the length of the shortest sequence of calls that completes the set-to-set broadcast from a set \( A \) of originators to a set \( B \) of receivers, within a connected graph \( G \). \( F(A, B, G) \) is also called the cost of an algorithm. We present bounds on \( F(A, B, G) \) for weighted and for non-weighted graphs.
Let \( \gamma_c(G) \) denote the connected domination number of the graph \( G \). A graph \( G \) is said to be connected domination edge critical, or simply \( \gamma_c \)-critical, if \( \gamma_c(G + e) < \gamma_c(G) \) for each edge \( e \in E(\overline{G}) \). We answer a question posed by Zhao and Cao concerning \( \gamma_c \)-critical graphs with maximum diameter.
For a positive integer \(d \geq 1\), an \(L(d, 1)\)-labeling of a graph \(G\) is an assignment of nonnegative integers to \(V(G)\) such that the difference between labels of adjacent vertices is at least \(d\), and the difference between labels of vertices that are distance two apart is at least \(1\). The span of an \(L(d, 1)\)-labeling of a graph \(G\) is the difference between the maximum and minimum integers used by it. The minimum span of an \(L(d, 1)\)-labeling of \(G\) is denoted by \(\lambda(G)\). In [17], we obtained that \(r\Delta + 1 \leq \lambda(G(rP_5)) \leq r\Delta + 2\), \(\lambda(G(rP_k)) = r\Delta + 1\) for \(k \geq 6\); and \(\lambda(G(rP_4)) \leq (\Delta + 1)r + 1\), \(\lambda(G(rP_3)) \leq (\Delta + 1)r + \Delta\) for any graph \(G\) with maximum degree \(\Delta\). In this paper, we will focus on \(L(d, 1)\)-labelings of the edge-multiplicity-path-replacement \(G(rP_k)\) of a graph \(G\) for \(r \geq 2\), \(d \geq 3\), and \(k \geq 3\). And we show that the class of graphs \(G(rP_k)\) with \(k \geq 3\) satisfies the conjecture proposed by Havet and Yu [7].
Let \(R\) be a commutative ring with non-zero identity. The cozero-divisor graph of \(R\), denoted by \(\Gamma'(R)\), is a graph with vertex-set \(W^*(R)\), which is the set of all non-zero non-unit elements of \(R\), and two distinct vertices \(a\) and \(b\) in \(W^*(R)\) are adjacent if and only if \(a \not\in Rb\) and \(b \not\in Ra\), where for \(c \in R\), \(Rc\) is the ideal generated by \(c\). In this paper, we completely determine all finite commutative rings \(R\) such that \(\Gamma'(R)\) is planar, outerplanar and a ring graph.
The Wiener index is the sum of distances between all pairs of vertices in a connected graph. A cactus is a connected graph in which any two of its cycles have at most one common vertex. In this article, we present some graphic transformations and derive the formulas for calculating the Wiener index of new graphs. With these transformations, we characterize the graphs having the smallest Wiener index among all cacti given matching number and cycle number.
A Roman dominating function on a graph \(G\) is a function \(f: V(G) \to \{0, 1, 2\}\) satisfying the condition that every vertex \(u\) of \(G\) for which \(f(u) = 0\) is adjacent to at least one vertex \(v\) of \(G\) for which \(f(v) = 2\). The weight of a Roman dominating function is the value \(f(V(G)) = \sum_{u \in V(G)} f(u)\). The Roman domination number, \(\gamma_R(G)\), of \(G\) is the minimum weight of a Roman dominating function on \(G\). A graph \(G\) is said to be Roman domination edge critical, or simply \(\gamma_R\)-edge critical, if \(\gamma_R(G + e) < \gamma_R(G)\) for any edge \(e \not\in E(G)\). In this paper, we characterize all \(\gamma_R\)-edge critical connected graphs having precisely two cycles.
An \(h\)-edge-coloring (block-coloring) of type \(s\) of a graph \(G\) is an assignment of \(h\) colors to the edges (blocks) of \(G\) such that for every vertex \(x\) of \(G\), the edges (blocks) incident with \(x\) are colored with \(s\) colors. For every color \(i\), \(\xi_{x,i}\) (\(\mathcal{B}_{x,i}\)) denotes the set of all edges (blocks) incident with \(x\) and colored by \(i\). An \(h\)-edge-coloring (\(h\)-block-coloring) of type \(s\) is equitable if for every vertex \(x\) and for colors \(i\), \(j\), \(||\xi_{x,i}| – |\xi_{x,j}|| \leq 1\) (\(||\mathcal{B}_{x,i}| – |\mathcal{B}_{x,j}|| \leq 1\)). In this paper, we study the existence of \(h\)-edge-colorings of type \(s = 2,3\) of \(K_t\) and then show that the solution of this problem induces the solution of the existence of a \(C_4\)-\(_tK_2\)-design having an equitable \(h\)-block-coloring of type \(s = 2,3\).
G. Chartrand et al. [3] define a graph \(G\) without isolated vertices to be the least common multiple (lcm) of two graphs \(G_1\) and \(G_2\) if \(G\) is a graph of minimum size such that \(G\) is both \(G_1\)-decomposable and \(G_2\)-decomposable. A bi-star \(B_{m,n}\) is a caterpillar with spine length one. In this paper, we discuss a good lower bound for \(lcm(B_{m,n}, G)\), where \(G\) is a simple graph. We also investigate \(lcm(B_{m,n}, rK_2)\) and provide a good lower bound and an appropriate upper bound for \(lcm(B_{m,n}, P_{r+1})\) for all \(m \geq 1\), \(n \geq 1\), and \(r \geq 1\).
A path in an edge-colored graph is said to be a rainbow path if no two edges on the path share the same color. An edge-colored graph \(G\) is rainbow connected if there exists a \(u-v\) rainbow path for any two vertices \(u\) and \(v\) in \(G\). The rainbow connection number of a graph \(G\), denoted by \(rc(G)\), is the smallest number of colors that are needed in order to make \(G\) rainbow connected. For any two vertices \(u\) and \(v\) of \(G\), a rainbow \(u-v\) geodesic in \(G\) is a rainbow \(u\)–\(v\) path of length \(d(u,v)\), where \(d(u,v)\) is the distance between \(u\) and \(v\). The graph \(G\) is strongly rainbow connected if there exists a rainbow \(u-v\) geodesic for any two vertices \(u\) and \(v\) in \(G\). The strong rainbow connection number of \(G\), denoted by \(src(G)\), is the smallest number of colors that are needed in order to make \(G\) strongly rainbow connected.
In this paper, we determine the precise (strong) rainbow connection numbers of ladders and Möbius ladders. Let \(p\) be an odd prime; we show the (strong) rainbow connection numbers of Cayley graphs on the dihedral group \(D_{2p}\) of order \(2p\) and the cyclic group \(\mathbb{Z}_{2p}\) of order \(2p\). In particular, an open problem posed by Li et al. in [8] is solved.
Given a collection of graphs \(\mathcal{H}\), an \(\mathcal{H}\)-decomposition of \(\lambda K_v\) is a decomposition of the edges of \(\lambda K_v\) into isomorphic copies of graphs in \(\mathcal{H}\). A kite is a triangle with a tail consisting of a single edge. In this paper, we investigate the decomposition problem when \(\mathcal{H}\) is the set containing a kite and a \(4\)-cycle, that is, this paper gives a complete solution to the problem of decomposing \(\lambda K_v\) into \(r\) kites and \(s\) \(4\)-cycles for every admissible values of \(v\), \(r,\lambda\), and \(s\).
A \(b\)-coloring of a graph \(G\) with \(k\) colors is a proper coloring of \(G\) using \(k\) colors in which each color class contains a color dominating vertex, that is, a vertex which has a neighbor in each of the other color classes. The largest positive integer \(k\) for which \(G\) has a \(b\)-coloring using \(k\) colors is the \(b\)-chromatic number \(\beta(G)\) of \(G\). The \(b\)-spectrum \(\mathcal{S}_b(G)\) of a graph \(G\) is the set of positive integers \(k\), \(\chi(G) \leq k \leq b(G)\), for which \(G\) has a \(b\)-coloring using \(k\) colors. A graph \(G\) is \(b\)-continuous if \(\mathcal{S}_b(G) = \{\chi(G), \ldots, b(G)\}\). It is known that for any two graphs \(G\) and \(H\), \(b(G \Box H) \geq \max\{b(G), b(H)\}\), where \(\Box\) stands for the Cartesian product. In this paper, we determine some families of graphs \(G\) and \(H\) for which \(b(G \Box H) \geq b(G) + b(H) – 1\). Further, we show that if \(O_k,i=1,2,\ldots,n\) are odd graphs with \(k_i \geq 4\) for each \(i\), then \(O_{k_1} \Box O_{k_2} \Box \ldots \Box O_{k_n}\) is \(b\)-continuous and \(b(O_{k_1} \Box O_{k_2} \Box \ldots \Box O_{k_n}) = 1 + \sum\limits_{i=1}^{n} k_i\).
We study the number of elements \(x\) and \(y\) of a finite group \(G\) such that \(x \otimes y = 1_{G \oplus G}\) in the nonabelian tensor square \(G \otimes G\) of \(G\). This number, divided by \(|G|^2\), is called the tensor degree of \(G\) and has connections with the exterior degree, introduced a few years ago in [P. Niroomand and R. Rezaei, On the exterior degree of finite groups, Comm. Algebra \(39 (2011), 335–343\)]. The analysis of upper and lower bounds of the tensor degree allows us to find interesting structural restrictions for the whole group.
For a (molecular) graph \(G\), the general sum-connectivity index \(\chi_\alpha(G)\) is defined as the sum of the weights \((d_u + d_v)^\alpha\) of all edges \(uv\) of \(G\), where \(d_u\) (or \(d_v\)) denotes the degree of a vertex \(u\) (or \(v\)) in \(G\) and \(\alpha\) is an arbitrary real number. In this paper, we give an efficient formula for computing the general sum-connectivity index of polyomino chains and characterize the extremal polyomino chains with respect to this index, which generalizes one of the main results in [Z. Yarahmadi, A. Ashrafi, S. Moradi, Extremal polyomino chains with respect to Zagreb indices, Appl. Math. Lett. 25 (2012): 166-171].
In this paper, we investigate the basis number for the wreath product of wheels with paths. Also, as a related problem, we construct a minimum cycle basis of the same.
Let\(ex(m, C_{\leq n})\) denote the maximum size of a graph of order \(m\) and girth at least \(n+1\), and \(EX(m, C_{\leq n})\) be the set of all graphs of girth at least \(n+1\) and size \(ex(m, C_{\leq n})\). The Ramsey number \(R(C_{\leq n}, K_m)\) is the smallest \(k\) such that every graph of order \(k\) contains either a cycle of order \(n\) for some \(3 \leq l \leq n\) or a set of \(m\) independent vertices. It is known that \(ex(2n, C_{\leq n}) = 2n + 2\) for \(n \geq 4\), and the exact values of \(R(C_{\leq n}, K_m)\) for \(n \geq m\) are known. In this paper, we characterize all graphs in \(EX(2n, C_{\leq n})\) for \(n \geq 5\), and then obtain the exact values of \(R(C_{\leq n}, K_m)\) for \(m \in \{n, n+1\}\).
Since their desirable features, variable-weight optical orthogonal codes (VWOOCs) have found wide ranges of applications in various optical networks and systems. In recent years, optimal \(2\)-CP\((W, 1, Q; n)\)s are used to construct optimal VWOOCs. So far, some works have been done on optimal \(2\)-CP\((W, 1, Q; n)\)s with \(w_{\max} \leq 6\), where \(w_{\max} = \max\{w: w \in W\}\). As far as the authors are aware, little is known for explicit constructions of optimal \(2\)-CP\((W, 1, Q; n)\)s with \(w_{\max} \geq 7\) and \(|W| = 3\). In this paper, two explicit constructions of \(2\)-CP\((\{3, 4, 7\}, 1, Q; n)\)s are given, and two new infinite classes of optimal VWOOCs are obtained.
In this study, it has been researched which Euclidean regular polyhedrons are also taxicab regular and which are not. The existence of non-Euclidean taxicab regular polyhedrons in the taxicab \(3\)-space has also been investigated.
As a generalization of attenuated space, the concept of singular linear spaces was firstly introduced in [1]. In this paper, we construct a family of error-correcting pooling designs with the incidence matrix of two types of subspaces of singular linear space over finite fields, and exhibit their disjunct properties. Moreover, we show that the new construction gives better ratio of efficiency than the former ones under certain conditions. Finally, the paper gives a brief introduction about the relationship between the columns (rows) of the matrix and the related parameters.
A map is unicursal if all its vertices are even-valent except two odd-valent vertices. This paper investigates the enumeration of rooted nonseparable unicursal planar maps and provides two functional equations satisfied by its generating functions with the number of nonrooted vertices, the number of inner faces (or the number of edges) and the valencies of the two odd vertices of maps as parameters.
Let \(\sigma_k(G)\) denote the minimum degree sum of \(k\) independent vertices of a graph \(G\). A spanning tree with at most \(3\) leaves is called a spanning \(3\)-ended tree. In this paper, we prove that for any \(k\)-connected claw-free graph \(G\) with \(|G| = n\), if \(\sigma_{k+3}(G) \geq n – k\), then \(G\) contains a spanning \(3\)-ended tree.
As a promotion of the channel assignment problem, an \(L(1,1,1)\)-labeling of a graph \(G\) is an assignment of nonnegative integers to \(V(G)\) such that the difference between labels of adjacent vertices is at least \(1\), and the difference between labels of vertices that are distance two and three apart is at least \(1\). About \(10\) years ago, many mathematicians considered colorings (proper, general, total or from lists) such that vertices (all or adjacent) are distinguished either by sets or multisets or sums. In this paper, we will study \(L(1,1,1)\)-labeling-number and \(L(1,1)\)-edge-labeling-number of the edge-path-replacement. From this, we will consider the total-neighbor-distinguishing coloring and the neighbor-distinguishing coloring of the edge-multiplicity-paths-replacements, give a reference for the conjectures: \(\text{tndis-}_\Sigma(G) \leq \Delta + 3\), \(\text{ndi}_\Sigma(G) \leq \Delta + 2\), and \(\text{tndi}_S(G) \leq \Delta + 3\) for the edge-multiplicity-paths-replacements \(G(rP_k)\) with \(k \geq 3\) and \(r \geq 1\).
A \(T\)-shape tree is a tree with exactly one of its vertices having maximal degree \(3\). In this paper, we consider a class of tricyclic graphs which is obtained from a \(T\)-shape tree by attaching three identical odd cycles \(C_ks\) to three vertices of degree \(1\) of the \(T\)-shape tree, respectively, where \(k \geq 3\) is odd. It is shown that such graphs are determined by their adjacency spectrum.
In this paper, we have proved that if a contraction critical \(8\)-connected graph \(G\) has no vertices of degree \(8\), then for every vertex \(x\) of \(G\), either \(x\) is adjacent to a vertex of degree \(9\), or there are at least \(4\) vertices of degree \(9\) such that every one of them is at distance \(2\) from \(x\).
The crossing number of a graph \(G\) is the minimum number of pairwise intersections of edges in a drawing of \(G\). The \(n\)-dimensional locally twisted cubes \(LTQ_n\), proposed by X.F. Yang, D.J. Evans and G.M. Megson, is an important interconnection network with good topological properties and applications. In this paper, we mainly obtain an upper bound on the crossing number of \(LTQ_n\), no more than \(\frac{265}{6}4^{n-4} – (n^2 + \frac{15+(-1)^{n-1}}{6}2^{n-3}\).
Let \(G\) be an infinite geometric graph; in particular, a graph whose vertices are a countable discrete set of points on the plane, with vertices \(u, v\) adjacent if their Euclidean distance is less than 1. A “fire” begins at some finite set of vertices and spreads to all neighbors in discrete steps; in the meantime, \(f\) vertices can be deleted at each time-step. Let \(f(G)\) be the least \(f\) for which any fire on \(G\) can be stopped in finite time. We show that if \(G\) has bounded density, in the sense that no open disk of radius \(r\) contains more than \(\lambda\) vertices, then \(f(G)\) is bounded above by ceiling of a universal constant times \(\frac{\lambda}{r^2}\). Similarly, if the density of \(G\) is bounded from below in the sense that every open disk of radius \(r\) contains at least \(\beta\) vertices, then \(f(G)\) is bounded below by \(\kappa\) times the square of the floor of a universal constant times \(\frac{1}{r}\).
Let \(G\) be a graph, and let \(k \geq 2\) be an integer. A graph \(G\) is fractional independent-set-deletable \(k\)-factor-critical (in short, fractional ID-\(k\)-factor-critical) if \(G – I\) has a fractional \(k\)-factor for every independent set \(I\) of \(G\). In this paper, a Fan-type condition for fractional ID-\(k\)-factor-critical graphs is given.
The crossing number of a graph \(G\) is the smallest number of pairwise crossings of edges among all the drawings of \(G\) in the plane. The pancake graph is an important network topological structure for interconnecting processors in parallel computers. In this paper, we prove the exact crossing number of the pancake graph \(P_4\) is six.
A planar graph is called \(C_4\)-free if it has no cycles of length four. Let \(f(n,C_4)\) denote the maximum size of a \(C_4\)-free planar graph with order \(n\). In this paper, it is shown that \(f(n,C_4) = \left\lfloor \frac{15}{7}(n-2) \right\rfloor – \mu\) for \(n \geq 30\), where \(\mu = 1\) if \(n \equiv 3 \pmod{7}\) or \(n = 32, 33, 37\), and \(\mu = 0\) otherwise.
The Harary spectral radius \(\rho(G)\) of a graph \(G\) is the largest eigenvalue of the Harary matrix \(RD(G)\). In this paper, we determine graphs with the largest Harary spectral radius in four classes of simple connected graphs with \(n\) vertices: with given matching number, vertex connectivity, edge connectivity, and chromatic number, respectively.
We consider the relationship between the minimum degree \(\delta(G\) of a graph and the complexity of recognizing if a graph is \(T\)-tenacious. Let \(T \geq 1\) be a rational number. We first show that if \(\delta(G) \geq \frac{Tn}{T+1}\) then \(G\) is \(T\)-tenacious. On the other hand, for any fixed \(\epsilon > 0\), we show that it is NP-hard to determine if \(G\) is \(T\)-tenacious, even for the class of graphs with \(\delta(G) \geq (\frac{T}{T+1} – \epsilon)n\).
Let \(G\) be a finite group and let \(S\) be a nonempty subset of \(G\). For any positive integer \(k\), let \(S^k\) be the subset product given by the set \(\{s_1 \cdots s_k \mid s_1, \ldots, s_k \in S\}\). If there exists a positive integer \(n\) such that \(S^n = G\), then \(S\) is said to be exhaustive. Let \(e(S)\) denote the smallest positive integer \(n\), if it exists, such that \(S^n = G\). We call \(e(S)\) the exhaustion number of the set \(S\). If \(S^n \neq G\) for any positive integer \(n\), then \(S\) is said to be non-exhaustive. In this paper, we obtain some properties of exhaustive and non-exhaustive subsets of finite groups.
A graph \(G\) is called \(H\)-equipackable if every maximal \(H\)-packing in \(G\) is also a maximum \(H\)-packing in \(G\). In 2012, \(P_k\)-equipackable paths and cycles, \(M_k\)-equipackable paths and cycles are characterized. In this paper, \(P_m \cup P_k\)-equipackable paths and cycles are characterized.
If \(X\) is a geodesic metric space and \(x_1, x_2, x_3 \in X\), a geodesic triangle \(T = \{x_1, x_2, x_3\}\) is the union of the three geodesics \([x_1x_2]\), \([x_2x_3]\) and \([x_3x_1]\) in \(X\). The space \(X\) is \(\delta\)-hyperbolic (in the Gromov sense) if any side of \(T\) is contained in a \(\delta\)-neighborhood of the union of the two other sides, for every geodesic triangle \(T\) in \(X\). The study of hyperbolic graphs is an interesting topic since the hyperbolicity of a geodesic metric space is equivalent to the hyperbolicity of a graph related to it. Regular graphs are a very interesting class of graphs with many applications. The main aim of this paper is to obtain information about the hyperbolicity constant of regular graphs. We obtain several bounds for this parameter; in particular, we prove that \(\delta(G) \leq \frac{\Delta n}{8(\Delta-1)+1}\) for any \(4\)-regular graph \(G\) with \(n\) vertices. Furthermore, we show that for each \(\Delta \geq 2\) and every possible value \(t\) of the hyperbolicity constant, there exists a \(\Delta\)-regular graph \(G\) with \(\delta(G) = t\). We also study the regular graphs \(G\) with \(\delta(G) \leq 1\), i.e., the graphs which are like trees (in the Gromov sense). Besides, we prove some inequalities involving the hyperbolicity constant and domination numbers for regular graphs.
When \(G\) and \(F\) are graphs, \(v \in V(G)\) and \(\varphi\) is an orbit of \(V(F)\) under the action of the automorphism group of \(F\), \(s(F,G,v,\varphi)\) denotes the number of induced subgraphs of \(G\) isomorphic to \(F\) such that \(v\) lies in orbit \(\theta\) of \(F\). Vertices \(v \in V(G)\) and \(w \in V(H)\) are called \(k\)-vertex subgraph equivalent (\(k\)-SE), \(2 \leq k < n = |V(G)|\), if for each graph \(F\) with \(k\) vertices and for every orbit \(\varphi\) of \(F\), \(s(F,G,v,\varphi) = s(F,H,w,\varphi)\), and they are called similar if there is an isomorphism from \(G\) to \(H\) taking \(v\) to \(w\). We prove that \(k\)-SE vertices are \((k-1)\)-SE and several parameters of \((n-1)\)-SE vertices are equal. It is also proved that in many situations, “(n-1)-SE between vertices is equivalent to their similarity'' and it is true always if and only if Ulam's Graph Reconstruction Conjecture is true.
External Difference Families \((EDFs)\) are a new type of combinatorial designs originated from cryptography. In this paper, some constructions of \(EDFs\) are presented by using Gauss sums. Several classes of \(EDFs\) and related combinatorial designs are obtained.
The crossing number problem is in the forefront of topological graph theory. At present, there are only a few results concerning crossing numbers of join of some graphs. In this paper, for the special graph \(Q\) on six vertices, we give the crossing numbers of its join with \(n\) isolated vertices, as well as with the path \(P_n\) on \(n\) vertices and with the cycle \(C_n\).
In this article, we give a generalization of the multiparameter non-central Stirling numbers of the first and second kinds, Lah numbers, and harmonic numbers. Some new combinatorial identities, new explicit formulas, and many relations between different types of Stirling numbers and generalized harmonic numbers are found. Moreover, some interesting special cases of the generalized multiparameter non-central Stirling numbers are deduced. Furthermore, a matrix representation of the results obtained is given and a computer program is written using Maple and executed for calculating \(GMPNSN-1\) and their inverse \((GMPNSN-2)\), along with some of their interesting special cases.
Suppose \(G\) is a graph. Let \(u\) be a vertex of \(G\). A vertex \(v\) is called an \(i\)-neighbor of \(u\) if \(d_G(u,v) = i\). A \(1\)-neighbor of \(u\) is simply called a neighbor of \(u\). Let \(s\) and \(t\) be two nonnegative integers. Suppose \(f\) is an assignment of nonnegative integers to the vertices of \(G\). If the following three conditions are satisfied, then \(f\) is called an \((s, t)\)-relaxed \(L(2,1)\)-labeling of \(G\): (1) for any two adjacent vertices \(u\) and \(v\) of \(G\), \(f(u) \neq f(v)\); (2) for any vertex \(u\) of \(G\), there are at most \(s\) neighbors of \(u\) receiving labels from \(\{f(u) – 1, f(u)+ 1\}\); (3) for any vertex \(u\) of \(G\), the number of \(2\)-neighbors of \(u\) assigned the label \(f(u)\) is at most \(t\). The minimum span of \((s, t)\)-relaxed \(L(2,1)\)-labelings of \(G\) is called the \((s,t)\)-relaxed \(L(2,1)\)-labeling number of \(G\), denoted by \(\lambda_{2,1}^{s,t}(G)\). It is clear that \(\lambda_{2,1}^{0,0}(G)\) is the so-called \(L(2, 1)\)-labeling number of \(G\). In this paper, the \((s, t)\)-relaxed \(L(2, 1)\)-labeling number of the hexagonal lattice is determined for each pair of two nonnegative integers \(s\) and \(t\). And this provides a series of channel assignment schemes for the corresponding channel assignment problem on the hexagonal lattice.
As an additive weight version of the Harary index, the reciprocal degree distance of a simple connected graph \(G\) is defined as \(RDD(G) = \sum\limits_{u,v \subseteq V(G)} \frac{d_G(u)+d_G(v)}{d_G(u,v)}\), where \(d_G(u)\) is the degree of \(u\) and \(d_G(u,v)\) is the distance between \(u\) and \(v\) in \(G\). In this paper, we respectively characterize the extremal graphs with the maximum \(RDD\)-value among all the graphs of order \(n\) with given number of cut vertices and cut edges. In addition, an upper bound on the reciprocal degree distance in terms of the number of cut edges is provided.
In this work, linear codes over \(\mathbb{Z}_{2^s}\) are considered together with the extended Lee weight, which is defined as
\[w_L(a) = \begin{cases}
a & \text{if } a \leq 2^{s-1}, \\
2^s – x & \text{if } a > 2^{s-1}.
\end{cases}\]
The ideas used by Wilson and Yildiz are employed to obtain divisibility properties for sums involving binomial coefficients and the extended Lee weight. These results are then used to find bounds on the power of 2 that divides the number of codewords whose Lee weights fall in the same congruence class modulo \(2^e\). Comparisons are made with the results for the trivial code and the results for the homogeneous weight.
In this paper we study the Laplacian spectral radius of bicyclic graphs with given independence number and characterize the extremal graphs completely.
In this paper, we obtain some analytical expressions and give two simple formulae for the expected values of the Wiener indices of the random Phenylene and Spiro hexagonal chains.
Let \(G\) be a bicyclic graph. Bicyclic graphs are connected graphs in which the number of edges equals the number of vertices plus one. In this paper, we determine the graph with the maximal signless Laplacian spectral radius among all the bicyclic graphs with \(n\) vertices and diameter \(d\).
The harmonic index \(H(G)\) of a graph \(G\) is defined as the sum of the weights \(\frac{2}{d_u+ d_v}\) of all edges \(uv\) of \(G\), where \(d_u\) denotes the degree of a vertex \(u\) in \(G\). We determine the \(n\)-vertex trees with the second and third maximum harmonic indices for \(n \geq 7\), the fourth maximum harmonic index for \(n \geq 10\), and fifth maximum harmonic index for $n \geq 11\), and unicyclic graphs with the second and third maximum harmonic indices for \(n \geq 5\), the fourth maximum harmonic index for \(n \geq 7\), and fifth maximum harmonic index for \(n \geq 8\), and bicyclic graphs with the maximum harmonic index for \(n \geq 6\), the second and third maximum harmonic indices for \(n \geq 7\), and fourth maximum harmonic index for \(n \geq 9\).
Graph embedding has been known as a powerful tool for implementation of parallel algorithms and simulation of different interconnection networks. In this paper, we obtain the minimum wirelength of embedding circulant networks into necklace and windmill graphs. The algorithms for obtaining the same are of \(O(2n)\)-linear time.
In this paper, a reliable symbolic computational algorithm is presented for inverting a general companion matrix by using parallel computing along with recursion. The computational cost of the algorithm is \(O(n^2)\). The algorithm is implementable to the Computer Algebra System (CAS) such as MAPLE, MATLAB, and MATHEMATICA. Three examples are presented for the sake of illustration.
Let \(K_r\) be the complete graph on \(r\) vertices in which there exists an edge between every pair of vertices, \(K_{m,n}\) be the complete bipartite graph with \(m\) vertices in one partition and \(n\) vertices in the other partition, where each vertex in one partition is adjacent to each vertex in the other partition, and \(K(n, r)\) be the complete \(r\)-partite graph \(K_{n,n,…,n}\) where each partition has \(n\) vertices. In this paper, we determine the minimum number of monochromatic stars \(K_{1,p}\), \( \forall p \geq 2\), in any \(t\)-coloring (\(t \geq 2\)) of edges of \(K_r\), \(K_{m,n}\), and \(K(n, r)\). Also, we prove that these lower bounds are sharp for all values of \(m, n, p, r\), and \(t\) by giving explicit constructions.
In this paper, we prove that if the toughness of a \(k\)-tree \(G\) is at least \(\frac{k+1}{3}\), then \(G\) is panconnected for \(k \geq 3\), or \(G\) is vertex pancyclic for \(k = 2\). This result improves a result of Broersma, Xiong, and Yoshimoto.
Since the Wiener index has been successful in the study of benzenoid systems and boiling points of alkanes, it is natural to examine this number for the study of fullerenes, most of whose cycles are hexagons. This topological index is equal to the sum of distances between all pairs of vertices of the respective graph. It was introduced in \(1947\) by one of the pioneers of this area, Harold Wiener, who realized that there are correlations between the boiling points of paraffins and the structure of the molecules. The present paper is the first attempt to compute the Wiener index of an infinite class of fullerenes. Further, we obtain a correlation between the values of the Wiener index and the boiling point of such fullerenes for the first time.
A graph is said to be symmetric if its automorphism group is transitive on its arcs. A complete classification is given of pentavalent symmetric graphs of order \(40p\) for each prime \(p\). It is shown that a connected pentavalent symmetric graph of order \(40p\) exists if and only if \(p = 3\), and up to isomorphism, there are only two such graphs.
A broadcast on a graph \(G\) is a function \(f: V \to \{0, \dots, diam(G)\}\) such that for every vertex \(v \in V(G)\), \(f(v) \leq e(v)\), where \(diam(G)\) denotes the diameter of \(G\) and \(e(v)\) denotes the eccentricity of vertex \(v\). The upper broadcast domination number of a graph is the maximum value of \(\sum_{v \in V} f(v)\) among all minimal broadcasts \(f\) for which each vertex of the graph is within distance \(f(v)\) from some vertex \(v\) having \(f(v) \geq 1\). We give a new upper bound on the upper broadcast domination number which improves a previous result of Dunbar et al. in [Broadcasts in graphs, Discrete Applied Mathematics 154 (2006) 59-75]. We also prove that the upper broadcast domination number of any grid graph \(G_{m,n} = P_m \Box P_n\) equals \(m(n – 1)\).
For a vertex \(v\) of a graph \(G\), Zhu, Li, and Deng introduced the concept of implicit degree \(id(v)\), according to the degrees of the neighbors of \(v\) and the vertices at distance \(2\) with \(v\) in \(G\). For a subset \(S \subseteq V(G)\), let \(i\Delta_2(G, S)\) denote the maximum value of the implicit degree sum of two vertices of \(S\). In this paper, we will prove: Let \(G\) be a \(2\)-connected graph on \(n \geq 3\) vertices and \(d\) be a nonnegative integer. If \(i\Delta_2(G, S) \geq d\) for each independent set \(S\) of order \(\kappa(G) + 1\), then \(G\) has a cycle of length at least \(\min\{d, n\}\).
For a nonempty graph \(G = (V(G), E(G))\), a signed cycle dominating function on \(G\) is introduced by Xu in 2009 as a function \(f : E(G) \to \{1, -1\}\) such that \(\sum_{e \in E(C)} f(e) \geq 1\) for any induced cycle \(C\) of \(G\). A set \(\{f_1, f_2, \dots, f_d\}\) of distinct signed cycle dominating functions on \(G\) with the property that \(\sum_{i=1}^{d} f_i(e) \leq 1\) for each \(e \in E(G)\), is called a signed cycle dominating family (of functions) on \(G\). The maximum number of functions in a signed cycle dominating family on \(G\) is the signed cycle domatic number of \(G\), denoted by \(d’_{sc}(G)\). In this paper, we study the signed cycle domatic numbers in graphs and present sharp bounds for \(d’_{sc}(G)\). In addition, we determine the signed cycle domatic number of some special graphs.
Using partition theoretic methods we combinatorially interpret the four Ae Rogers—Ramanujan identities of Andrews, Schilling and Wamaar.
Let \(p > 165\) be a prime and let \(G\) be a cyclic group of order \(p\). Let \(S\) be a minimal zero-sum sequence with elements over \(G\), i.e., the sum of elements in \(S\) is zero, but no proper nontrivial subsequence of \(S\) has sum zero. We call \(S\) unsplittable, if there do not exist \(g \in S\) and \(x, y \in G\) such that \(g = x + y\) and \(Sg^{-1}x y\) is also a minimal zero-sum sequence. In this paper, we determine the structure of \(S\) which is an unsplittable minimal zero-sum sequence of length \(\frac{p-1}{2}\) or \(\frac{p-3}{2}\). Furthermore, if \(S\) is a minimal zero-sum sequence with \(|S| \geq \frac{p-3}{2}\), then \(ind(S) \leq 2\).
For two given graphs \(G_1\) and \(G_2\), the Ramsey number \(R(G_1, G_2)\) is the smallest integer \(x\) such that for any graph \(G\) of order \(n\), either \(G\) contains \(G_1\) or the complement of \(G\) contains \(G_2\). In this paper, we study a large class of trees \(T\) as studied by Cockayne in [3], including paths and trees which have a vertex of degree one adjacent to a vertex of degree two, as special cases. We evaluate some \(R(T’_m, B_m)\), where \(T’_n \in \mathbb{T}\) and \(B_m\) is a book of order \(m+2\). Besides, some bounds for \(R(T’_n, B_n)\) are obtained.
Graceful labeling of graphs is used in radar codes. In this work, we introduce a new version of gracefulness, which we call edge-even graceful labeling of graphs. We establish a necessary and sufficient condition for edge-even graceful labeling of path graphs \(P_n\), cycle graphs \(C_n\), and star graphs \(K_{1,n}\). We also prove some necessary and sufficient conditions for some path and cycle-related graphs, namely, Friendship, Wheel, Double wheel, and Fan graphs.
The Hamiltonian problem is a classical problem in graph theory. Most of the research on the Hamiltonian problem is looking for sufficient conditions for a graph to be Hamiltonian. For a vertex \(v\) of a graph \(G\), Zhu, Li, and Deng introduced the concept of implicit degree \(id(v)\), according to the degrees of its neighbors and the vertices at distance \(2\) with \(v\) in \(G\). In this paper, we will prove that: Let \(G\) be a \(2\)-connected graph on \(n \geq 3\) vertices. If the maximum value of the implicit degree sums of \(2\) vertices in \(S\) is more than or equal to \(n\) for each independent set \(S\) with \(\kappa(G) + 1\) vertices, then \(G\) is Hamiltonian.
Let \((d_1, d_2, \dots, d_n)\) be a sequence of positive integers with \(n-1 \geq d_1 \geq d_2 \geq \dots \geq d_n\). We give a characterization of \((d_1, d_2, \dots, d_n)\) that is the degree sequence of a graph with cyclomatic number \(k\). This simplifies the characterization of Erdős-Gallai.
We explore new combinatorial properties of overpartitions, which are natural generalizations of integer partitions. Building on recent work, we state general combinatorial identities between standard partition, overpartition, and regular partition functions. We provide both generating function and bijective proofs. We also prove congruences for certain overpartition functions combinatorially.
Let \(G\) be a simple graph on \(n\) vertices. The Laplacian Estrada index of \(G\) is defined as \(LEE(G) = \sum_{i=1}^{n} e^{\mu_i}\), where \(\mu_1, \mu_2, \dots, \mu_n\) are the Laplacian eigenvalues of \(G\). In this paper, threshold graphs on \(n\) vertices and \(m\) edges having maximal and minimal Laplacian Estrada index are determined, respectively.
In this paper, formulas of the resistance distance for the arbitrary two-vertex resistance of \(G\), \(H = G_1 \boxdot G_2\) and \(G_1 \boxminus G_2\) in the electrical networks are obtained in a much simpler way. Furthermore, \(K_f(G_1 \boxdot G_2)\) and \(K_f(G_1 \boxminus G_2)\) can be expressed as a combination of \(K_f(G_1)\) and \(K_f(G_2)\).
Networks are important structures and appear in many different applications and settings. The vulnerability value of a communication network shows the resistance of the network after the disruption of some centers or connection lines until a communication breakdown. Centrality parameters play an important role in the field of network analysis. Numerous studies have proposed and analyzed several centrality measures. These concepts measure the importance of a node’s position in a network. In this paper, vertex residual closeness \((VRC)\) and normalized vertex residual closeness \((NVRC)\) of some splitting networks modeled by splitting graphs are obtained.
Let \(T\) be an isosceles right triangle and let \(S_1, S_2, S_3, \dots\) be the homothetic copies of a square \(S\). In this paper, we consider the parallel covering and packing of \(T\) with the sequence \(\{S_n\}\) of squares.
A radio labeling of a simple connected graph \( G \) is a function \( f: V(G) \to \mathbb{Z}^+ \) such that for every two distinct vertices \( u \) and \( v \) of \( G \),
$$distance(u, v) + |f(u) – f(v)| \geq 1 + diameter(G).$$
The radio number of a graph \( G \) is the smallest integer \( M \) for which there exists a labeling \( f \) with \( f(v) \leq M \) for all \( v \in V(G) \). An edge-balanced caterpillar graph is a caterpillar graph that has an edge such that removing this edge results in two components with an equal number of vertices. In this paper, we determine the radio number of particular edge-balanced caterpillars as well as improve the lower bounds of the radio number of other edge-balanced caterpillars.
It is known that an ordered \(\rho\)-labeling of a bipartite graph \( G \) with \( n \) edges yields a cyclic \( G \)-decomposition of \( K_{2nx+1} \) for every positive integer \( x \). We extend the concept of an ordered \(\rho\)-labeling to bipartite digraphs and show that an ordered directed \(\rho\)-labeling of a bipartite digraph \( D \) with \( n \) arcs yields a cyclic \( D \)-decomposition of \( K_{nx+1}^* \) for every positive integer \( x \). We also find several classes of bipartite digraphs that admit an ordered directed \(\rho\)-labeling.
Compressed sensing (CS), which is a rising technique of signal processing, successfully manages the huge expenditure of increasing the sampling rate as well as the intricate issues to our work. Hence, more and more attention has been paid to CS during recent years. In this paper, we construct a family of error-correcting pooling designs based on singular linear space over finite fields, which can be efficiently applied to signal processing in terms of CS.
It is proven that for all positive integers \( k \), \( n \), and \( r \), every sufficiently large positive integer is the sum of \( r \) or more \( k \)th powers of distinct elements of \(\{n,n + 1,n + 2,\ldots\}\). The case \( n = 1 \) is the conjecture in the title of [1].
In 1770, Waring conjectured that for each positive integer \( k \) there exists a \( g(k) \) such that every positive integer is a sum of \( g(k) \) or fewer \( k \)th powers of positive integers. Hilbert proved this theorem in 1909, giving rise to Waring’s problem, which asks, for each \( k \), what is the smallest \( g(k) \) such that the statement holds. For further details, see [3].
As a natural question arising from this problem, Johnson and Laughlin [1] proposed what they called an anti-Waring conjecture, which is the following: If \( k \) and \( r \) are positive integers, then every sufficiently large positive integer is the sum of \( r \) or more distinct \( k \)th powers of positive integers. When this holds for a pair \( k, r \), let \( N(k,r) \) denote the smallest positive integer such that each integer \( n \) greater than or equal to \( N(k,r) \) is the sum of \( r \) or more \( k \)th powers of distinct positive integers. As noted in [1], it is easy to see that, for all \( r \), \( N(1,r) = 1 + 2 + \cdots + r = \frac{r(r+1)}{2} \). It is also shown in [1] that \( N(2,1) = N(2, 2) = N(2,3) = 129 \).
Johnson and Laughlin further posed the question of whether given any positive integers \( k \), \( n \), \( r \), there exists an integer \( N(k,n,r) \) such that every integer \( z \) greater than or equal to \( N(k,n,r) \) can be written as a sum of \( r \) or more distinct elements from the set \( \{m^k \mid m \in \mathbb{N}, m \geq n\} \). The aim of this paper is to prove both this statement and the anti-Waring conjecture to be true.
We consider an optimization problem motivated by the tradeoff between connectivity and resilience in key predistribution schemes (KPS) for sensor networks that are based on certain types of combinatorial designs. For a specific class of designs, we show that there is no real disadvantage in requiring the underlying design to be regular.
We define three new pebbling parameters of a connected graph \( G \), the \( r \)-, \( g \)-, and \( u \)-\emph{critical pebbling numbers}. Together with the pebbling number, the optimal pebbling number, the number of vertices \( n \) and the diameter \( d \) of the graph, this yields \( 7 \) graph parameters. We determine the relationships between these parameters. We investigate properties of the \( r \)-critical pebbling number, and distinguish between greedy graphs, thrifty graphs, and graphs for which the \( r \)-critical pebbling number is \( 2^d \).
A set \( S \subset V \) of vertices in a graph \( G = (V, E) \) is called open irredundant if for every vertex \( v \in S \) there exists a vertex \( w \in V \setminus S \) such that \( w \) is adjacent to \( v \) but to no other vertex in \( S \). The upper open irredundance number \( OIR(G) \) equals the maximum cardinality of an open irredundant set in \( G \). A real-valued function \( g: V \to [0,1] \) is called open irredundant if for every vertex \( v \in V \), \( g(v) > 0 \) implies there exists a vertex \( w \) adjacent to \( v \) such that \( g(N[w]) = 1 \). An open irredundant function \( g \) is maximal if there does not exist an open irredundant function \( h \) such that \( g \neq h \) and \( g(v) \leq h(v) \), for every \( v \in V \). The fractional upper open irredundance number equals \( OIR_f(G) = \sup\{|g|: g \text{ is an open irredundant function on } G\} \). In this paper we prove that for any graph \( G \), \( OIR(G) = OIR_f(G) \).
A graph \( G \) is \( H \)-saturated if \( G \) does not contain \( H \) as a subgraph, but the addition of any edge between two nonadjacent vertices in \( G \) results in a copy of \( H \) in \( G \). The saturation number \( \operatorname{sat}(n, H) \) is the smallest possible number of edges in an \( n \)-vertex \( H \)-saturated graph. The values of saturation numbers for small graphs and \( H \) are obtained computationally, and some general results for specific path unions are also obtained.
A path \( P \) in a graph \( G \) is said to be a degree monotone path if the sequence of degrees of the vertices of \( P \) in the order in which they appear on \( P \) is monotonically non-decreasing. The length of the longest degree monotone path in \( G \) is denoted by \( \operatorname{mp}(G) \). This parameter was first studied in an earlier paper by the authors where bounds in terms of other parameters of \( G \) were obtained.
In this paper we concentrate on the study of how \( \operatorname{mp}(G) \) changes under various operations on \( G \). We first consider how \( \operatorname{mp}(G) \) changes when an edge is deleted, added, contracted or subdivided. We similarly consider the effects of adding or deleting a vertex. We sometimes restrict our attention to particular classes of graphs.
Finally we study \( \operatorname{mp}(G \times H) \) in terms of \( \operatorname{mp}(G) \) and \( \operatorname{mp}(H) \) where \( \times \) is either the Cartesian product or the join of two graphs.
In all these cases we give bounds on the parameter \( \operatorname{mp} \) of the modified graph in terms of the original graph or graphs and we show that all the bounds are sharp.
Given a graph \( G \), a \( k \)-ranking is a labeling of the vertices such that any path connecting two vertices with the same label contains a vertex with a larger label. A \( k \)-ranking is minimal if and only if reducing any label violates the ranking property. The arank number of a graph \( \psi_r(G) \), is the maximum \( k \) such that \( G \) has a minimal \( k \)-ranking. The arank number of a cycle was first investigated by Kostyuk and Narayan. They determined precise arank numbers for most cycles, and determined the arank number within \( 1 \) for all other cases. In this paper we introduce a new concept called the flanking number, which is used to solve all open cases. We prove that \( \psi_r(C_n) = \lfloor\log_2(n + 1)\rfloor + \lfloor\log_2 \left(\frac{n+2}{3}\right)\rfloor + 1 \) for all \( n > 6 \), which completely solves the problem that has been open since \( 2003 \).
A DI-pathological graph is a graph in which every minimum dominating set intersects every maximal independent set. DI-pathological graphs are related to the Inverse Domination Conjecture; hence, it is useful to characterize properties of them. One characterization question is how large or small a graph can be relative to the domination number. Two useful characterizations of size seem relevant, namely the number of vertices and the number of edges. In this paper, we provide two results related to this question. In terms of the number of vertices, we show that every connected, DI-pathological graph has at least \(2\gamma(G) + 4\) vertices except \(K_{3,3}\), \(K_{3,4}\), and six graphs on nine vertices and show that our lower bound is best possible. We then show that with one exception, every connected, DI-pathological graph with no isolated vertices has at least \(2\gamma(G) + 5\) edges and show that our lower bound is best possible.
A dominating set in a graph \( G \) is a subset \( S \) of vertices such that any vertex not in \( S \) is adjacent to some vertex of \( S \). The domination number, \( \gamma(G) \), of \( G \) is the minimum cardinality of a dominating set. A dominating set of cardinality \( \gamma(G) \) is called a \( \gamma(G) \)-set. A fair dominating set in a graph \( G \) (or FD-set) is a dominating set \( S \) such that all vertices not in \( S \) are dominated by the same number of vertices from \( S \); that is, every two vertices not in \( S \) have the same number of neighbors in \( S \). The fair domination number, \( fd(G) \), of \( G \) is the minimum cardinality of an FD-set. A fair dominating set of \( G \) of cardinality \( fd(G) \) is called an \( fd(G) \)-set. We say that \( fd(G) \) and \( \gamma(G) \) are \emph{strongly equal} and denote by \( fd(G) \equiv \gamma(G) \), if every \( \gamma(G) \)-set is an \( fd(G) \)-set. In this paper, we provide a constructive characterization of trees \( T \) with \( fd(T) \equiv \gamma(T) \).
We consider edge-colorings of complete graphs in which each color induces a subgraph that does not contain an induced copy of \( K_{1,t} \), for some \( t \geq 3 \). It turns out that such colorings, if the underlying graph is sufficiently large, contain spanning monochromatic \( k \)-connected subgraphs. Furthermore, there exists a color, say blue, such that every vertex has very few incident edges in colors other than blue.
Multi-sender authentication codes allow a group of senders to construct an authenticated message for a receiver such that the receiver can verify the authenticity of the received message. In this paper, we construct one multi-sender authentication code from polynomials over finite fields. Some parameters and the probabilities of deceptions of this code are also compute
If \(T\) is a tree on \(n\) vertices, \(n \geq 3\), and if \(G\) is a connected graph such that \(d(u) + d(v) + d(u,v) \geq 2n\) for every pair of distinct vertices of \(G\), it has been conjectured that \(G\) must have a non-separating copy of \(T\). In this note, we prove this result for the special case in which \(d(u) + d(v) + d(u,v) \geq 2n + 2\) for every pair of distinct vertices of \(G\), and improve this slightly for trees of diameter at least four and for some trees of diameter three.
Let \(G\) be a graph and \(H\) a subgraph of \(G\). A \(D(G, H, \lambda)\) design is a collection \(\mathcal{D}\) of subgraphs of \(G\) each isomorphic to \(H\) so that every \(2\)-path (path of length \(2\)) in \(G\) lies in exactly \(\lambda\) subgraphs in \(\mathcal{D}\). The problem of constructing \(D(K_n,C_n,1)\) designs is the so-called Dudeney’s round table problem. We denote by \(C_k\), a cycle on \(k\) vertices and by \(P_k\), a path on \(k\) vertices.
In this paper, we construct \(D(K_{n,n},C_{2n},1)\) designs and \(D(K_n,P_n,1)\) designs when \(n \equiv 0,1,3 \pmod{4}\); and \(D(K_{n,n},C_{2n},2)\) designs and \(D(K_n,P_n,2)\) designs when \(n \equiv 2 \pmod{4}\). The existence problems of \(D(K_{n,n},C_{2n},1)\) designs and \(D(K_n,P_n,1)\) designs for \(n \equiv 2 \pmod{4}\) remain open.
The spread of a graph \(G\) is defined as the difference between the largest and smallest eigenvalues of \(G\). Using the lower bounds obtained by Liu and Liu in [4] on the spread of a graph, we in this note present spread conditions for some Hamiltonian properties of a graph.
A \({vertex \;cover}\) of a graph \(G = (V, E)\) is a subset \(S \subseteq V\) such that every edge is incident with at least one vertex in \(S\), and \(\alpha(G)\) is the cardinality of a smallest vertex cover. For a given vertex cover \(S\), a defense by \(S\) to an attack on an edge \(e = \{v, w\}\), where \(v \in S\), is a one-to-one function \(f : S \to V\), such that:
Informally, a set is an \({eternal\; vertex \;cover}\) if it can defend an “attack” on any edge and the process can be repeated indefinitely. The cardinality of a smallest eternal vertex cover is denoted \(\alpha_{m}^\infty(G)\). A set of vertices which is not an eternal vertex cover is \({mortal}\). A formal definition of eternal vertex cover is provided and demonstrated to be equivalent to a characterization using closed families of vertex covers.
Eternal vertex covers are shown to be closed under taking supersets and a lower bound for \(\alpha_{m}^\infty(G)\) is given which depends on the vertex connectivity number and the independent domination number. A corresponding upper bound is given for the size of a mortal set. The \({death \;spiral\; number}\) of a mortal vertex cover is defined and used to partition the collection of all mortal sets. Mortal sets are shown to be closed under taking subsets implying the collection of mortal sets for a graph with at least one edge is an independence system. The death spiral number of a graph is the maximum of the death spiral numbers of all mortal sets.
An optimal attack/defense strategy is determined for a set of size \(\alpha_{m}^\infty(T) – 1\) in a tree \(T\), along with a polynomial labeling algorithm which computes its death spiral number.
A graph \(G\) is called super edge-magic if there exists a bijective function \(f: V(G) \cup E(G) \rightarrow \{1, 2, \dots, |V(G)| + |E(G)|\}\) such that \(f(V(G)) = \{1, 2, \dots, |V(G)|\}\) and \(f(u) + f(v) + f(uv)\) is a constant for each \(uv \in E(G)\). The super edge-magic deficiency, \(\mu_s(G)\), of a graph \(G\) is defined as the smallest nonnegative integer \(n\) with the property that the graph \(G \cup nK_1\) is super edge-magic, or \(+\infty\) if there exists no such integer \(n\). In this paper, the super edge-magic deficiency of certain 2-regular graphs with two components is computed, which leads us to a conjecture on the super edge-magic deficiency of graphs in this class.
From a computer search, new minimum sizes for the maximal partial spreads in \(PG(3,q)\) have been obtained for \(q = 8, 9, 16\) and for every \(q\) such that \(25 \leq q \leq 101\). Furthermore, density results in the cases \(q = 8, 9, 16, 19, 23, 25, 27\) have been obtained. Finally, the already known exceptional size \(45\) for \(q = 7\) has been found again.
We decompose the complete multigraph \(K(v, \lambda)\) into copies of a graph \(H_i\) (\(i = 1, 2, 3\)). Each \(H_i\) is a near-triangle in that it is connected and has \(3\) vertices. In several cases, the decompositions are completed using classical combinatorial sequences due to Langford and Skolem.
It may be desired to seat \(n\) people along a row (as at a lunch counter), or \(n+1\) people around a circular table, in \(n\) consecutive rounds of seating, so that each person \(x\) has every other person \(y\) on their right exactly once, and on their left exactly once, in one of the seatings. Alternatively, it may be desired to seat \(2n\) people along a row, or \(2n + 1\) people around a circular table, in only \(n\) consecutive rounds, so that each person \(x\) is adjacent to every other person \(y\) (either on the right or the left) exactly once. We show that these problems are solved using the rows of Tuscan squares to specify the successive rounds of seatings.
Let \(\mathbb{F}_q^(n+1)\) denote the \((n+l)\)-dimensional projective space over a finite field \(\mathbb{F}_q\). For a fixed integer \(m \leq \min\{n,l\}\), denote by \(\mathcal{L}_o^m(\mathbb{F}_q^{n+1})\) the set of all subspaces of type \((t,t_1)\), where \(t_1 \leq t \leq m\). Partially ordered by ordinary inclusion, one family of quasi-regular semilattices is obtained. Moreover, we compute all its parameters.
If \(X\) is a geodesic metric space and \(x_1, x_2, x_3 \in X\), a geodesic triangle \(T = \{x_1, x_2, x_3\}\) is the union of the three geodesics \([x_1x_2]\), \([x_2x_3]\) and \([x_3x_1]\) in \(X\). The space \(X\) is \(\delta\)-hyperbolic (in the Gromov sense) if any side of \(T\) is contained in a \(5\delta\)-neighborhood of the union of the two other sides, for every geodesic triangle \(T\) in \(X\). We denote by \(\delta(X)\) the sharp hyperbolicity constant of \(X\), i.e., \(\delta(X) := \inf\{\delta \geq 0: X \text{ is } \delta\text{-hyperbolic}\}\). The main result of this paper is the inequality \(\delta(G) \leq \delta(\mathcal{L}(G))\) for the line graph \(\mathcal{L}(G)\) of every graph \(G\). We prove also the upper bound \(\delta(L(G)) \leq 5\delta(G) + 3l_{\max}\), where \(\max\) is the supremum of the lengths of the edges of \(G\). Furthermore, if every edge of \(G\) has length \(k\), we obtain \(\delta(G) \leq \delta(\mathcal{L}(G)) \leq 5\delta(G) + 5k/2\).
For graphs \(G\) and \(H\), the size-balanced Ramsey multipartite number \(m_j(G, H)\) is defined as the smallest positive integer \(s\) such that any arbitrary red/blue coloring of the graph \(K_{s,s}\) forces the appearance of a red \(G\) or a blue \(H\). In the main case of this paper, we generalize methods used in finding bipartite Ramsey numbers for \(b(nK_2, mK_2)\) to finding the balanced Ramsey multipartite number \(m_j(nK_2, mK_2)\).
The subdivision graph \(S(G)\) of a graph \(G\) is the graph obtained by inserting a new vertex into every edge of \(G\). Let \(G_1\) and \(G_2\) be two vertex-disjoint graphs. The subdivision-vertex corona of \(G_1\) and \(G_2\), denoted by \(G_1 \odot G_2\), is the graph obtained from \(S(G_1)\) and \(|V(G_1)|\) copies of \(G_2\), all vertex-disjoint, by joining the \(i\)th vertex of \(V(G_1)\) to every vertex in the \(i\)th copy of \(G_2\). The subdivision-edge corona of \(G_1\) and \(G_2\), denoted by \(G_1 \ominus G_2\), is the graph obtained from \(S(G_1)\) and \(|I(G_1)|\) copies of \(G_2\), all vertex-disjoint, by joining the \(i\)th vertex of \(I(G_1)\) to every vertex in the \(i\)th copy of \(G_2\), where \(I(G_1)\) is the set of inserted vertices of \(S(G_1)\). In this paper, we determine the generalized characteristic polynomial of \(G_1 \odot G_2\) (respectively, \(G_1 \ominus G_2\)). As applications, the results on the spectra of \( G_1 \odot G_2\) (respectively, \(G_1 \ominus G_2\)) enable us to construct infinitely many pairs of \(\Phi\)-cospectral graphs. The adjacency spectra of \(G_1 \odot G_2\) (respectively, \(G_1 \ominus G_2\)) help us to construct many infinite families of integral graphs. By using the Laplacian spectra, we also obtain the number of spanning trees and Kirchhoff index of \(G_1 \odot G_2\) and \(G_1 \ominus G_2\), respectively.
In this paper, we study arc-transitive pentavalent graphs of order \(4p^n\), where \(p\) is a prime and \(n\) is a positive integer. It is proved that no such graph exists for each prime \(p \geq 5\), and all such graphs with \(p = 2\) or \(3\) which are \(G\)-basic (that is, \(G\) has no non-trivial normal subgroup such that the graph is a normal cover of the corresponding normal quotient graph) are determined. Moreover, as an application, arc-transitive pentavalent graphs of order \(4p^2\) and \(4p^3\) are determined.
In \(1982\), Beutelspacher and Brestovansky determined the \(2\)-color Rado number of the equation \[x_1+ x_2 x + \ldots +x_{m-1} =x_{ m} \] for all \(m \geq 3\). Here we extend their result by determining the 2-color Rado number of the equation \[x_1 +x_2 + \dots + x_n = y_1 +y_2+ \ldots + y_k\] for all \(n \geq 2\) and \(k \geq 2\). As a consequence, we determine the 2-color Rado number of \[x_1+ x_2 + \ldots + x_n = a_1 y_1 + \dots + a_\ell y_\ell\] in all cases where \(n \geq 2\) and \(n \geq a_1 + \dots + a_\ell\), and in most cases where \(n \geq 2\) and \(2n \geq a_1 + \dots + a_\ell\).
The subdivision graph \(S(G)\) of a graph \(G\) is the graph obtained by inserting a new vertex into every edge of \(G\). The set of inserted vertices of \(S(G)\) is denoted by \(I(G)\). Let \(G_1\) and \(G_2\) be two vertex-disjoint graphs. The subdivision-edge-vertex join of \(G_1\) and \(G_2\), denoted by \(G_1 \odot G_2\), is the graph obtained from \(S(G_1)\) and \(S(G_2)\) by joining every vertex in \(I(G_1)\) to every vertex in \(V(G_2)\). The subdivision-edge-edge join of \(G_1\) and \(G_2\), denoted by \(G_1 \ominus G_2\), is the graph obtained from \(S(G_1)\) and \(S(G_2)\) by joining every vertex in \(I(G_1)\) to every vertex in \(I(G_2)\). The subdivision-vertex-edge join of \(G_1\) and \(G_2\), denoted by \(G_1 \odot G_2\), is the graph obtained from \(S(G_1)\) and \(S(G_2)\) by joining every vertex in \(V(G_1)\) to every vertex in \(I(G_2)\). In this paper, we obtain the formulas for resistance distance of \(G_1 \odot G_2\), \(G_1 \ominus G_2\), and \(G_1 \odot G_2\).
A hypergraph is intersecting if any two different edges have exactly one common vertex, and an \(n\)-quasicluster is an intersecting hypergraph with \(n\) edges, each one containing at most \(n\) vertices, and every vertex is contained in at least two edges. The Erdős-Faber-Lovász Conjecture states that the chromatic number of any \(n\)-quasicluster is at most \(n\). In the present note, we prove the correctness of the conjecture for a new infinite class of \(n\)-quasiclusters using a specific edge coloring of the complete graph.
Let \(G\) be a graph of order \(n\) and let \(\Phi(G, \lambda) = \det(\lambda I_n – L(G)) = \sum_{k=0}^{n}(-1)^k c_k(G) \lambda^{n-k}\) be the characteristic polynomial of the Laplacian matrix of a graph \(G\). In this paper, we identify the minimal Laplacian coefficients of unicyclic graphs with \(n\) vertices and diameter \(d\). Finally, we characterize the graphs with the smallest and the second smallest Laplacian-like energy among the unicyclic graphs with \(n\) vertices and fixed diameter \(d\).
The balanced hypercube, which is a variant of the hypercube, is proposed as a novel inter-processor network. Among the attractive properties of the balanced hypercube, the most special one is that each processor has a backup processor sharing the same neighborhood. A connected graph \(G\) with at least \(2m + 2\) vertices is said to be \(m\)-extendable if it possesses a matching of size \(m\) and every such matching can be extended to a perfect matching of \(G\). In this paper, we prove that the balanced hypercube \(BH_n\) is \(m\)-extendable for every \(m\) with \(1 \leq m \leq 2n – 2\), and our result is optimal.
A set \(W \subseteq V(G)\) is called a resolving set, if for each two distinct vertices \(u, v \in V(G)\) there exists \(w \in W\) such that \(d(u, w) \neq d(v, w)\), where \(d(x, y)\) is the distance between the vertices \(x\) and \(y\). A resolving set for \(G\) with minimum cardinality is called a metric basis. A graph with a unique metric basis is called a unique basis graph. In this paper, we study some properties of unique basis graphs.
In this paper, we study the number of 1-factors and edge-colorings of the Möbius ladder graphs. We find exact formulae for such numbers and show that there are exponentially many 1-factors and edge-colorings in such graphs. As applications, we show that every “man-made” triangular embedding for \(K_{12m+7}\), by combining the current graphs with those of Youngs and Ringel, permits exponentially many “Grünbaum colorings” (i.e., 3-edge-colored triangulations in such a way that each triangle receives three distinct colors).
Multi-receiver authentication codes with dynamic sender (\(DMRA\)-codes) are extensions of traditional group communication systems in which any member of a group can broadcast an authenticated message such that all other group members can individually verify its authenticity, and some malicious participants of the group cannot successfully impersonate the potential sender, or substitute a transmitted message. In this paper, a construction of \(DMRA\)-code will be given using linear code and its unconditional security is also guaranteed.
We consider the problem of finding quasiperiodicities in Fibonacci strings. A factor \(u\) of a string \(y\) is a cover of \(y\) if every letter of \(y\) falls within some occurrence of \(u\) in \(y\). A string \(v\) is a seed of \(y\) if it is a cover of a superstring of \(y\). A left seed of a string \(y\) is a prefix of \(y\) that is a cover of a superstring of \(y\). Similarly, a right seed of a string \(y\) is a suffix of \(y\) that is a cover of a superstring of \(y\). In this paper, we present some interesting results regarding quasiperiodicities in Fibonacci strings; we identify all covers, left/right seeds, and seeds of a Fibonacci string and all covers of a circular Fibonacci string.
We investigate a modifications of the well-known irregularity strength of graphs, namely the total edge irregularity strength and the total vertex irregularity strength. In this paper, we determine the exact value of the total edge (vertex) irregularity strength for convex polytope graphs with pendent edges.
Two \(G\)-designs \((X, \mathcal{A}_1)\) and \((X, \mathcal{A}_2)\) are said to intersect in \(m\) blocks if \(|\mathcal{A}_1 \cap \mathcal{A}_2| = m\). In this paper, we complete the solution of the intersection problem for \(G\)-designs, where \(G\) is a connected graph of size five which contains a cycle.
In this paper we discuss how the addition of a new edge affects the total edge irregularity strength of a graph.
Let \(G\) be a connected graph and \(k > 1\) be an integer. The local \(k\)-restricted edge connectivity \(\lambda_k(X,Y)\) of \(X,Y\) in \(G\) is the maximum number of edge-disjoint \(X\)-\(Y\) paths for \(X,Y \subseteq V\) with \(|X| = |Y| = k\), \(X \cap Y = \emptyset\), \(G[X]\) and \(G[Y]\) are connected. The \(k\)-restricted edge connectivity of \(G\) is defined as \(\lambda_k(G) = \min\{\lambda_k(X,Y) : X,Y \subseteq V, |X| = |Y| = k, X \cap Y = \emptyset, G[X] \text{ and } G[Y]\) are connected. Then \(G\) is local optimal \(k\)-restricted edge connected if \(\lambda_k(X,Y) = \min\{w(X), w(Y)\}\) for all \(X,Y \subseteq V\) with \(|X| = |Y| = k\), \(G[X]\) and \(G[Y]\) are connected, where \(w(X) = |E(X, \overline{X})|\). If \(\lambda_k(G) = \xi_k(G)\), where \(\xi_k(G) = \min\{w(X) : U \subset V, |U| = k \text{ and } G[U] \text{ is connected}\}\), then \(G\) is called \(\lambda_k\)-optimal. In this paper, we obtain several sufficient conditions for a graph to be \(3\)-optimal (or local optimal \(k\)-restricted edge connected).
A graph \(G\) is called edge-magic if there exists a bijective function \(f: V(G) \cup E(G) \to \{1, 2, \ldots, |V(G)| + |E(G)|\}\) such that \(f(u) + f(v) + f(uv)\) is a constant for each \(uv \in E(G)\). Also, \(G\) is called super edge-magic if \(f(V(G)) = \{1, 2, \ldots, |V(G)|\}\). Moreover, the super edge-magic deficiency, \(\mu_s(G)\), of a graph \(G\) is defined to be the smallest nonnegative integer \(n\) with the property that the graph \(G \cup nK_1\) is super edge-magic, or \(+\infty\) if there exists no such integer \(n\). In this paper, we introduce the notion of the sequential number, \(\sigma(G)\), of a graph \(G\) without isolated vertices to be either the smallest positive integer \(n\) for which it is possible to label the vertices of \(G\) with distinct elements from the set \(\{0, 1, \ldots, n\}\) in such a way that each \(uv \in E(G)\) is labeled \(f(u) + f(v)\) and the resulting edge labels are \(|E(G)|\) consecutive integers, or \(+\infty\) if there exists no such integer \(n\). We prove that \(\sigma(G) = \mu_s(G) + |V(G)| – 1\) for any graph \(G\) without isolated vertices, and \(\sigma(K_{m,n}) = mn\) for every two positive integers \(m\) and \(n\), which allows us to settle the conjecture that \(\mu_s(K_{m,n}) = (m-1)(n-1)\) for every two positive integers \(m\) and \(n\).
Let \(G = (V, E)\) be a graph. An edge labeling \(f: E \to \mathbb{Z}_2\) induces a vertex labeling \(f^*: V \to \mathbb{Z}_2\) defined by \(f^*(v) = \sum_{uv \in E} f(uv) \pmod{2}\). For each \(i \in \mathbb{Z}_2\), define \(E_i(f) = |f^{-1}(i)|\) and \(V_i(f) = |(f^*)^{-1}(i)|\). We call \(f\) edge-friendly if \(|E_1(f) – E_0(f)| \leq 1\). The edge-friendly index \(I_f(G)\) is defined as \(V_1(f) – V_0(f)\), and the full edge-friendly index set \(FEFI(G)\) is defined as \(\{I_f(G): f \text{ is an edge-friendly labeling}\}\). Further, the edge-friendly index set \(EFI(G)\) is defined as \(\{|I_f(G)|: f \text{ is an edge-friendly labeling}\}\). In this paper, we study the full edge-friendly index set of the star \(K_{1,n}\), \(2\)-regular graph, wheel \(W_n\), and \(m\) copies of path \(mP_n\), \(m \geq 1\).
An acyclic total coloring is a proper total coloring of a graph \(G\) such that there are at least \(4\) colors on vertices and edges incident with a cycle of \(G\). The acyclic total chromatic number of \(G\), \(\chi”_a(G)\), is the least number of colors in an acyclic total coloring of \(G\). In this paper, we prove that for every plane graph \(G\) with maximum degree \(\Delta\) and girth \(g(G)\), \(\chi_a(G) = \Delta+1\) if (1) \(\Delta \geq 9\) and \(g(G) \geq 4\); (2) \(\Delta \geq 6\) and \(g(G) \geq 5\); (3) \(\Delta \geq 4\) and \(g(G) \geq 6\); (4) \(\Delta \geq 3\) and \(g(G) \geq 14\).
Codes in \(l_{p\gamma}\)-spaces, introduced by the author in [3], are a natural generalization of one-dimensional codes in \(RT\)-spaces [6] to block coding and have applications in different areas of combinatorial/discrete mathematics, e.g., in the theory of uniform distribution, experimental designs, cryptography, etc. In this paper, we introduce various types of weight enumerators in \(l_{p\gamma}\)-codes, viz., exact weight enumerator, complete weight enumerator, block weight enumerator, and \(\gamma\)-weight enumerator. We obtain the MacWilliams duality relation for the exact and complete weight enumerators of an \(l_{p\gamma}\)-code.
We introduce a theorem on bipartite graphs, and some theorems on chains of two and three complete graphs, considering when they are combination or non-combination graphs, present some families of combination graphs. We give a survey for trees of order \(\leq 10\), which are all combination graphs.
A set of vertices in a graph \(G\) without isolated vertices is a total dominating set (TDS) of \(G\) if every vertex of \(G\) is adjacent to some vertex in \(S\). The minimum cardinality of a TDS of \(G\) is the total domination number \(\gamma_t(G)\) of \(G\). In this paper, the total domination number of generalized \(n\)-graphs and \(m \times n\) ladder graphs is determined.
We identify a graph without proper cycles, which is comatching with a cycle,The result is then extended to certain general families of graphs with cyclomatic number \(1\), formed by attaching trees to cycles.
A vertex \(v \in V(G)\) is said to be a self vertex switching of \(G\) if \(G\) is isomorphic to \(G^v\), where \(G^v\) is the graph obtained from \(G\) by deleting all edges of \(G\) incident to \(v\) and adding all edges incident to \(v\) which are not in \(G\). In [6], the author characterized connected unicyclic graphs each with a self vertex switching. In this paper, we characterize disconnected unicyclic graphs each with a self vertex switching.
An \(f\)-coloring of a graph \(G\) is an edge-coloring of \(G\) such that each color appears at each vertex \(v \in V(G)\) at most \(f(v)\) times. A multi-wheel graph is a graph obtained from \(s\) cycles \(C_{n_1}, C_{n_2}, \ldots, C_{n_s}\) (\(s \geq 1\)) by adding a new vertex, say \(w\), and edges joining \(w\) to all the vertices of the \(s\) cycles. In this article, we solve a conjecture posed by Yu et al. in 2006 and prove that it is not always true. Furthermore, the classification problem of multi-wheel graphs on \(f\)-colorings is solved completely.
For a connected graph \(G = (V, E)\) of order at least two, a chord of a path \(P\) is an edge joining two non-adjacent vertices of \(P\). A path \(P\) is called a monophonic path if it is a chordless path. A longest \(x\)-\(y\) monophonic path is called an \(x\)-\(y\) detour monophonic path. A set \(S\) of vertices of \(G\) is a detour monophonic set of \(G\) if each vertex \(v\) of \(G\) lies on an \(x\)-\(y\) detour monophonic path for some \(x\) and \(y\) in \(S\). The minimum cardinality of a detour monophonic set of \(G\) is the detour monophonic number of \(G\) and is denoted by \(dm(G)\). For any two vertices \(u\) and \(v\) in \(G\), the monophonic distance \(dm(u,v)\) from \(u\) to \(v\) is defined as the length of a \(u\)-\(v\) detour monophonic path in \(G\). The monophonic eccentricity \(em(v)\) of a vertex \(v\) in \(G\) is the maximum monophonic distance from \(v\) to a vertex of \(G\). The monophonic radius \(rad_{m}(G)\) of \(G\) is the minimum monophonic eccentricity among the vertices of \(G\), while the monophonic diameter \(diam_{m}(G)\) of \(G\) is the maximum monophonic eccentricity among the vertices of \(G\). It is shown that for positive integers \(r\), \(d\), and \(n \geq 4\) with \(r < d\), there exists a connected graph \(G\) with \(rad_{m}(G) = r\), \(diam_{m}(G) = d\), and \(dm(G) = n\). Also, if \(p\), \(d\), and \(n\) are integers with \(2 \leq n \leq p-d+4\) and \(d \geq 3\), there is a connected graph \(G\) of order \(p\), monophonic diameter \(d\), and detour monophonic number \(n\). Further, we study how the detour monophonic number of a graph is affected by adding some pendant edges to the graph.
In this paper we introduce a new kind of two-parameters generalization of Pell numbers. We give two distinct graph interpretations and prove some identities for these numbers. Moreover we define matrix generators and derive the generalized Cassini formula for the introduced numbers.
A graph is said to be a neighbourly irregular graph (or simply an NI graph) if no two adjacent vertices have the same degree. In this paper, we introduce the neighbourly regular strength of a graph. Let \(G\) be a simple graph of order \(n\). Let \(NI(G)\) denote the set of all NI graphs in which \(G\) is an induced subgraph. The neighbourly regular strength of \(G\) is denoted by \(NRS(G)\) and is defined as the minimum \(k\) for which there is an NI graph \(NI(G)\) of order \(n+k\) in \(NI(G)\). We prove that the \(NRS(G)\) is at most \(n-1\), with possible equality only if \(G\) is complete. In addition, we determine the \(NRS\) for some well-known graphs.
We first introduce the concept of \((k, k’, k”)\)-domination numbers in graphs, which is a generalization of many domination parameters. Then we find lower and upper bounds for this parameter, which improve many well-known results in the literature.
We are interested in ordering the elements of a subset \( A \) of the non-zero integers modulo \( n \) in such a way that all the partial sums are distinct. We conjecture that this can always be done, and we prove various partial results about this problem.
A graph \( G \) with maximum degree \( \Delta \) and edge chromatic number \( \chi'(G) > \Delta \) is \emph{edge-\(\Delta\)-critical} if \( \chi'(G-e) = \Delta \) for each \( e \in E(G) \). In this article, we provide a new proof of adjacency Lemmas on edge-critical graphs such that Vizing’s adjacency lemma becomes a corollary of our results.
This paper surveys recent results for flag enumeration of polytopes, Bruhat graphs, balanced digraphs, Whitney stratified spaces and quasi-graded posets.
A bipancyclic graph on \( v \) vertices is a bipartite graph that contains, as subgraphs, cycles of length \( n \) for every even integer \( n \) such that \( 4 \leq n \leq v \). Such a graph is uniquely bipancyclic if it contains exactly one subgraph of each permissible length.
In this paper, we find all uniquely bipancyclic graphs on 30 or fewer vertices.
A balanced complete bipartite graph is a complete bipartite graph where the degrees of its vertices differ by at most 1. In a red-blue-green coloring of the edges of a graph \( G \), every edge of \( G \) is colored red, blue, or green. For three graphs \( F_1 \), \( F_2 \), and \( F_3 \), the 2-Ramsey number \( R_2(F_1, F_2, F_3) \) of \( F_1 \), \( F_2 \), and \( F_3 \), if it exists, is the smallest order of a balanced complete bipartite graph \( G \) such that every red-blue-green coloring of the edges of \( G \) contains a red \( F_1 \), a blue \( F_2 \), or a green \( F_3 \). In this note, we determine that
\[
20 \leq R_2(C_4, C_4, C_4) \leq 21.
\]
A Hamiltonian graph \( G \) is said to be \(\ell\)-path-Hamiltonian, where \(\ell\) is a positive integer less than or equal to the order of \( G \), if every path of order \(\ell\) in \( G \) is a subpath of some Hamiltonian cycle in \( G \). The Hamiltonian cycle extension number of \( G \) is the maximum positive integer \(\ell\) for which every path of order \(\ell\) or less is a subpath of some Hamiltonian cycle in \( G \). If the order of \( G \) equals \( n \), then it is known that \( \text{hce}(G) = n \) if and only if \( G \) is a cycle or a regular complete bipartite graph (when \( n \) is even) or a complete graph. We present a complete characterization of Hamiltonian graphs of order \( n \) that are \(\ell\)-path-Hamiltonian for each \(\ell \in \{n-3, n-2, n-1, n\}\).
Let \( G \) be an edge-colored connected graph. A path \( P \) is a proper path in \( G \) if no two adjacent edges of \( P \) are colored the same. An edge coloring is a proper-path coloring of \( G \) if every pair \( u, v \) of distinct vertices of \( G \) is connected by a proper \( u-v \) path in \( G \). The minimum number of colors required for a proper-path coloring of \( G \) is the proper connection number \( \text{pc}(G) \) of \( G \). We study proper-path colorings in those graphs obtained by some well-known graph operations, namely line graphs, powers of graphs, coronas of graphs, and vertex or edge deletions. Proper connection numbers are determined for all iterated line graphs and powers of a given connected graph. For a connected graph \( G \), sharp lower and upper bounds are established for the proper connection number of (i) the \( k \)-iterated corona of \( G \) in terms of \( \text{pc}(G) \) and \( k \), and (ii) the vertex or edge deletion graphs \( G-v \) and \( G-e \), where \( v \) is a non-cut-vertex of \( G \) and \( e \) is a non-bridge of \( G \), in terms of \( \text{pc}(G) \) and the degree of \( v \). Other results and open questions are also presented.
A red-blue coloring of a graph \( G \) is an edge coloring of \( G \) in which every edge of \( G \) is colored red or blue. Let \( F \) be a connected graph of size 2 or more with a red-blue coloring, at least one edge of each color, where some blue edge of \( F \) is designated as the root of \( F \). Such an edge-colored graph \( F \) is called a color frame. An \( F \)-coloring of a graph \( G \) is a red-blue coloring of \( G \) in which every blue edge of \( G \) is the root edge of a copy of \( F \) in \( G \). The \( F \)-chromatic index \( \chi_F'(G) \) of \( G \) is the minimum number of red edges in an \( F \)-coloring of \( G \). A minimal \( F \)-coloring of \( G \) is an \( F \)-coloring with the property that if any red edge of \( G \) is re-colored blue, then the resulting red-blue coloring of \( G \) is not an \( F \)-coloring of \( G \). The maximum number of red edges in a minimal \( F \)-coloring of \( G \) is the upper \( F \)-chromatic index \( \chi_F”(G) \) of \( G \). For integers \( k \) and \( m \) with \( 1 \leq k < m \) and \( m \geq 3 \), let \( S_{k,m} \) be the color frame of the star \( K_{1,m} \) of size \( m \) such that \( S_{k,m} \) has exactly \( k \) red edges and \( m-k \) blue edges. For a positive integer \( k \), a set \( X \) of edges of a graph \( G \) is a \( \Delta_k \)-set if \( \Delta(G[X]) = k \), where \( G[X] \) is the subgraph of \( G \) induced by \( X \). The maximum size of a \( \Delta_k \)-set in \( G \) is referred to as the \( k \)-matching number of \( G \) and is denoted by \( a_k'(G) \). A \( \Delta_k \)-set \( X \) is maximal if \( X \cup \{e\} \) is not a \( \Delta_k \)-set for every \( e \in E(G) – X \). The minimum size of a maximal \( \Delta_k \)-set of \( G \) is the lower \( k \)-matching number of \( G \) and is denoted by \( a_k''(G) \). In this paper, we consider \( S_{k,m} \)-colorings of a graph and study relations between \( S_{k,m} \)-colorings and \( \Delta_k \)-sets in graphs. Bounds are established for the \( S_{k,m} \)-chromatic indexes \( \chi_{S_{k,m}}'(G) \) and \( \chi_{S_{k,m}}''(G) \) of a graph \( G \) in terms of the \( k \)-matching numbers \( a_k'(G) \) and \( a_k''(G) \) of the graph. Other results and questions are also presented.
Let \( G \) be a Hamiltonian graph of order \( n \geq 3 \). For an integer \(\ell\) with \(1 \leq \ell \leq n\), the graph \( G \) is \(\ell\)-path-Hamiltonian if every path of order \(\ell\) lies on a Hamiltonian cycle in \( G \). The Hamiltonian cycle extension number of \( G \) is the maximum positive integer \(\ell\) for which every path of order \(\ell\) or less lies on a Hamiltonian cycle of \( G \). For an integer \(\ell\) with \(2 \leq \ell \leq n-1\), the graph \( G \) is \(\ell\)-path-pancyclic if every path of order \(\ell\) in \( G \) lies on a cycle of every length from \(\ell+1\) to \(n\). (Thus, a \(2\)-path-pancyclic graph is edge-pancyclic.) A graph \( G \) of order \( n \geq 3 \) is path-pancyclic if \( G \) is \(\ell\)-path-pancyclic for each integer \(\ell\) with \(2 \leq \ell \leq n-1\). In this paper, we present a brief survey of known results on these two parameters and investigate the \(\ell\)-path-Hamiltonian graphs and \(\ell\)-path-pancyclic graphs having small minimum degree and small values of \(\ell\). Furthermore, highly path-pancyclic graphs are characterized and several well-known classes of \(2\)-path-pancyclic graphs are determined. The relationship among these two parameters and other well-known Hamiltonian parameters is investigated along with some open questions in this area of research.
Let \( G \) be a graph with vertex set \( V(G) \) and edge set \( E(G) \). A \((p, q)\)-graph \( G = (V, E) \) is said to be AL(\(k\))-traversal if there exists a sequence of vertices \((v_1, v_2, \ldots, v_p)\) such that for each \( i = 1, 2, \ldots, p-1 \), the distance between \( v_i \) and \( v_{i+1} \) is equal to \( k \). We call a graph \( G \) a 2-steps Hamiltonian graph if it has an AL(2)-traversal in \( G \) and \( d(v_p, v_1) = 2 \). In this paper, we characterize some cubic graphs that are 2-steps Hamiltonian. We show that no forbidden subgraph characterization for non-2-steps-Hamiltonian cubic graphs is available by demonstrating that every cubic graph is a homeomorphic subgraph of a non-2-steps Hamiltonian cubic graph.
For a graph \(G = (V, E)\) and a coloring \(f : V(G) \to \mathbb{Z}_2\), let \(v_f(i) = |f^{-1}(i)|\). \(f\) is said to be friendly if \(|v_f(1) – v_f(0)| \leq 1\). The coloring \(f : V(G) \to \mathbb{Z}_2\) induces an edge labeling \(f_+ : E(G) \to \mathbb{Z}_2\) defined by \(f_+(xy) = |f(x) – f(y)|\), for all \(xy \in E(G)\). Let \(e_f(i) = |f_+^{-1}(i)|\). The friendly index set of the graph \(G\), denoted by \(FI(G)\), is defined by
\[
FI(G) = \{ |e_f(1) – e_f(0)| : f \text{ is a friendly vertex labeling of } G \}.
\]
In this paper, we determine the friendly index set of certain classes of trees and introduce a few classes of fully cordial trees.
Let \( G = (V(G), E(G)) \) be a simple, finite, and undirected graph with \( n \) vertices. Given a bijection \( f : V(G) \to \{1, \dots, n\} \), one can associate two integers \( S = f(u) + f(v) \) and \( D = |f(u) – f(v)| \) with every edge \( uv \in E(G) \). The labeling \( f \) induces an edge labeling \( f’ : E(G) \to \{0, 1\} \) such that for any edge \( uv \) in \( E(G) \), \( f'(uv) = 1 \) if \(\gcd(S, D) = 1\), and \( f'(uv) = 0 \) otherwise. Such a labeling is called an SD-prime labeling if \( f'(uv) = 1 \) for all \( uv \in E(G) \). We say that \( G \) is SD-prime if it admits an SD-prime labeling. A graph \( G \) is said to be a \({strongly \;SD-prime \;graph}\) if for every vertex \( v \) of \( G \) there exists an SD-prime labeling \( f \) satisfying \( f(v) = 1 \). In this paper, we first give some sufficient conditions for a theta graph to be strongly SD-prime. We then provide constructions of new SD-prime graphs from known SD-prime graphs and investigate the SD-primality of some general graphs.
Recently, the authors proposed a fundamental theorem for the decomposing of a complete bipartite graph. They applied the theorem to obtain complete results on the decomposition of a complete bipartite graph into connected subgraphs on four vertices and up to four edges. In this paper, we decompose a complete multi-bipartite graph into its subgraphs of four vertices and five edges. We show that necessary conditions are sufficient for the decompositions, with some exceptions where decompositions do not exist
The authors previously defined the Stanton-type graph \(S(n,m)\) and demonstrated how to decompose \(\lambda K_n\) (for the appropriate minimal values of \(\lambda\)) into Stanton-type graphs \(S(4, 3)\) of the LOE-, OLE-, LEO-, and ELO-types. Sarvate and Zhang showed that for all possible values of \(\lambda\), the necessary conditions are sufficient for LOE- and OLE-decompositions. In this paper, we show that for all possible values of \(\lambda\), the necessary conditions are sufficient for LEO- and ELO-decompositions.
Let \(G\) be a graph with vertex set \(V(G)\) and edge set \(E(G)\). A \((p, q)\)-graph \(G = (V, E)\) is said to be \(AL(k)\)-traversal if there exists a sequence of vertices \(\{v_1, v_2, \ldots, v_p\}\) such that for each \(i = 1, 2, \ldots, p-1\), the distance between \(v_i\) and \(v_{i+1}\) is equal to \(k\). We call a graph \(G\) a \(k\)-steps Hamiltonian graph if it has an \(AL(k)\)-traversal in \(G\) and the distance between \(v_p\) and \(v_1\) is \(k\). In this paper, we completely classify whether a subdivision graph of a cycle with a chord is \(2\)-steps Hamiltonian.
The spectrum problem for decomposition of trees with up to eight edges was introduced and solved in 1978 by Huang and Rosa. Additionally, the packing problem was settled for all trees with up to six edges by Roditty. For the first time, we consider obtaining all possible leaves in a maximum tree-packing of \(K_n\), which we refer to as the spectrum problem for packings of complete graphs. In particular, we completely solve this problem for trees with at most five edges. The packing designs are used in developing optimal error-correcting codes, which have applications in biology, such as in DNA sequencing.
Let \(G\) be a graph with vertex set \(V(G)\) and edge set \(E(G)\). A labeling \(f\) of a graph \(G\) is said to be edge-friendly if \(|e_f(0) – e_f(1)| \leq 1\), where \(e_f(i) = \text{card}\{e \in E(G) : f(e) = i\}\). An edge-friendly labeling \(f : E(G) \to \mathbb{Z}_2\) induces a partial vertex labeling \(f^+ : V(G) \to A\) defined by \(f^+(x) = 0\) if the edges incident to \(x\) are labeled \(0\) more than \(1\). Similarly, \(f^+(x) = 1\) if the edges incident to \(x\) are labeled \(1\) more than \(0\). \(f^+(x)\) is not defined if the edges incident to \(x\) are labeled \(1\) and \(0\) equally. The edge-balance index set of the graph \(G\), \(EBI(G)\), is defined as \(\{|v_f(0) – v_f(1)| : \text{the edge labeling } f \text{ is edge-friendly}\}\), where \(v_f(i) = \text{card}\{v \in V(G) : f^+(v) = i\}\).
An \(n\)-wheel is a graph consisting of \(n\) cycles, with each vertex of the cycles connected to one central hub vertex. The edge-balance index sets of \(n\)-wheels are presented.
A set of vertices \(W\) \({locally\; resolves}\) a graph \(G\) if every pair of adjacent vertices is uniquely determined by its coordinate of distances to the vertices in \(W\). The minimum cardinality of a local resolving set of \(G\) is called the \({local\; metric\; dimension}\) of \(G\). A graph \(G\) is called a \(k\)-regular graph if every vertex of \(G\) is adjacent to \(k\) other vertices of \(G\). In this paper, we determine the local metric dimension of an \((n-3)\)-regular graph \(G\) of order \(n\), where \(n \geq 5\).
Let \(\mathcal{G}_n\) be the set of all simple loopless undirected graphs on \(n\) vertices. Let \(T\) be a linear mapping, \(T: \mathcal{G}_n \to \mathcal{G}_n\), such that the dot product dimension of \(T(G)\) is the same as the dot product dimension of \(G\) for any \(G \in \mathcal{G}_n\). We show that \(T\) is necessarily a vertex permutation. Similar results are obtained for mappings that preserve sets of graphs with specified dot product dimensions.
A permutation \(\pi\) on a set of positive integers \(\{a_1, a_2, \ldots, a_n\}\) is said to be graphical if there exists a graph containing exactly \(a_i\) vertices of degree \(\pi(a_i)\) for each \(i\) (\(1 \leq i \leq n\)). It has been shown that for positive integers with \(a_1 < a_2 < \ldots < a_n\), if \(\pi(a_n) = a_n\), then the permutation \(\pi\) is graphical if and only if the sum \(\sum_{i=1}^n a_i \pi(a_i)\) is even and \(a_n \leq \sum_{i=1}^{n-1} a_i\pi(a_i)\).
We use a criterion of Tripathi and Vijay to provide a new proof of this result and to establish a similar result for permutations \(\pi\) such that \(\pi(a_{n-1}) = a_n\). We prove that such a permutation is graphical if and only if the sum \(\sum_{i=1}^n a_i \pi(a_i)\) is even and \(a_na_{n-1} \leq a_{n-1}(a_{n-1} – 1) + \sum_{i\neq n-1} a_i\pi(a_i)\). We also consider permutations such that \(\pi(a_n) = a_{n-1}\) and, more generally, those such that \(\pi(a_n) = a_{n-j}\) for some \(j\) (\(1 < j < n\)).
A \(k\)-labeling of a graph is a labeling of the vertices of the graph by \(k\)-tuples of non-negative integers such that two vertices of \(G\) are adjacent if and only if their label \(k\)-tuples differ in each coordinate. The dimension of a graph \(G\) is the least \(k\) such that \(G\) has a \(k\)-labeling.
Lovász et al. showed that for \(n \geq 3\), the dimension of a path of length \(n\) is \((\log_2 n)^+\). Lovász et al. and Evans et al. determined the dimension of a cycle of length \(n\) for most values of \(n\). In the present paper, we obtain the dimension of a caterpillar or provide close bounds for it in various cases.
Let \(G\) be a graph of order \(n\). In [A. Saito, Degree sums and graphs that are not covered by two cycles, J. Graph Theory 32 (1999), 51–61.], Saito characterized the graphs with \(\sigma_3(G) \geq n-1\) that are not covered by two cycles. In this paper, we characterize the graphs with \(\sigma_4(G) \geq n-1\) that are not covered by three cycles. Moreover, to prove our main theorem, we show several new results which are useful in the study of this area.
Let \(\mathcal{B}(n, a)\) be the set of bicyclic graphs on \(n\) vertices with matching number \(\alpha\). In this paper, we characterize the extremal bicyclic graph with minimal Hosoya index and maximal Merrifield-Simmons index in \(\mathcal{B}(n, a)\).
A word has a shape determined by its image under the Robinson-Schensted-Knuth correspondence. We show that when a word \(w\) contains a separable (i.e., \(3142\)- and \(2413\)-avoiding) permutation \(\sigma\) as a pattern, the shape of \(w\) contains the shape of \(\sigma\). As an application, we exhibit lower bounds for the lengths of supersequences of sets containing separable permutations.
Two graphs are defined to be adjointly equivalent if their complements are chromatically equivalent. In \([2, 7]\), Liu and Dong et al. give the first four coefficients \(b_0\), \(b_1\), \(b_2\), \(b_3\) of the adjoint polynomial and two invariants \(R_1\), \(R_2\), which are useful in determining the chromaticity of graphs. In this paper, we give the expression of the fifth coefficient \(b_4\), which brings about a new invariant \(R_3\). Using these new tools and the properties of the adjoint polynomials, we determine the chromatic equivalence class of \(\overline{B_{n-9,1,5}}\).
Given a tournament \(T = (V, A)\), a subset \(X\) of \(V\) is an interval of \(T\) provided that for any \(a, b \in X\) and \(x \in V – X\), \((a, x) \in A\) if and only if \((b, x) \in A\). For example, \(\emptyset\), \(\{x\}\) (\(x \in V\)), and \(V\) are intervals of \(T\), called trivial intervals. A tournament whose intervals are trivial is indecomposable; otherwise, it is decomposable. With each indecomposable tournament \(T\), we associate its indecomposability graph \(\mathbb{I}(T)\) defined as follows: the vertices of \(\mathbb{I}(T)\) are those of \(T\) and its edges are the unordered pairs of distinct vertices \(\{x, y\}\) such that \(T -\{x, y\}\) is indecomposable. We characterize the indecomposable tournaments \(T\) whose \(\mathbb{I}(T)\) admits a vertex cover of size \(2\).
Let \(G\) be a simple graph. A harmonious coloring of \(G\) is a proper vertex coloring such that each pair of colors appears together on at most one edge. The harmonious chromatic number \(h(G)\) is the least number of colors in such a coloring. In this paper, it is shown that if \(T\) is a tree of order \(n\) and \(\Delta(T) \geq \frac{n}{2}\), then \(h(T) = \Delta(T) + 1\), where \(\Delta(T)\) denotes the maximum degree of \(T\). Let \(T_1\) and \(T_2\) be two trees of order \(n_1\) and \(n_2\), respectively, and \(F = T_1 \cup T_2\). In this paper, it is shown that if \(\Delta(T_i) = \Delta_i\) and \(\Delta_i \geq \frac{n_i}{2}\), for \(i = 1, 2\), then \(h(F) \leq \Delta(F) + 2\). Moreover, if \(\Delta_1 = \Delta_2 = \Delta \geq \frac{n_i}{2}\), for \(i = 1, 2\), then \(h(F) = \Delta + 2\).
Hamming graph \(H(n, k)\) has as vertex set all words of length \(n\) with symbols taken from a set of \(k\) elements. Suppose \(L\) denotes the set \(\bigcup_{i=0}^{n+1}\Omega_l\) with \(\Omega_l=\{\sum\limits_{i\in I_1}e_i^1+\sum\limits_{i\in I_2}e_i^2+\ldots+\sum\limits_{i\in I_k}e_i^k|I_j\cap I_j’=\emptyset (j\neq j’),|\bigcup_{j=1}^kI_j|=l\}\) for \(0\leq l\leq n\) and \(\Omega_{n+1}\). For any two elements \(x, y \in L\), define \(x \leq y\) if and only if \(y = I\) or \(I^x_j \leq I^y_j\) for some \(1 \leq j \leq k\). Then \(L\) is a lattice, denoted by \(L_o\). Reversing the above partial order, we obtain the dual of \(L_o\), denoted by \(L_r\). This article discusses their geometric properties and computes their characteristic polynomials.
The paper considers two-dimensional linear codes with sub-block structure in RT-spaces \([2-5,7]\) whose error location techniques are described in terms of various sub-blocks. Upper and lower-bounds are given for the number of check digits required with any error locating code in RT-spaces.
Let \(k \geq 0\) be an integer. Oblong (pronic) numbers are numbers of the form \(O_k = k(k+1)\). In this work, we set a new integer sequence \(B = B_n(k)\) defined as \(B_0 = 0\), \(B_1 = 1\), and \(B_n = O_k B_{n-1} – B_{n-2}\) for \(n \geq 2\), and then derive some algebraic relations on it. Later, we give some new results on balancing numbers via oblong numbers.
This note deals with the computation of the factorization number \(F_2(G)\) of a finite group \(G\). By using the Möbius inversion formula, explicit expressions of \(F_2(G)\) are obtained for two classes of finite abelian groups, improving the results of “Factorization numbers of some finite groups”, Glasgow Math. J. (2012).
Given a set of vertices \(S = \{v_1, v_2, \ldots, v_k\}\) of a connected graph \(G\), the metric representation of a vertex \(v\) of \(G\) with respect to \(S\) is the vector \(r(v|S) = (d(v, v_1), d(v, v_2), \ldots, d(v, v_k))\), where \(d(v, v_i)\), \(i \in \{1, \ldots, k\}\), denotes the distance between \(v\) and \(v_i\). \(S\) is a resolving set of \(G\) if for every pair of distinct vertices \(u, v\) of \(G\), \(r(u|S) \neq r(v|S)\). The metric dimension \(\dim(G)\) of \(G\) is the minimum cardinality of any resolving set of \(G\). Given an ordered partition \(\Pi = \{P_1, P_2, \ldots, P_t\}\) of vertices of a connected graph \(G\), the partition representation of a vertex \(v\) of \(G\), with respect to the partition \(\Pi\), is the vector \(r(v|\Pi) = (d(v, P_1), d(v, P_2), \ldots, d(v, P_t))\), where \(d(v, P_i)\), \(1 \leq i \leq t\), represents the distance between the vertex \(v\) and the set \(P_i\), that is \(d(v, P_i) = \min_{u \in P_i} \{d(v, u)\}\). \(\Pi\) is a resolving partition for \(G\) if for every pair of distinct vertices \(u, v\) of \(G\), \(r(u|\Pi) \neq r(v|\Pi)\). The partition dimension \(\mathrm{pd}(G)\) of \(G\) is the minimum number of sets in any resolving partition for \(G\). Let \(G\) and \(H\) be two graphs of order \(n\) and \(m\), respectively. The corona product \(G \odot H\) is defined as the graph obtained from \(G\) and \(H\) by taking one copy of \(G\) and \(n\) copies of \(H\) and then joining, by an edge, all the vertices from the \(i\)-th copy of \(H\) with the \(i\)-th vertex of \(G\). Here, we study the relationship between \(\mathrm{pd}(G \odot H)\) and several parameters of the graphs \(G \odot H\), \(G\), and \(H\), including \(\dim(G \odot H)\), \(\mathrm{pd}(G)\), and \(\mathrm{pd}(H)\).
We study: combination and permutation graphs. We introduce some familes to be: combination graphs and permutation graphs.
An \(L(2, 1)\)-labeling of a graph \(G\) is a function \(f\) from the vertex set \(V(G)\) to the set of all nonnegative integers such that \(|f(x) – f(y)| \geq 2\) if \(d(x, y) = 1\) and \(|f(x) – f(y)| \geq 1\) if \(d(x, y) = 2\), where \(d(x, y)\) denotes the distance between \(x\) and \(y\) in \(G\). The \(L(2, 1)\)-labeling number, \(\lambda(G)\), of \(G\) is the smallest number \(k\) such that \(G\) has an \(L(2, 1)\)-labeling \(f\) with \(\max\{f(v) : v \in V(G)\} = k\). In this paper, we present a new characterization on \(d\)-disk graphs for \(d > 1\). As an application, we give upper bounds on the \(L(2, 1)\)-labeling number for these classes of graphs.
The Randić index \(R(G)\) of a graph \(G\) is the sum of the weights \((d_u d_v)^{-\frac{1}{2}}\) over all edges \(uv\) of \(G\), where \(d_u\) denotes the degree of the vertex \(u\). In this paper, we determine the first ten, eight, and six largest values for the Randić indices among all trees, unicyclic graphs, and bicyclic graphs of order \(n \geq 11\), respectively. These extend the results of Du and Zhou [On Randić indices of trees, unicyclic graphs, and bicyclic graphs, International Journal of Quantum Chemistry, 111 (2011), 2760–2770].
A paired-dominating set of a graph \(G\) is a dominating set of vertices whose induced subgraph has a perfect matching. The paired-domination number is the minimum cardinality of a paired-dominating set of \(G\). In this paper, we investigate the paired-domination number in claw-free graphs with minimum degree at least four. We show that a connected claw-free graph \(G\) with minimum degree at least four has paired-domination number at most \(\frac{4}{7}\) its order.
Given a set of vertices \(S = \{v_1, v_2, \ldots, v_k\}\) of a connected graph \(G\), the metric representation of a vertex \(v\) of \(G\) with respect to \(S\) is the vector \(r(v|S) = (d(v, v_1), d(v, v_2), \ldots, d(v, v_k))\), where \(d(v, v_i)\), \(i \in \{1, \ldots, k\}\), denotes the distance between \(v\) and \(v_i\). \(S\) is a resolving set of \(G\) if for every pair of distinct vertices \(u, v\) of \(G\), \(r(u|S) \neq r(v|S)\). The metric dimension \(\dim(G)\) of \(G\) is the minimum cardinality of any resolving set of \(G\). Given an ordered partition \(\Pi = \{P_1, P_2, \ldots, P_t\}\) of vertices of a connected graph \(G\), the partition representation of a vertex \(v\) of \(G\), with respect to the partition \(\Pi\), is the vector \(r(v|\Pi) = (d(v, P_1), d(v, P_2), \ldots, d(v, P_t))\), where \(d(v, P_i)\), \(1 \leq i \leq t\), represents the distance between the vertex \(v\) and the set \(P_i\), that is \(d(v, P_i) = \min_{u \in P_i} \{d(v, u)\}\). \(\Pi\) is a resolving partition for \(G\) if for every pair of distinct vertices \(u, v\) of \(G\), \(r(u|\Pi) \neq r(v|\Pi)\). The partition dimension \(\mathrm{pd}(G)\) of \(G\) is the minimum number of sets in any resolving partition for \(G\). Let \(G\) and \(H\) be two graphs of order \(n\) and \(m\), respectively. The corona product \(G \odot H\) is defined as the graph obtained from \(G\) and \(H\) by taking one copy of \(G\) and \(n\) copies of \(H\) and then joining, by an edge, all the vertices from the \(i\)-th copy of \(H\) with the \(i\)-th vertex of \(G\). Here, we study the relationship between \(\mathrm{pd}(G \odot H)\) and several parameters of the graphs \(G \odot H\), \(G\), and \(H\), including \(\dim(G \odot H)\), \(\mathrm{pd}(G)\), and \(\mathrm{pd}(H)\).
A bridge graph is a special one of those graphs with more than one cut-edge. In this paper, we compute Wiener, hyper-Wiener, \(PI\) and vertex \(PI\) indices of graphs with more than one cut-edge, which generalize results in [12, 13, 14].
For an ordered set \(W = \{w_1, w_2, \ldots, w_k\}\) of vertices and a vertex \(v\) in a connected graph \(G\), the ordered \(k\)-vector \(r(v|W) := (d(v, w_1), d(v, w_2), \ldots, d(v, w_k))\) is called the (metric) representation of \(v\) with respect to \(W\), where \(d(x, y)\) is the distance between the vertices \(x\) and \(y\). The set \(W\) is called a resolving set for \(G\) if distinct vertices of \(G\) have distinct representations with respect to \(W\). A minimum resolving set for \(G\) is a basis of \(G\) and its cardinality is the metric dimension of \(G\). The resolving number of a connected graph \(G\) is the minimum \(k\) such that every \(k\)-set of vertices of \(G\) is a resolving set. A connected graph \(G\) is called randomly \(k\)-dimensional if each \(k\)-set of vertices of \(G\) is a basis. In this paper, along with some properties of randomly \(k\)-dimensional graphs, we prove that a connected graph \(G\) with at least two vertices is randomly \(k\)-dimensional if and only if \(G\) is a complete graph \(K_{k+1}\) or an odd cycle.
We say that \(G\) is nearly claw-free if for every \(v \in A\), the set of centers of claws of \(G\), there exist two vertices \(x, y \in N(v)\) such that \(x, y \notin A\) and \(N_G(v) \subseteq N_G(x) \cup N_G(y) \cup \{x, y\}\). A graph \(G\) is triangularly connected if for every pair of edges \(e_1, e_2 \in E(G)\), \(G\) has a sequence of \(3\)-cycles \(C_1, C_2, \ldots, C_r\) such that \(e_1 \in C_1, e_2 \in C_l\) and \(E(C_i) \cap E(C_{i+1}) \neq \emptyset\) for \(1 \leq i \leq l-1\). In this paper, we will show that (i) every triangularly connected \(K_{1,4}\)-free nearly claw-free graph on at least three vertices is fully cycle extendable if the clique number of the subgraph induced by the set of centers of claws of \(G\) is at most \(2\), and (ii) every \(4\)-connected line graph of a nearly claw-free graph is hamiltonian connected.
In this paper, we will find a combinatorial formula that relates the power of a \(k\)-Fibonacci number, \(F_{k,n}^p\), to the number \(F_{k,an}\). From this formula, and if \(p\) is odd, we will find a new formula that allows expressing the \(k\)-Fibonacci number \(F_{k,(2r+1)n}\) as a combination of odd powers of \(F_{k,n}\). If \(p\) is even, the formula is similar but for the even \(k\)-Lucas numbers \(L_{k,2rn}\).
The resistance distance between two vertices of a connected graph \(G\) is defined as the effective resistance between them in the corresponding electrical network constructed from \(G\) by replacing each edge of \(G\) with a unit resistor. The Kirchhoff index \(Kf(G)\) is the sum of resistance distances between all pairs of vertices of the graph \(G\). In this paper, we determine the tricyclic graphs with the smallest and the second smallest Kirchhoff indices.
The basis number of a graph \(G\) is defined to be the least non-negative integer \(d\) such that there is a basis \(\mathcal{B}\) of the cycle space of \(G\) such that each edge of \(G\) is contained in at most \(d\) members of \(\mathcal{B}\). In this paper, we determine the basis number of the wreath product of different ladders.
The \(Co-PI\) index have been introduced by Hasani et al. recently. In this paper, we present a new version for the \(Co-PI\) index, and the Cartesian product, Corona product and join of graphs under this new index are computed.
In this paper we give upper bounds of the number of edges in four types of labeled graphs of known orders.
In this paper, some lattices generated by the orbits of the subspaces under finite classical groups are considered. the characteristic polynomials of these lattices are obtained by using the effective approach by Aigner in \([2]\) , and their expressions are also determined.
In this paper, we study \(CT\)-burst array error \([6]\) detection and correction in row-cyclic array codes \([8]\).
In this work, we define a new integer sequence related to Fibonacci and Pell sequences with four parameters and then derived some algebraic identities on it including, the sum of first non-zero terms, recurrence relations, rank of its terms, powers of companion matrix and the limit of cross-ratio of four consecutive terms of it.
The hexagonal system considered here, denoted by \({E}_n^2\), is formed by \(3n\) (\(n \geq 2\)) hexagons shown in Fig. 2(a). In this paper, we give the explicit expression of the characteristic polynomial \(\Phi_A({E}_n^2, x)\). Subsequently, we obtain the multiplicity of eigenvalues \(+1\), the spectral radius, and the nullity of \({E}_n^2\). Furthermore, the energy, Estrada index, and the number of Kekulé structures of \({E}_n^2\) are determined.
The frequency assignment problem originated in researching mobile communication networks. A proper total coloring of a graph \(G\) is a coloring of both edges and vertices of \(G\) such that no two adjacent or incident elements receive the same color. As known, the vertex distinguishing total coloring is one of the suitable tools for investigating the frequency assignment problem. We introduce a new graph total coloring, called \((4)\)-adjacent vertex distinguishing total coloring (\((4)\)-AVDTC), in this paper. Our coloring contains the adjacent vertex distinguishing total coloring. The minimum number of colors required for every \((4)\)-AVDTC of \(G\) is called the \((4)\)-AVDTC chromatic number of \(G\). We will show that using at most \(\Delta(G) + 4\) colors can achieve at least \(4\) different adjacent vertex distinguishing actions for some communication networks \(G\). The exact \((4)\)-AVDTC chromatic numbers of several classes of graphs are determined here and a problem is presented.
Let \(R\) be a commutative ring with identity and \(T(\Gamma(R))\) its total graph. The subject of this article is the investigation of the properties of the corresponding line graph \(L(T(\Gamma(R)))\). The classification of all commutative rings whose line graphs are planar or toroidal is given. It is shown that for every integer \(g \geq 0\) there are only finitely many commutative rings such that \(\gamma(L(T(\Gamma(R)))) = g\).
Sparse anti-magic squares are useful in constructing vertex-magic labelings for bipartite graphs. An \(n \times 7\) array based on \(\{0, 1, \ldots, nd\}\) is called a sparse anti-magic square of order \(n\) with density \(d\) (\(d < n\)), denoted by SAMS\((n, d)\), if its row-sums, column-sums, and two main diagonal sums constitute a set of \(2n + 2\) consecutive integers. A SAMS\((n, d)\) is called regular if there are \(d\) positive entries in each row, each column, and each main diagonal. In this paper, some constructions of regular sparse anti-magic squares are provided and it is shown that there exists a regular SAMS\((n, d-1)\) if and only if \(n \geq 4\).
In this paper, we perform a further investigation for the \(q\)-analogues of the classical Bernoulli numbers and polynomials. By applying summation transform techniques, we establish some new recurrence relations for these type numbers and polynomials. We also present some illustrative special cases as well as immediate consequences of the main results.
The toughness \(t(G)\) of a noncomplete graph \(G\) is defined as \[t(G) = \min\left\{\frac{|S|}{w(G – S)} \mid S \subset V(G), w(G – S) \geq 2\right\}\] and the toughness of a complete graph is \(\infty\), where \(w(G – S)\) is the number of connected components of \(G – S\). In this paper, we give the sharp upper and lower bounds for the Kronecker product of a complete graph and a tree. Moreover, we determine the toughness of the Kronecker product of a complete graph and a star, a path, respectively.
For a vertex \(v\) of a graph \(G\), Zhu, Li, and Deng introduced the concept of implicit degree \(id(v)\), according to the degrees of its neighbors and the vertices at distance \(2\) with \(v\) in \(G\). For \(S \subset V(G)\), let \(i\Delta_2(S)\) denote the maximum value of the implicit degree sum of two vertices of \(S\). In this paper, we will prove the following result: Let \(G\) be a \(2\)-connected graph on \(n \geq 3\) vertices. If \(i\Delta_2(S) \geq d\) for each independent set \(S\) of order \(\kappa(G) + 1\), then \(G\) has a cycle of length at least \(\min\{d, n\}\). This result generalizes one result of Yamashita [T. Yamashita, On degree sum conditions for long cycles and cycles through specified vertices, Discrete Math., \(308 (2008) 6584-6587]\).
For a given graph \(G = (V, E)\), by \(f(v)\), we denote the sum of the color on the vertex \(v\) and the colors on the edges incident with \(v\). A proper \(k\)-total coloring \(\phi\) of a graph \(G\) is called a neighbor sum distinguishing \(k\)-total coloring if \(f(u) \neq f(v)\) for each edge \(uv \in E(G)\). The smallest number \(k\) in such a coloring of \(G\) is the neighbor sum distinguishing total chromatic number, denoted by \(\chi”_{\sum}(G)\). The maximum average degree of \(G\) is the maximum of the average degree of its non-empty subgraphs, which is denoted by \(\mathrm{mad}(G)\). In this paper, by using the Combinatorial Nullstellensatz and the discharging method, we prove that if \(G\) is a graph with \(\Delta(G) \geq 6\) and \(\mathrm{mad}(G) < \frac{18}{5}\), then \(\chi''_{\sum}(G) \leq \Delta(G) + 2\). This bound is sharp.
A two-character set is a set of points of a finite projective space that has two intersection numbers with respect to hyperplanes. Two-character sets are related to strongly regular graphs and two-weight codes. In the literature, there are plenty of constructions for (non-trivial) two-character sets by considering suitable subsets of quadrics and Hermitian varieties. Such constructions exist for the quadrics \(Q^{+}(2n-1,4) \subseteq PG(2n-1,q)\), \(Q^{-}(2n+1,4) \subseteq PG(2n+1,q)\) and the Hermitian varieties \(H(2n-1,q^{2}) \subseteq PG(2n-1,q^{2})\), \(H(2n,q^{2}) \subseteq PG(2n,q^{2})\). In this note, we show that every two-character set of \(PG(2n,q)\) that is contained in a given nonsingular parabolic quadric \(Q(2n,q) \subseteq PG(2n,q)\) is a subspace of \(PG(2n,q)\). This offers some explanation for the absence of the parabolic quadrics in the above-mentioned constructions.
Using the companion matrices, we get more identities and Hessenberg matrices about Fibonacci and Tribonacci numbers.
By Fibonacci and Tribonacci numbers we can evaluate the determinants and permanents of some special Hessenberg matrices.
Let \(G\) be a graph with vertex set \(V(G)\) and edge set \(E(G)\). A function \(f: E(G) \rightarrow \{-1, 1\}\) is said to be a signed star dominating function of \(G\) if \(\sum_{e \in E_G(v)} f(e) \geq 1\) for every \(v \in V(G)\), where \(E_G(v) = \{uv \in E(G) | u \in V(G)\}\). The minimum of the values of \(\sum_{e \in E(G)} f(e)\), taken over all signed dominating functions \(f\) on \(G\), is called the signed star domination number of \(G\) and is denoted by \(\gamma_{SS}(G)\). In this paper, we prove that \(frac{n}{2}\leq \gamma_{SS}(T) \leq n-1\) for every tree \(T\) of order \(n\), and characterize all trees on \(n\) vertices with signed star domination number \(\frac{n}{2}\), \(\frac{n+1}{2}\), \(n-1\), or \(n-3\).
The concept of rainbow connection was introduced by Chartrand et al. in 2008. The rainbow connection number, \(rc(G)\), of a connected graph \(G = (V, E)\) is the minimum number of colors needed to color the edges of \(E\), so that each pair of vertices in \(V\) is connected by at least one path in which no two edges are assigned the same color. The rainbow vertex-connection number, \(rvc(G)\), is the vertex version of this problem. In this paper, we introduce mixed integer programming models for both versions of the problem. We show the validity of the proposed models and test their efficiency using a nonlinear programming solver.
A graph of order \(n\) is \(p\)-factor-critical, where \(p\) is an integer with the same parity as \(n\), if the removal of any set of \(p\) vertices results in a graph with a perfect matching. It is well known that a connected vertex-transitive graph is \(1\)-factor-critical if it has odd order and is \(2\)-factor-critical or elementary bipartite if it has even order. In this paper, we show that a connected non-bipartite vertex-transitive graph \(G\) with degree \(k \geq 6\) is \(p\)-factor-critical, where \(p\) is a positive integer less than \(k\) with the same parity as its order, if its girth is not less than the bigger one between \(6\) and \( \frac{k(p-1)+8}{2(k-2)}\).
In this paper, the completely regular endomorphisms of a split graph are investigated. We give necessary and sufficient conditions the completely regular endomorphisms of a split graph form a monoid.
In this paper, we interpret a generalized basic series as the generating function of two different combinatorial objects, viz., a restricted \(n\)-colour partition function, which we call a two-colour partition function, and a weighted lattice path function. This leads to infinitely many combinatorial identities. Our main result has the potential of yielding many Rogers-Ramanujan-MacMahon type combinatorial identities. This is illustrated by an example.
Let \(u\) and \(v\) be two vertices in a graph \(G\). We say vertex \(u\) dominates vertex \(v\) if \(N(v) \subseteq N(u) \cup \{u\}\). If \(u\) dominates \(v\) or \(v\) dominates \(u\), then \(u\) and \(v\) are comparable. The Dilworth number of a graph \(G\), denoted \(\text{Dil}(G)\), is the largest number of pairwise incomparable vertices in the graph \(G\). A graph \(G\) is called \(\{H_1, H_2, \ldots, H_k\}\)-free if \(G\) contains no induced subgraph isomorphic to any \(H_i\), \(1 \leq i \leq k\). A graph \(G\) is called an \(L_1\)-graph if, for each triple of vertices \(u\), \(v\), and \(w\) with \(d(u,v) = 2\) and \(w \in N(u) \cap N(v)\), \(d(u)+d(v) \geq |N(u) \cup N(v) \cup N(w)| – 1\). Let \(G\) be a \(k\) (\(k \geq 2\))-connected \(L_2\)-graph. If \(G\) is \(\{K_{1,5}, K_{1,5+e}\}\)-free and \(\text{Dil}(G) \leq 2k-1\), then \(G\) is Hamiltonian or \(G \in \mathcal{F}\), where \(K_{1,5}+e\) is a graph obtained by joining a pair of nonadjacent vertices in \(K_{s,s}\) and \(\mathcal{F} = \{G : K_{p,p-1} \subseteq G \subseteq K_{p} \vee (p+1)K_1, 2 \leq p \leq 3\}\), where \(\vee\) denotes the join operation of two graphs.
For a simple digraph \(D\) with \(n\) vertices, the energy of \(D\) is defined as \(E(D) = \sum_{i=1}^{n} |\Re(z_i)|\), where \(z_1, z_2, \ldots, z_n\) are the eigenvalues of \(D\). This paper first gives an improved lower bound on the spectral radius of \(D\), which is used to obtain some upper bounds for the energy \(E(D)\). These results improve and generalize some known results on upper bounds of the energy of digraphs.
A vertex \(v \in V(G)\) is said to be a self vertex switching of \(G\) if \(G\) is isomorphic to \(G^v\), where \(G^v\) is the graph obtained from \(G\) by deleting all edges of \(G\) incident to \(v\) and adding all edges incident to \(v\) which are not in \(G\). The set of all self vertex switchings of \(G\) is denoted by \({SS_1}(G)\) and its cardinality by \(ss_1(G)\). In [6], the number \(ss_1(G)\) is calculated for the graphs cycle, path, regular graph, wheel, Euler graph, complete graph, and complete bipartite graphs. In this paper, for a vertex \(v\) of a graph \(G\), the graph \(G^v\) is characterized for tree, star, and forest with a given number of components. Using this, we characterize trees and forests, each with a self vertex switching.
Permutation tableaux were introduced in the study of totally positive Grassmannian cells, and are connected with the steady state of asymmetric exclusion process, which is an important model from statistical mechanics. In this paper, we firstly establish a shape-preserving involution on the set of permutation tableaux of length \(n\), which directly shows that the number of permutation tableaux of length \(n\) with \(k\) essential 1’s equals the number of permutation tableaux of length \(n\) with \(n-k\) unrestricted rows. In addition, we introduce three combinatorial structures, called free permutation tableaux, restricted set partitions, and labeled Dyck paths. We discuss the properties of their internal structures and present the correspondence between the set of free permutation tableaux of length \(n\) and the set of restricted set partitions of \(\{1,2,\ldots,n\}\), and we also give a bijection between the set of restricted set partitions of \(\{1,2,\ldots,n\}\) and the set of labeled Dyck paths of length \(2n\). Finally, we make a generalization of the latter bijection.
Let \(G = (V, E)\) be a connected multigraph with order \(n\). \(\delta(G)\) and \(\lambda(G)\) are the minimum degree and edge connectivity, respectively. The multigraph \(G\) is called maximally edge-connected if \(\lambda(G) = \delta(G)\) and super edge-connected if every minimum edge-cut consists of edges incident with a vertex of minimum degree. A sequence \(D = (d_1, d_2, \ldots, d_n)\) with \(d_1 \geq d_2 \geq \ldots \geq d_n\) is called a multigraphic sequence if there is a multigraph with vertices \(v_1, v_2, \ldots, v_n\) such that \(d(v_i) = d_i\) for each \(i = 1, 2, \ldots, n\). The multigraphic sequence \(D\) is super edge-connected if there exists a super edge-connected multigraph \(G\) with degree sequence \(D\). In this paper, we present that a multigraphic sequence \(D\) with \(d_n = 1\) is super edge-connected if and only if \(\sum\limits_{i=1}^{n} d_i \geq 2n\) and give a sufficient and necessary condition for a multigraphic sequence \(D\) with \(d_n = 2\) to be super edge-connected. Moreover, we show that a multigraphic sequence \(D\) with \(d_n \geq 3\) is always super edge-connected.
The general sum-connectivity index is defined as \(\chi_\alpha(G) = \sum_{uv \in E(G)} (d_G(u) + d_G(v))^\alpha\). Let \(\mathcal{T}(n, \beta)\) be the class of trees of order \(n\) with given matching number \(\beta\). In this paper, we characterize the structure of the trees with a given order and matching number that have maximum general sum-connectivity index for \(0 < \alpha < 1\) and give a sharp upper bound for \(\alpha \geq 1\).
The hyper-Wiener index is a graph invariant that is used as a structure descriptor for predicting physicochemical properties of organic compounds. We determine the n-vertex unicyclic graphs with the third smallest and the third largest hyper-Wiener indices for \(n\geq 5\).
A graph \(G\) with no isolated vertex is total restrained domination vertex critical if for any vertex \(v\) of \(G\) that is not adjacent to a vertex of degree one, the total restrained domination number of \(G – v\) is less than the total restrained domination number of \(G\). We call these graphs \(\gamma_{tr}\)-vertex critical. If such a graph \(G\) has total restrained domination number \(k\), we call it \(k\)-\(\gamma_{tr}\)-vertex critical. In this paper, we study matching properties in \(4\)-\(\gamma_{tr}\)-vertex critical graphs of minimum degree at least two.
A generalized weighted digraph \(G = (V, E)\) is a digraph with \(n\) vertices and \(m\) arcs without loops and multiarcs, where each arc is assigned a weight that is a non-negative and symmetric matrix of order \(p\). In this paper, we give a sharp upper bound for the spectral radius of generalized weighted digraphs (see Theorem 2.7), which generalizes some other results on the spectral radius of weighted digraphs in [4], [11], and [16].
It has been shown by Bennett et al. in 1998 that a holey Schröder design with \(n\) holes of size 2 and one hole of size \(u\), i.e., of type \(2^n u\), exists if \(1 \leq u \leq 4\) and \(n \geq u+1\) with the exception of \((n,u) \in \{(2, 1), (3, 1), (3, 2)\}\), or \(u \geq 16\) and \(n \geq \left\lceil \frac{5u}{4} \right\rceil + 14\). In this paper, we extend this result by showing that, for \(1 \leq u \leq 16\), a holey Schröder design of type \(2^n u\) exists if and only if \(n \geq u+1\), with the exception of \((n,u) \in \{(2, 1), (3, 1), (3, 2)\}\) and with the possible exception of \((n,u) \in \{(7,5), (7,6), (11,9), (11,10)\}\). For general \(u\), we prove that there exists an HSD(\(2^n u\)) for all \(u \geq 17\) and \(n \geq \left\lceil \frac{5u}{4} \right\rceil + 4\). Moreover, if \(u \geq 35\), then an HSD(\(2^n u\)) exists for all \(n \geq \left\lceil \frac{5u}{4} \right\rceil + 1\); if \(u \geq 95\), then an HSD(\(2^n u\)) exists for all \(n \geq \left\lceil \frac{5u}{4} \right\rceil – 2\). We also improve a well-known result on the existence of holey Schröder designs of type \(h^n\) by removing the remaining possible exception of type \(64\).
A vertex of a graph is said to be total domination critical if its deletion decreases the total domination number. A graph is said to be total domination vertex critical if all of its vertices, except the supporting vertices, are total domination vertex critical. We show that if \(G\) is a connected total domination vertex critical graph with total domination number \(k \geq 4\), then the diameter of \(G\) is at most \(\lfloor \frac{5k-7}{3}\rfloor\).
By computer-assisted approaches and inductive arguments, two curious sums of triple multiplication of binomial coefficients are established in the present paper. The two curious sums arise in proving Melham’s conjecture on odd power sums of Fibonacci numbers, which was confirmed by Xie, Yang and the present author. However, being different from their’s technical way, the method used in the paper is more elementary.
Let \(G\) be a graph and \(u\) be a vertex of \(G\). The transmission index of \(u\) in \(G\), denoted by \(T_G(u)\), is the sum of distances from \(u\) to all the other vertices in graph \(G\), i.e., \(T(u) = T_G(u) = \sum_{v \in V} d_G(u,v)\). The Co-PI index [1] is defined as \(Co\text{-}PI(G) = \sum_{uv \in E(G)} |T(u) – T(v)|\). In this paper, we give some upper bounds for the Co-PI indices of the join, composition, disjunction, symmetric difference, and corona graph \(G_1 \circ G_2\).
The purpose of this note is the study of the hypergroups associated with binary relations. New types of matrices, called \(i\)-very good and regular reversible matrices, are introduced in order to give some properties of the Rosenberg hypergroups related to them. A program written in MATLAB computes the number of these hypergroups up to isomorphism.
Let \(A_n\) be the alternating group of degree \(n\) with \(n > 4\). Set \(T = \{(1 2 3), (1 3 2), (1 2)(3 i) \mid 4 \leq i \leq n\}\). The alternating group network, denoted by \(AN_n\), is defined as the Cayley graph on \(A_n\) with respect to \(T\). Some properties of \(AN_n\) have been investigated in [App. Math.—JCU, Ser. A 14 (1998) 235-239; IEEE Trans. Comput. 55 (2006) 1645-1648; Inform. Process. Lett. 110 (2010) 403-409; J. Supercomput. 54 (2010) 206-228]. In this paper, it is shown that the full automorphism group of \(AN_n\) is the semi-direct product \(R(A_n) \rtimes \text{Aut}(A_n, T)\), where \(R(A_n)\) is the right regular representation of \(A_n\) and \(\text{Aut}(A_n, T) = \{\alpha \in \text{Aut}(A_n) \mid T^\alpha = T\} \cong S_{n-3} \times S_2\).
The harmonic index of a graph \(G\) is defined as the sum of weights \(\frac{2}{\deg(v) + \deg(u)}\) of all edges \(uv\) in \(E(G)\), where \(\deg(v)\) denotes the degree of a vertex \(v\) in \(V(G)\). In this note, we generalize results of [L. Zhong, The harmonic index on graphs, Appl. Math. Lett. 25 (2012), 561-566] and establish some upper and lower bounds on the harmonic index of \(G\).
Let \(\lambda_1, \lambda_2, \ldots, \lambda_n\) be the eigenvalues of the distance matrix of a connected graph \(G\). The distance Estrada index of \(G\) is defined as \(DEE(G) = \sum_{i=1}^{n} e^{\lambda_i}\). In this note, we present new lower and upper bounds for \(DEE(G)\). In addition, a Nordhaus-Gaddum type inequality for \(DEE(G)\) is given.
The splitting-off operation has important applications for graph connectivity problems. Shikare, Dalvi, and Dhotre [splitting-off operation for binary matroids and its applications, Graphs and Combinatorics, \(27(6) (2011), 871–882\)] extended this operation to binary matroids. In this paper, we provide a sufficient condition for preserving \(n\)-connectedness of a binary matroid under the splitting-off operation.
For positive integers \(j\) and \(k\) with \(j > k\), an \(L(j,k)\)-labelling is a generalization of classical graph coloring where adjacent vertices are assigned integers at least \(j\) apart, and vertices at distance two are assigned integers at least \(k\) apart. The span of an \(L(j,k)\)-labelling of a graph \(G\) is the difference between the maximum and minimum integers assigned to its vertices. The \(L(j,k)\)-labelling number of \(G\), denoted by \(\lambda_{j,k}(G)\), is the minimum span over all \(L(j,k)\)-labellings of \(G\). An \(m\)-\((j,k)\)-circular labelling of \(G\) is a function \(f: V(G) \to \{0,1,\ldots,m-1\}\) such that \(|f(u)-f(v)|_m \geq j\) if \(u\) and \(v\) are adjacent, and \(|f(u)-f(v)|_m \geq k\) if \(u\) and \(v\) are at distance two, where \(|x|_m = \min\{|x|,m-|x|\}\). The span of an \(m\)-\((j,k)\)-circular labelling of \(G\) is the difference between the maximum and minimum integers assigned to its vertices. The \(m\)-\((j,k)\)-circular labelling number of \(G\), denoted by \(\sigma_{j,k}(G)\), is the minimum span over all \(m\)-\((j,k)\)-circular labellings of \(G\). The \(L'(j,k)\)-labelling is a one-to-one \(L(j,k)\)-labelling, and the \(m\)-\((j,k)’\)-circular labelling is a one-to-one \(m\)-\((j,k)\)-circular labelling. Denote \(\lambda’_{j,k}(G)\) the \(L'(j,k)\)-labelling number and \(\sigma’_{j,k}(G)\) the \(m\)-\((j,k)’\)-circular labelling number. When \(j=d, k=1\), \(L(j,k)\)-labelling becomes \(L(d,1)\)-labelling. [Discrete Math. 232 (2001) 163-169] determined the relationship between \(\lambda_{2,1}(G)\) and \(\sigma_{2,1}(G)\) for a graph \(G\). We generalized the concept of path covering to the \(t\)-group path covering (Inform Process Lett (2011)) of a graph. In this paper, using group path covering, we establish relationships between \(\lambda_{4,1}(G)\) and \(\sigma_{4,1}(G)\) and between \(\lambda_{j,k}(G)\) and \(\sigma_{j,k}(G)\) for a graph \(G\) with diameter 2. Using these results, we obtain shorter proofs for the \(\sigma’_{j,k}\)-number of Cartesian products of complete graphs [J Comb Optim (2007) 14: 219-227].
We prove the following Turdn-Type result: If there are more than \(9mn/16\) edges in a simple and bipartite Eulerian digraph with vertex partition size m and n, then the graph contains a directed cycle of length \(4\) or \(6\). By using this result, we improve an upper bound for the diameter of interchange graphs.
The well known infinite families of prisms and antiprisms on the sphere were, for long time, not considered as Archimedean solids for reasons not fully understood. In this paper we describe the first two infinite families of Archimedean maps on higher genera which we call “generalized” prisms and “generalized” antiprisms.
Let \(G\) be a graph with vertex set \(V(G)\) and edge set \(E(G)\). A vertex labeling \(f: V(G) \to \mathbb{Z}_2\) induces an edge labeling \(f^*: E(G) \to \mathbb{Z}_2\) defined by \(f^*(x,y) = f(x) + f(y)\), for each edge \((x,y) \in E(G)\). For each \(i \in \mathbb{Z}_2\), let \(v_f(i) = |\{v \in V(G) : f(v) = i\}|\) and \(e_f(i) = |\{e \in E(G) : f^*(e) = i\}|\). A vertex labeling \(f\) of a graph \(G\) is said to be friendly if \(|v_f(1) – v_f(0)| \leq 1\). The friendly index set of the graph \(G\), denoted by \(FI(G)\), is defined as \(\{|v_f(1) – v_f(0)| : \text{the vertex labeling } f \text{ is friendly}\}\). The full friendly index set of the graph \(G\), denoted by \(FFI(G)\), is defined as \(\{|e_f(1) – e_f(0)| : \text{the vertex labeling } f \text{ is friendly}\}\). In this paper, we determine \(FFI(G)\) and \(FI(G)\) for a class of cubic graphs which are twisted products of Möbius.
Let \(k\) be a non-negative integer. Two digraphs \(G = (V, A)\) and \(G’ = (V, A’)\) are \(\{k\}\)-hypomorphic if for all \(k\)-element subsets \(K\) of \(V\), the subdigraphs \(G[K]\) and \(G'[K]\) induced on \(K\) are isomorphic. The equivalence relation \(\mathcal{D}_{G,G’}\) on \(V\) is defined by: \(x \mathcal{D}_{G,G’} y\) if \(x = y\) or there exists a sequence \(x_0 = x, \ldots, x_n = y\) of elements of \(V\) satisfying \((x_i, x_{i+1}) \in A\) if and only if \((x_i, x_{i+1}) \in A’\), for all \(i\), \(0 < i k + 6\). If \(G\) and \(G’\) are two digraphs, \(\{4\}\)-hypomorphic and \(\{v – k\}\)-hypomorphic on the same vertex set \(V\) of \(uv\) vertices, and \(C\) is an equivalence class of the equivalence relation \(\mathcal{D}_{G,G’}\), then \(G'[C \setminus A]\) and \(G[C \setminus A]\) are isomorphic for all subsets \(A\) of \(V\) of at most \(k\) vertices. In particular, \(G'[C]\) and \(G[C]\) are \(\{v – k – h\}\)-hypomorphic for all \(h \in \{1, 2, \ldots, k\}\), and \(G'[C]\) and \(G[C]\) (resp. \(G’\) and \(G\)) are isomorphic. In particular, for \(k = 1\) and \(k = 4\) we obtain the result of G. Lopez and C. Rauzy [7]. As an application of the main result, we have: If \(G\) and \(G’\) are \(\{v – 4\}\)-hypomorphic on the same vertex set \(V\) of \(v > 10\) vertices, then \(G[X]\) and \(G'[X]\) are isomorphic for all subsets \(X\) of \(V\); the particular case of tournaments was obtained by Y. Boudabbous [2].
We prove that each graph in two infinite families is fixed uniquely by just two of its maximal induced subgraphs, with each of which the degree of the missing vertex is also given. One of these families contains all separable self-complementary graphs and a self-complementary graph of diameter \(3\) and order \(n\) for each \(n \geq 5\) such that \(n \equiv 0\) or \(1 \pmod{4}\). The other contains a Hamiltonian self-complementary graph of diameter \(2\) and order \(n\) for each admissible \(n \geq 8\).
Restricted strong partially balanced \(t\)-designs were first formulated by Pei, Li, Wang, and Safavi-Naini in their investigation of authentication codes with arbitration. We recently proved that optimal splitting authentication codes that are multi-fold perfect against spoofing can be characterized in terms of restricted strong partially balanced \(t\)-designs. This article investigates the existence of optimal restricted strong partially balanced 2-designs, ORSPBD\((v, 2 \times 4, 1)\), and shows that there exists an ORSPBD\((v, 2 \times 4, 1)\) for even \(v\). As its application, we obtain a new infinite class of 2-fold perfect \(4\)-splitting authentication codes.
The \(3\)-consecutive vertex coloring number \(\psi_{3c}(G)\) of a graph \(G\) is the maximum number of colors permitted in a coloring of the vertices of \(G\) such that the middle vertex of any path \(P_3\) in \(G\) has the same color as one of the ends of that \(P_3\). This coloring constraint exactly means that no \(P_3\) subgraph of \(G\) is properly colored in the classical sense. The \(3\)-consecutive edge coloring number \(\psi’_{3c}\) is the maximum number of colors permitted in a coloring of the edges of \(G\) such that the middle edge of any sequence of three edges (in a path \(P_3\) or cycle \(C_3\)) has the same color as one of the other two edges. For graphs \(G\) of minimum degree at least \(2\), denoting by \(L(G)\) the line graph of \(G\), we prove that there is a bijection between the \(3\)-consecutive vertex colorings of \(G\) and the \(3\)-consecutive edge colorings of \(L(G)\), which keeps the number of colors unchanged, too. This implies that \(\psi_{3c} = \psi’_{3c}(L(G))\); i.e., the situation is just the opposite of what one would expect at first sight.
We consider a discrete-time dynamic problem in graphs in which the goal is to maintain a dominating set over an infinite sequence of time steps. At each time step, a specified vertex in the current dominating set must be replaced by a neighbor. In one version of the problem, the only change to the current dominating set is replacement of the specified vertex. In another version of the problem, other vertices in the dominating set can also be replaced by neighbors. A variety of results are presented relating these new parameters to the eternal domination number, domination number, and independence number of a graph.
The Turán number \(ex(m, G)\) of the graph \(G\) is the maximum number of edges of an \(m\)-vertex simple graph having no \(G\) as a subgraph. A \emph{star} \(S_r\) is the complete bipartite graph \(K_{1,r}\) (or a tree with one internal vertex and \(r\) leaves) and \(pS_r\) denotes the disjoint union of \(p\) copies of \(S_r\). A result of Lidický et al. (Electron. J. Combin. \(20(2)(2013) P62\)) implies that \(ex(m,pS_r) = \left\lfloor\frac{(m-p+1)(r-1)}{2}\right\rfloor + (p-1)m – \binom{p}{2}\) for \(m\) sufficiently large. In this paper, we give another proof and show that \(ex(m,pS_r) = \left\lfloor \frac{(m-p+1)(r-1)}{2}\right\rfloor + (p-1)m – \binom{p}{2}\) for all \(r \geq 1\), \(p \geq 1\), and \(m \geq \frac{1}{2}r^2p(p – 1) + p – 2 + \max\{rp, r^2 + 2r\}\).
Following a problem introduced by Schurch [M. Schurch, \({On\; the\; Depression\; of\; Graphs}\), Doctoral Dissertation, University of Victoria, 2013], we find exact values of the minimum number of colours required to properly edge colour \( K_n \), \( n \geq 6 \), using natural numbers, such that the length of a shortest maximal path of increasing edge labels is equal to three. This result improves the result of Breytenbach and Mynhardt [A. Breytenbach and C. M. Mynhardt, On the \(\varepsilon\)-to appear-Ascent Chromatic Index of Complete Graphs, \({Involve}\), to appear].
A tree \( T \), in an edge-colored graph \( G \), is called a \({rainbow\; tree}\) if no two edges of \( T \) are assigned the same color. A \( k \)-\({rainbow\; coloring}\) of \( G \) is an edge coloring of \( G \) having the property that for every set \( S \) of \( k \) vertices of \( G \), there exists a rainbow tree \( T \) in \( G \) such that \( S \subseteq V(T) \). The minimum number of colors needed in a \( k \)-rainbow coloring of \( G \) is the \( k \)-\({rainbow\; index}\) of \( G \), denoted by \( \text{rx}_k(G) \). In this paper, we investigate the \(3\)-rainbow index \( \text{rx}_3(G) \) of a connected graph \( G \). For a connected graph \( G \), it is shown that a sharp upper bound of \( \text{rx}_3(G) \) is \( \text{rx}_3(G[D]) + 4 \), where \( D \) is a connected 3-way dominating set and a connected 2-dominating set of \( G \). Moreover, we determine a sharp upper bound for \( K_{s,t} \) (\( 3 \leq s \leq t \)) and a better bound for \((P_5,C_5)\)-free graphs, respectively. Finally, a sharp bound for the \(3\)-rainbow index of general graphs is obtained.
A graph \( G \) admits an \( H \)-covering if every edge in \( E(G) \) belongs to a subgraph of \( G \) isomorphic to \( H \). The graph \( G \) is said to be \( H \)-magic if there exists a bijection \( f \) from \( V(G) \cup E(G) \) to \( \{1,2,\dots,|V(G)| + |E(G)|\} \) such that for every subgraph \( H’ \) of \( G \) isomorphic to \( H \), \( \sum_{v\in V(H’)} f(v) + \sum_{e\in E(H’)} f(e) \) is constant. When \( f(V(G)) = \{1,2,\dots,|V(G)|\} \), then \( G \) is said to be \( H \)-supermagic. In this paper, we investigate path-supermagic cycles. We prove that for two positive integers \( m \) and \( t \) with \( m > t \geq 2 \), if \( C_m \) is \( P_t \)-supermagic, then \( C_{3m} \) is also \( P_t \)-supermagic. Moreover, we show that for \( t \in \{3, 4, 9\} \), \( C_n \) is \( P_t \)-supermagic if and only if \( n \) is odd with \( n > t \).
Let \( G \) be an edge-colored connected graph. A path \( P \) is a proper path in \( G \) if no two adjacent edges of \( P \) are colored the same. If \( P \) is a proper \( u \) — \( v \) path of length \( d(u,v) \), then \( P \) is a proper \( u \) — \( v \) geodesic. An edge coloring \( c \) is a proper-path coloring of a connected graph \( G \) if every pair \( u,v \) of distinct vertices of \( G \) are connected by a proper \( u \) — \( v \) path in \( G \) and \( c \) is a strong proper coloring if every two vertices \( u \) and \( v \) are connected by a proper \( u \) — \( v \) geodesic in \( G \). The minimum number of colors used in a proper-path coloring and strong proper coloring of \( G \) are called the proper connection number \( \text{pc}(G) \) and strong proper connection number \( \text{spc}(G) \) of \( G \), respectively. These concepts are inspired by the concepts of rainbow coloring, rainbow connection number \( \text{rc}(G) \), strong rainbow coloring, and strong connection number \( \text{src}(G)\) of a connected graph \(G\). The numbers \(\text{pc}(G)\) and \(\text{spc}(G)\) are determined for several well-known classes of graphs \(G\). We investigate the relationship among these four edge colorings as well as the well-studied proper edge colorings in graphs. Furthermore, several realization theorems are established for the five edge coloring parameters, namely \(\text{pc}(G)\), \(\text{spc}(G)\), \(\text{rc}(G)\), \(\text{src}(G)\) and the chromatic index of a connected graph \(G\).
We are given suppliers and customers, and a set of tables. Every evening of the forthcoming days, there will be a dinner. Each customer must eat with each supplier exactly once, but two suppliers may meet at most once at a table. The number of customers and the number of suppliers who can sit together at a table are bounded above by fixed parameters. What is the minimum number of evenings to be scheduled in order to reach this objective? This question was submitted by a firm to the Junior company of a French engineering school some years ago. Lower and upper bounds are given in this paper, as well as proven optimal solutions with closed-form expressions for some cases.
In this paper, we mainly discuss the monotonicity of some sequences related to the hyperfibonacci sequences \( \{F_{n}^{[r]}\}_{n\geq 0} \) and the hyperlucas sequences \( \{L_{n}^{[r]}\}_{n\geq 0} \), where \( r \) is a positive integer. We prove that \( \{\sqrt[n]{F_{n}^{[1]}}\}_{n\geq 1} \) and \( \{\sqrt[n]{F_{n}^{[2]}}\}_{n\geq 1} \) are unimodal and \( \{\sqrt[n]{L_{n}^{[1]}}\}_{n\geq 1} \), \( \{\sqrt[n]{F_{n+1}^{[1]}/{F_{n}^{[1]}}}\}_{n\geq 1} \), and \( \{\sqrt[n]{L_{n+1}^{[1]}/{L_{n}^{[1]}}}\}_{n\geq 2} \) are decreasing. Furthermore, we discuss the monotonicity of the sequences
\[
\left\{\frac{\sqrt[n+1]{F_{n+1}^{[1]}}}{\sqrt[n]{F_{n}^{[1]}}}\right\}_{n\geq 1} \text{ and } \left\{\frac{\sqrt[n+1]{L_{n+1}^{[1]}}}{\sqrt[n]{L_{n}^{[1]}}}\right\}_{n\geq 1}
\]
The Ramsey number \( R(C_4, K_m) \) is the smallest \( n \) such that any graph on \( n \) vertices contains a cycle of length four or an independent set of order \( m \). With the help of computer algorithms, we obtain the exact values of the Ramsey numbers \( R(C_4, K_9) = 30 \) and \( R(C_4, K_{10}) = 36 \). New bounds for the next two open cases are also presented.
We describe the construction of transitive \( 2 \)-designs and strongly regular graphs defined on the conjugacy classes of the maximal and second maximal subgroups of the symplectic group \( S(6, 2) \). Furthermore, we present linear codes invariant under the action of the group \( S(6, 2) \) obtained as the codes of the constructed designs and graphs.
Gionfriddo and Lindner detailed the idea of the metamorphosis of \( 2 \)-fold triple systems with no repeated triples into \( 2 \)-fold \( 4 \)-cycle systems of all orders where each system exists in [3]. In this paper, this concept is expanded to address all orders \( n \) such that \( n \equiv 5, 8, \text{ or } 11 \pmod{12} \). When \( n \equiv 11 \pmod{12} \), a maximum packing of \( 2K_n \) with triples has a metamorphosis into a maximum packing of \( 2K_n \) with \( 4 \)-cycles, with the leave of a double edge being preserved throughout the metamorphosis. For \( n \equiv 5 \text{ or } 8 \pmod{12} \), a maximum packing of \( 2K_n \) with triples has a metamorphosis into a \( 2 \)-fold \( 4 \)-cycle system of order \( n \), except for when \( n = 5 \text{ or } 8 \), when no such metamorphosis is possible.
Eternal domination of a graph requires the positioning of guards to protect against an infinitely long sequence of attacks where, in response to an attack, each guard can either remain in place or move to a neighbouring vertex, while keeping the graph dominated. This paper investigates the \( m \)-eternal domination numbers for \( 5 \times n \) grid graphs. The values, previously known for \( 1 \leq n \leq 5 \), are determined for \( 6 \leq n \leq 12 \), and lower and upper bounds derived for \( n > 12 \).
A graph \( G = (V, E) \) with \( p \) vertices and \( q \) edges is said to be odd graceful if there is an injection \( f \) from the vertex set of \( G \) to \( \{0, 1, 2, \dots, 2q – 1\} \) such that when each edge \( xy \) is assigned the label \( |f(x) – f(y)| \), the resulting edge labels are distinct and induce the set \( \{1, 3, 5, \dots, 2q – 1\} \). In 2009, Barrientos conjectured that every bipartite graph is odd graceful. In this paper, we partially solve Barrientos’ conjecture by showing that the following graphs are odd graceful:
In this paper, \( q \)-analogs of covering designs and Steiner systems based on the subspaces of type \( (m,0) \) and the subspaces of type \( (m_1,0) \) in singular linear space \( \mathbb{F}_q^{(n+l)} \) over \( \mathbb{F}_q \) are presented, where \( m_1 < m \). Then the properties about \( q \)-analogs of covering designs and Steiner systems are discussed.
Let \( G \) be a graph with \( q \) edges. A graph \( G^* \) is called an arbitrary supersubdivision of \( G \) if \( G^* \) is obtained from \( G \) by replacing every edge \( e_i \) of \( G \) by a complete bipartite graph \( K_{2,m_i} \), such a way that the end vertices of each \( e_i \) are identified with the two vertices of the 2-vertices part of \( K_{2,m_i} \), after removing the edge \( e_i \) from \( G \), where \( m_i \) of \( K_{2,m_i} \) may vary arbitrarily for each edge \( e_i \), \( 1 \leq i \leq q \).
As recognition of cordial graph is an NP-complete, it is interesting and significant to find the graphs whose arbitrary supersubdivision graphs are cordial. In this paper, we show that arbitrary supersubdivision of every bipartite graph is cordial. This result is obtained as a corollary of the general result that “Almost arbitrary supersubdivision of every graph is cordial”, where almost arbitrary supersubdivision is a relaxation of arbitrary supersubdivision graph.
Let \( G \) be a graph with edge set \( E(G) = E_1 \cup E_2 \) and \( E_1 \cap E_2 = \emptyset \). A graph \( G \) is called an almost arbitrary supersubdivision graph of \( G \) if \( G \) is obtained from \( G \) by replacing every edge \( e_i \in E \) by a complete bipartite graph \( K_{2,m_i} \), such a way that the end vertices of each \( e_i \) are merged with the two vertices of the 2-vertices part of \( K_{2,m_i} \), after removing the edge \( e_i \) from \( G \), where \( m_i \) is chosen as an arbitrary positive integer if \( e_i \in E_1 \) or else \( m_i \) is chosen as an arbitrary even positive integer if \( e_i \in E_2 \).
Under the conditions looser than previous works, this paper shows that the \( n \)-dimensional folded hypercube networks have a cycle with length at least \( 2^n – 2|F_v| \) when the number of faulty vertices and non-critical edges is at most \( 2n – 4 \), where \( |F_v| \) is the number of faulty vertices. Meanwhile, this paper proves that \( FQ_n \) contains a fault-free cycle with length at least \( 2^n – 2|F_v| \), under the constraints that (1) The number of both faulty nodes and faulty edges is no more than \( 2n – 3 \) and there is at least one faulty edge; (2) every node in \( FQ_n \) is incident to at least two fault-free links whose other end nodes are fault-free. These results have improved the present results with further theoretical evidence of the fact that \( FQ_n \) has excellent node-fault-tolerance and edge-fault-tolerance when used as a topology of large scale computer networks.
In this paper, \( (r, 2, k) \)-regular fuzzy graphs and totally \( (r, 2, k) \)-regular fuzzy graphs are defined and \( (r, 2, k) \)-regular fuzzy graphs and totally \( (r, 2, k) \)-regular fuzzy graphs are compared through various examples. A necessary and sufficient condition under which they are equivalent is provided. Also, \( (r, 2, k) \)-regularity on some fuzzy graphs whose underlying crisp graphs are a path on four vertices, a Barbell graph \( B_{n,n} \) \( (n > 1) \) and a cycle is studied with some specific membership functions.
The definition of \( E_k \)-cordial graphs is advanced by Cahit and Yilmaz\(^{[1]}\). Based on [1], a graph \( G \) is said to be \( E_3 \)-cordial if it is possible to label the edges with the numbers from the set \( \{0, 1, 2\} \) in such a way that, at each vertex \( v \), the sum of the labels on the edges incident with \( v \) modulo \( 3 \) satisfies the inequalities \( |v(i) – v(j)| \leq 1 \) and \( |e(i) – e(j)| \leq 1 \), where \( v(s) \) and \( e(t) \) are, respectively, the number of vertices labeled with \( s \) and the number of edges labeled with \( t \). In [1]-[3], authors discussed the \( E_3 \)-cordiality of \( P_n \) \( (n \geq 3) \); stars \( S_n \), \( |S_n| = n + 1 \); \( K_n \) \( (n \geq 3) \), \( C_n \) \( (n \geq 3) \), the one point union of any number of copies of \( K_n \) and \( K_m \odot K_m \). In this paper, we give the \( E_3 \)-cordiality of \( W_n \), \( P_m \times P_n \), \( K_{m,n} \) and trees.
In this paper, we give some new identities of symmetry for \(q\)-Bernoulli polynomials under the symmetric group of degree \(n\) arising from \(p\)-adic \(q\)-integrals on \(\mathbb{Z}_p\).
The covering and packing of a unit square (resp. cube) with squares (resp. cubes) are considered. In \(d\)-dimensional Euclidean space \(\mathbb{E}^d\), the size of a \(d\)-hypercube is given by its side length and the size of a covering is the total size of the \(d\)-hypercubes used to cover the unit hypercube. Denote by \(g_d(n)\) the smallest size of a minimal covering (consisting of \(n\) hypercubes) of a \(d\)-dimensional unit hypercube. In this paper, we consider the problem of covering a unit hypercube with hypercubes in \(\mathbb{E}^d\) for \(d \geq 4\) and determine the tight upper bound and lower bound for \(g_d(n)\).
Given the binomial transforms \(\{b_n\}\) and \(\{c_n\}\) of the sequences \(\{a_n\}\) and \(\{d_n\}\) correspondingly, we compute the binomial transform of the sequence \(\{a_nc_n\}\) in terms of \(\{b_n\}\) and \(\{d_n\}\). In particular, we compute the binomial transform of the sequences \(\{n{n-1}\ldots (n-1-m)a_n\}\) and \(\{a_k x^k\}\) in terms of \(\{b_n\}\). Further applications include new binomial identities with the binomial transforms of the products \(H_n B_n\), \(H_n F_n\), \(H_n L_n(X)\), and \(B_n F_n\), where \(H_n\), \(B_n\), \(F_n\), and \(L_n(X)\) are correspondingly the harmonic numbers, the Bernoulli numbers, the Fibonacci numbers, and the Laguerre polynomials.
A kite graph is a graph obtained from a \(3\)-cycle (or triple) by adding a pendent edge to a vertex of the \(3\)-cycle. A kite system of order \(v\) is a pair \((X, \mathcal{B})\), where \(\mathcal{B}\) is an edge-disjoint collection of kite graphs which partitions the edge set of \(K_v\). A kite system of order \(v\) is cyclic if it admits an automorphism of order \(v\), and 1-rotational if it admits an automorphism containing one fixed point and a cycle of length \(v – 1\). In this paper, we show that there exists a cyclic kite system of order \(v\) if and only if \(v \equiv 1 \pmod{8}\), and there exists a \(1\)-rotational kite system of order \(v\) if and only if \(v \equiv 0 \pmod{8}\).
A \(2\)-rainbow dominating function (2RDF) on a graph \(G = (V, E)\) is a function \(f\) from the vertex set \(V\) to the set of all subsets of the set \(\{1,2\}\) such that for any vertex \(v \in V\) with \(f(v) = \emptyset\) the condition \(\cup_{u \in N(v)} f(u) = \{1, 2\}\) is fulfilled. The weight of a 2RDF \(f\) is the value \(w(f) = \sum_{v \in V(G)} |f(v)|\). The \(2\)-rainbow domination number, denoted by \(\gamma_{r2}(G)\), is the minimum weight of a 2RDF on \(G\). The rainbow bondage number \(b_{r2}(G)\) of a graph \(G\) with maximum degree at least two, is the minimum cardinality of all sets \(E’ \subseteq E(G)\) for which \(\gamma_{r2}(G – E’) > \gamma_{r2}(G)\). Dehgardi, Sheikholeslami, and Volkmann [Discrete Appl. Math. \(174 (2014), 133-139]\) proved that the rainbow bondage number of a planar graph does not exceed 15. In this paper, we improve this result.
Let \(id(v)\) denote the implicit degree of a vertex \(v\) in a graph \(G\). We define \(G\) to be implicit claw-heavy if every induced claw of \(G\) has a pair of nonadjacent vertices such that their implicit degree sum is more than or equal to \(|V(G)|\). In this paper, we show that an implicit claw-heavy graph \(G\) is hamiltonian if we impose certain additional conditions on \(G\) involving numbers of common neighbors of some specific pair of nonadjacent vertices, or forbidden induced subgraphs. Our results extend two previous theorems of Chen et al. [B. Chen, 8. Zhang and S. Qiao, Hamilton cycles in claw-heavy graphs, Discrete Math., \(309 (2009) 2015-2019.]\) on the existence of Hamilton cycles in claw-heavy graphs.
A connected graph \(G\) is called a quasi-tree graph, if there exists \(v_0 \in V(G)\) such that \(G – v_0\) is a tree. In this paper, among all triangle-free quasi-tree graphs of order \(n\) with \(G – v_0\) being a tree and \(d(v_0) = d(v_0)\), we determine the maximal and the second maximal signless Laplacian spectral radii together with the corresponding extremal graphs. By an analogous manner, we obtain similar results on the spectral radius of triangle-free quasi-tree graphs.
The notation \(tK_3\) represents a graph with \(t\) copies of the complete graph \(K_3\). In this note, we discuss the goodness of path \(P_n\) or cycle \(C_n\) with respect to \(tK_3\). Furthermore, this result provides the computation of Ramsey number \(R(G, tK_3)\) when \(G\) is a set of disjoint paths or cycles.
A \(k\)-king in a digraph \(D\) is a vertex which can reach every other vertex by a directed path of length at most \(k\). Every tournament with no vertex of in-degree zero has at least three \(2\)-kings. In this paper, we present the structure of tournaments which have exactly three \(2\)-kings and prove that every strong tournament, containing at least \(k+2\) vertices with \(k \geq 3\), has at least \(k+1\) \(k\)-kings.
A kernel in a directed graph \(D(V, E)\) is a set \(S\) of vertices of \(D\) such that no two vertices in \(S\) are adjacent and for every vertex \(u\) in \(V \setminus S\) there is a vertex \(v\) in \(S\), such that \((u,v)\) is an arc of \(D\). The definition of kernel implies that the vertices in the kernel form an independent set. If the vertices of the kernel induce an independent set of edges, we obtain a variation of the definition of the kernel, namely a total-kernel. The problem of existence of a kernel is itself an NP-complete problem for a general digraph. But in this paper, we solve the strong total-kernel problem of an oriented Circular Ladder and Möbius Ladder in polynomial time.
For two vertices \(u\) and \(v\) of a nontrivial connected graph \(G\), the set \(I[u,v]\) consists of all vertices lying on some \(u-v\) geodesic in \(G\), including \(u\) and \(v\). For \(S \subseteq V(G)\), the set \(Z[S]\) is the union of all sets \(I[u,v]\) for \(u,v \in S\). A set \(S \subseteq V(G)\) is a connected geodetic set of \(G\) if \(Z[S] = V(G)\) and the subgraph in \(G\) induced by \(S\) is connected. The minimum cardinality of a connected geodetic set of \(G\) is the connected geodetic number \(g_c(G)\) of \(G\) and a connected geodetic set of \(G\) whose cardinality equals \(g_c(G)\) is a minimum connected geodetic set of \(G\). A subset \(T\) of a minimum connected geodetic set \(S\) is a forcing subset for \(S\) if \(S\) is the unique minimum connected geodetic set of \(G\) containing \(T\). The forcing connected geodetic number \(f(S)\) of \(S\) is the minimum cardinality of a forcing subset of \(S\) and the forcing connected geodetic number \(f(G)\) of \(G\) is the minimum forcing connected geodetic number among all minimum connected geodetic sets of \(G\). Therefore, \(0 \leq f_c(G) \leq g_c(G)\). We determine all pairs \((a,b)\) of integers such that \(f_c(G) = a\) and \(gc(G) = b\) for some nontrivial connected graph \(G\). We also consider a problem of realizable triples of integers.
Hovey [11] called a graph \(G\) \(A\)-cordial, where \(A\) is an additive Abelian group, and \(f: V(G) \to A\) is a labeling of the vertices of \(G\) with elements of \(A\) such that when the edges of \(G\) are labeled by the induced labeling \(f: E(G) \to A\) by \(f^*(xy) = f(x) + f(y)\), then the number of vertices (resp. edges) labeled with \(\alpha\) and the number of vertices (resp. edges) labeled with \(\beta\) differ by at most one for all \(\alpha, \beta \in A\). When \(A = \mathbb{Z}_k\), we call a graph \(G\) \(k\)-cordial instead of \(\mathbb{Z}_k\)-cordial. In this paper, we give a sufficient condition for the join of two \(k\)-cordial graphs to be \(k\)-cordial and we give also a necessary condition for certain Eulerian graphs to be \(k\)-cordial when \(k\) is even, and finally we complete the characterization of the \(4\)-cordiality of the complete tripartite graph.
Let \(*\) be a binary graph operation. We call \(*\) a Cayley operation if \(\Gamma_1 * \Gamma_2\) is a Cayley graph for any two Cayley graphs \(\Gamma_1\) and \(\Gamma_2\) . In this paper, we prove that the Cartesian, (categorical or tensor) direct, and lexicographic products are Cayley operations. We also investigate the following question: Under what conditions on a binary graph operation \(*\) and Cayley graphs \(\Gamma_1\) and \(\Gamma_2\), the graph product \(\Gamma_1 * \Gamma_2\) is again a Cayley graph. The latter question is studied for the union, join (sum), replacement, and zig-zag products of graphs.
Let \(R(G)\) be the graph obtained from \(G\) by adding a new vertex corresponding to each edge of \(G\) and by joining each new vertex to the end vertices of the corresponding edge. Let \(RT(G)\) be the graph obtained from \(R(G)\) by adding a new edge corresponding to every vertex of \(G\), and by joining the end vertices of each new edge to the corresponding vertex of \(G\). In this paper, we determine the Laplacian polynomials of \(RT(G)\) of a regular graph \(G\). Moreover, we derive formulae and lower bounds of Kirchhoff indices of the graphs. Finally, we also present the formulae for calculating the Kirchhoff indices of some special graphs as applications, which show the correction and efficiency of the proposed results.
For integer \(n \geq 2\), let \(a_1, a_2, a_3, \ldots, a_n\) be an increasing sequence of nonnegative integers, and define the \(n\)-star \(St(a_1, a_2, \ldots, a_n)\) as the disjoint union of the \(n\) star graphs \(K(1, a_1), K(1, a_2), \ldots, K(1, a_n)\). In this paper, we have partially settled the conjecture by Lee and Kong [4] that says for any odd \(n \geq 3\), the \(n\)-star \(St(a_1, a_2, \ldots, a_n)\) is super edge magic. We solve the two cases:
1. The \(n\)-star \(St(a_1, a_2, \ldots, a_n)\) is super edge magic where \(a_i = 1 + (i – 1)d\) for all integers \(1 \leq i \leq n\) and \(d\) is any positive integer.
2. An \(n\)-star \(St(a_1, a_2, \ldots, a_n)\) is not super edge magic when \(a_1 = 0\).
Let \(G\) be a simple connected graph with the vertex set \(V(G)\). The eccentric distance sum of \(G\) is defined as \(\xi^d(G) = \sum_{v \in V(G)} \varepsilon(v) D_G(v)\), where \(\varepsilon(v)\) is the eccentricity of the vertex \(v\) and \(D_G(v)\) is the sum of all distances from the vertex \(v\). The Harary index of \(G\) is defined as \(H(G) = \sum_{u,v \in V(G)} \frac{1}{d(u, v)}\), where \(d(u, v)\) is the distance between \(u\) and \(v\) in \(G\). The degree powers of \(G\) is defined as \(H(G) = \sum_{|u,v| \subseteq V(G)} \frac{1}{d(u,v)}\) for the natural number \(p \geq 1\). In this paper, we determine the extremal graphs with the minimal eccentric distance sum, the maximal Harary index, and the maximal degree powers among all graphs with given diameter.
Let \(\Gamma(V, E)\) be a graph of order \(n\), \(S \subset V\), and let \(B(S)\) be the set of vertices in \(V \setminus S\) that have a neighbor in \(S\). The differential of a set \(S\) is defined as \(\partial(S) = |B(S)| – |S|\), and the differential of the graph \(\Gamma\) is defined as \(\partial(\Gamma) = \max\{\partial(S) : S \subset V\}\). In this paper, we obtain several tight bounds for the differential in Cartesian product graphs. In particular, we relate the differential in Cartesian product graphs with some known parameters of \(\Gamma_1 \times \Gamma_2\), namely, its domination number, its maximum and minimum degree, and its order. Furthermore, we compute explicitly the differential of some classes of product graphs.
The necessary and sufficient conditions for a given sequence of positive integers \(d_1, d_2, \ldots, d_n\) to be the degree sequence of \(3\)-connected graphs and cactus graphs are proved respectively by S. L. Hakimi [5] and A. R. Rao [6]. In this note, we utilize these results to prove a formula for the functions \(d_{tc}(2m)\) and \(d_{ca}(2m)\), the number of degree sequences with degree sum \(2m\) by \(3\)-connected graphs and cactus graphs respectively. We give generating function proofs and elementary proofs of the formulas \(d_{tc}(2m)\) and \(d_{ca}(2m)\).
In this paper, the graphs with maximal (signless Laplacian) spectral radius among all connected graphs with given matching number are characterized.
Let \(c\) be a proper \(k\)-coloring of a connected graph \(G\) and \(\Pi = (V_1, V_2, \ldots, V_k)\) be an ordered partition of \(V(G)\) into the resulting color classes. For a vertex \(v\) of \(G\), the color code of \(v\) with respect to \(\Pi\) is defined to be the ordered \(k\)-tuple \(c_\Pi := (d(v, V_1), d(v, V_2), \ldots, d(v, V_k))\), where \(d(v, V_i) = \min\{d(v, x) \mid x \in V_i\}\) for \(1 \leq i \leq k\). If distinct vertices have distinct color codes, then \(c\) is called a locating coloring. The minimum number of colors needed in a locating coloring of \(G\) is the locating chromatic number of \(G\), denoted by \(\chi_L(G)\). In this paper, we study the locating chromatic numbers of grids, the cartesian product of paths and complete graphs, and the cartesian product of two complete graphs.
A graph \(\Gamma\) is said to be \((G, 2)\)-distance-transitive if, for \(i = 1, 2\) and for any two vertex pairs \((u_1, v_1)\) and \((u_2, v_2)\) with \(d_\Gamma(u_1, v_1) = d_\Gamma(u_2, v_2) = i\), there exists \(g \in G\) such that \((u_1, v_1)^g = (u_2, v_2)\). This paper classifies the family of \((G, 2)\)-distance-transitive graphs of valency \(7\).
We investigate the group choice number of a graph \(G\) and prove the group list coloring version of Brooks’ Theorem, the group list coloring version of Szekeres-Wilf extension of Brooks’ Theorem, and the Nordhaus-Gaddum inequalities for group choice numbers. Furthermore, we characterize all \(D\)-group choosable graphs and all \(3\)-group choosable complete bipartite graphs.
We study a poset of compositions restricted by part size under a partial ordering introduced by Björner and Stanley. We show that our composition poset \(C_{n, k}\) is isomorphic to the poset of words \(A_{d}^{*}\). This allows us to use techniques developed by Björner to study the Möbius function of \(C_{d+1}\). We use counting arguments and shellability as avenues for proving that the Möbius function is \(\mu(u, w) = (-1)^{|u|+|w|}{\binom{w}{u}}_{dn}\), where \({\binom{w}{u}}_{dn}\) is the number of \(d\)-normal embeddings of \(u\) in \(w\). We then prove that the formal power series whose coefficients are given by the zeta and the Möbius functions are both rational. Following in the footsteps of Björner and Reutenauer and Björner and Sagan, we rely on definitions to prove rationality in one case, and in another case we use finite-state automata.
The distribution of the set of embeddings of a graph into orientable or non-orientable surfaces is called the total embedding distribution. Chen, Gross, and Rieper [Discrete Math. \(128(1994) 73-94.]\) first used the overlap matrix for calculating the total embedding distributions of necklaces, closed-end ladders, and cobblestone paths. In this paper, also by using the overlap matrix, closed formulas of the total embedding distributions for two classes of graphs are given.
In this paper, we obtained two flag-transitive symmetric \((v, k, \lambda)\) designs admitting primitive automorphism groups of almost simple type with socle \(X = \mathrm{PSL}(12, 2)\).
In this paper, we present a new combinatorial problem, called the Nearly Perfect Bipartition Problem, which is motivated by a computer networks application. This leads to a new graph parameter, \(PN_p(G)\), which equals the maximum cardinality of a proper nearly perfect set. We show that the corresponding decision problem is \(NP\)-hard, even when restricted to graphs of diameter \(3\). We present several bounds for \(PN_p(G)\) and determine the value of \(PN_p(G)\) for several classes of graphs.
In this paper we determine the exact values of the signed domination number, signed total domination number, and minus domination number of complete multipartite graphs, which substantially generalizes some previous results obtained for special subclasses of complete multipartite graphs such as cliques and complete bipartite graphs.
In the paper, we discuss properties of the (super) vertex-graceful labeling of cycle \(C_n\), crown graph \(C_n \odot K_1\), and generalized crown graph \(C_n \odot K_{1,t}\), and prove that \(C_n\), \(C_{n} \odot K_1\), and \(C_n \odot K_{1,t}\) are vertex-graceful if \(n\) is odd; \(C_n\) is super vertex-graceful if \(n \neq 4, 6\); and \(C_{n} \odot K_1\) is super vertex-graceful if \(n\) is even. Moreover, we propose two conjectures on (super)vertex-graceful labeling.
For any integer \(m \geq 2\), let \(\mu_m\) be the group of \(m\)th roots of unity. Let \(p\) be a prime and \(a\) a positive integer. For \(m = p^\alpha\), it is shown that there is no \(n \times n\) matrix over \(\mu_m\) with vanishing permanent if \(n < p\).
A subset \(S \subseteq V(G)\) is an independent dominating set for \(G\) if \(S\) is independent and each vertex of \(G\) is either in \(S\) or adjacent to some vertex of \(S\). Let \(i(G)\) denote the minimum cardinality of an independent dominating set for \(G\). For a positive integer \(t\), a graph \(G\) is \(t\)-i-critical if \(i(G) = t\), but \(i(G + uv) < t\) for any pair of non-adjacent vertices \(u\) and \(v\) of \(G\). Further, for a positive integer \(k\), a graph \(G\) is \(k\)-factor-critical if for every \(S \subseteq V(G)\) with \(|S| = k\), \(G – S\) has a perfect matching. In this paper, we provide sufficient conditions for connected \(3\)-i-critical graphs to be \(k\)-factor-critical in terms of connectivity and minimum degree.
Let \(G = (V, E)\) be a simple graph, \(I(G)\) its incidence matrix. The incidence energy of \(G\), denoted by \(IE(G)\), is the sum of the singular values of \(I(G)\). The incidence energy \(IE(G)\) of a graph is a recently proposed quantity. However, \(IE(G)\) is closely related with the eigenvalues of the Laplacian and signless Laplacian matrices of \(G\). The trees with the maximal, the second maximal, the third maximal, the smallest, the second smallest, and the third smallest incidence energy were characterized. In this paper, the trees with the fourth and fifth smallest incidence energy are characterized by the quasi-order method and Coulson integral formula, respectively. In addition, the fourth maximal incidence energy among all trees on \(n\) vertices is characterized.
A Roman dominating function (or simply RDF) on a graph \(G = (V(G), E(G))\) is a labeling \(f: V(G) \to \{0, 1, 2\}\) satisfying the condition that every vertex with label \(0\) has at least a neighbor with label \(2\). The Roman domination number, \(\gamma_R(G)\), of \(G\) is the minimum of \(\sum_{v \in V(G)} f(v)\) over such functions. The Roman bondage number, \(b_R(G)\), of a graph \(G\) with maximum degree at least two is the minimum cardinality among all sets \(E \subseteq E(G)\) for which \(\gamma_R(G – E) > \gamma_R(G)\). It was conjectured that if \(G\) is a graph of order \(n\) with maximum degree at least two, then \(b_R(G) \leq n – 1\). In this paper, we settle this conjecture. More precisely, we prove that for every connected graph of order \(n \geq 3\), \(b_R(G) \leq \min\{n – 1, n – \gamma_R(G) + 5\}\).
Let \(G\) be a finite and simple graph with vertex set \(V(G)\), and let \(f: V(G) \to \{-1, 1\}\) be a two-valued function. If \(k \geq 1\) is an integer and \(\sum_{x\in N[v]}f(x) \geq k\) for each \(v \in V(G)\), where \(N[v]\) is the closed neighborhood of \(v$, then \(f\) is a signed \(k\)-dominating function on \(G\). A set \(\{f_1, f_2, \ldots, f_d\}\) of distinct signed \(k\)-dominating functions on \(G\) with the property that \(\sum_{i=1}{d}f_i(v) \leq j\) for each \(x \in V(G)\), is called a signed \((j, k)\)-dominating family (of functions) on \(G\), where \(j \geq 1\) is an integer. The maximum number of functions in a signed \((j, k)\)-dominating family on \(G\) is the signed \((j, k)\)-domatic number on \(G\), denoted by \(d_{jkS}(G)\).
The aim of this paper is to classify the vertex-primitive symmetric graphs of order \(6p\). These works were essentially done in \([1]\). But in \([1]\) there is no such situation: \(G = \mathrm{PSL}(2, 13)\) acting on the set of cosets of subgroup \(H \cong D_{14}\). Then \(m = |\Omega| = 78 = 6p\), \(G\) has rank \(9\), and the sub-orbits of \(G\) have one of length \(1\), five of length \(7\), and three of length \(14\). In this paper, we give a complete list of symmetric graphs of order \(6p\).
Let \(p\) be an odd prime and \(n\) be a positive integer. For any positive integer \(d \leq n\), let \(g_1(x) = 1 + x^{p^{n-d}} + x^{{2p}^{n-d}} + \ldots + x^{(p-1)p^{n-d}}\) and \(g_2(x) = 1 + x^{p^{n-d+1}} + x^{2p^{n-d+1}} + \ldots + x^{{(p^{d-1}-1)}{p^{n-d+1}}}\). In this paper, we provide a method to determine the weight distributions of binary cyclic codes of length \(p^n\) generated by the polynomials \(g_1(x)\) and \(g_01(x)g_2(x)\), which is effective for small values of \(p\) and \(d\).
A spanning tree with no vertices of degree two of a graph is called a homeomorphically irreducible spanning tree (or HIST) of the graph. It has been proved that every planar triangulation \(G\) with at least four vertices has a HIST \(H\) [1]. However, the previous result asserts nothing whether the degree of a fixed vertex \(v\) of \(G\) is at least three or not in \(H\). In this paper, we prove that if a planar triangulation \(G\) has \(2n\) (\(n \geq 2\)) vertices, then, for any vertex \(v\), \(G\) has a HIST \(H\) such that the degree of \(v\) is at least three in \(H\). We call such a spanning tree a rooted HIST of \(G\) with root \(v\).
A graph \(G\) is Hamiltonian connected, if there is a Hamiltonian path between every two distinct vertices of \(G\). A Hamiltonian connected graph \(G\) is called critical Hamiltonian connected (CHC), if for every edge \(e\) in \(G\), the graph \(G – e\) is not Hamiltonian connected. In this paper, we study the properties of CHC graphs.
A generalized \(\theta\)-graph is composed of at least three internal disjoint paths (at most one of them is with length 1) which have the same initial vertex and the same terminal vertex. If the initial vertex and the terminal vertex are the same in a generalized \(\theta\)-graph, then the generalized \(\theta\)-graph is called a degenerated \(\theta\)-graph or a petal graph. In this paper, two graft transformations that increase or decrease the \(Q\)-spectral radius of a graph are represented. With them, for the generalized \(\theta\)-graphs and petal graphs with order \(n\), the extremal graphs with the maximal \(Q\)-spectral radius and the extremal graphs with the minimal \(Q\)-spectral radius are characterized, respectively.
This paper discusses the permutations that are generated by rotating \(k \times k\) blocks of squares in a union of overlapping \(k \times (k + 1)\) rectangles. It is found that the single-rotation parity constraints effectively determine the group of accessible permutations. If there are \(m\) squares, and the space is partitioned as a checkerboard with \(m\) squares shaded and \(n – m\) squares unshaded, then the four possible cases are \(A_n\), \(S_n\), \(A_m \times A_{n-m}\), and the subgroup of all even permutations in \(S_m \times S_{n-m}\), with exceptions when \(k = 2\) and \(k = 3\).
The packing and covering numbers for the 4-stars were determined by Roditty in 1986. In this paper, we improve and extend these results by finding a corresponding maximum packing and minimum covering of the complete graph with 4-stars for every possible leave graph and excess graph.
We examine the Borda voting method, which has numerous interesting mathematical properties. We determine when a candidate can win a Borda election with all \(i\)th place votes and present a method of constructing ballots that yield such a victory. Then we present a connection between Borda elections and semi-magic squares. We show how a Borda election result gives rise to a semi-magic square, and we show that given any semi-magic square there exists at least one Borda election result corresponding to it.
In 2000, Rees and Shalaby constructed simple indecomposable two-fold cyclic triple systems for all \(v \equiv 0, 1, 3, 4, 7, \text{ and } 9 \pmod{12}\) where \(v = 4\) or \(v \geq 12\), using Skolem-type sequences.
We construct, using Skolem-type sequences, three-fold triple systems having the properties of being cyclic, simple, and indecomposable for all admissible orders \(v\), with some possible exceptions for \(v = 9\) and \(v = 24c + 57\), where \(c \geq 2\) is a constant.
Counting the number of maximal independent sets is \(\#P\)-complete even for chordal graphs. We prove that the number of maximal independent sets in a subclass \({G}_n^R\) (Right power set graphs) of chordal graphs can be computed in polynomial time using Golomb’s nonlinear recurrence relation. We provide a recursive construction of \({G}_n^R\) and prove that there are \(2^\frac{|V({G}_n^R)|+1}{4}\) maximum independent sets in \({G}_n^R\). We also provide a polynomial-time algorithm to solve the maximum independent set problem (MISP) in a superclass \(\mathcal{F}_n\) of the complement of \({G}_n^R\).
The eccentric connectivity index of the molecular graph \(G\) was proposed by Sharma, Goswami, and Madan in 1997 \cite{17}. This index is defined as \(\xi^c(G) = \sum_{v\in V(G)} \deg(v) \, ec(v)\), where \(\deg(v)\) is the degree of vertex \(v\) in \(G\) and eccentricity \(ec(v)\) is the largest distance between \(v\) and any other vertex of \(G\). Thus, in this paper, we established the general formulas for the eccentric connectivity index of joining a special graph to its paths and of joining two different graphs by a path. Proofs were also provided.
We present a design for a seven-game tournament of the \(7\)-player board game \({Diplomacy}\), in which each player plays each country one time and each pair of players shares a border either \(4\) or \(5\) times. It is impossible for each pair of players to share a border the same number of times in such a tournament, and so the tournament presented is the most “balanced” possible in this sense. A similarly balanced tournament can be constructed for a generalized version of the game involving an arbitrary number of countries. We also present an infinite family of graphs that cannot be balanced.
A graph \(G = (V, E)\) with \(p\) vertices and \(q\) edges is called a Harmonic mean graph if it is possible to label the vertices \(v \in V\) with distinct labels \(f(v)\) from \(1, 2, \dots, q+1\) in such a way that when each edge \(e = uv\) is labeled with \(f(e = uv) = \left\lceil\frac{2f(u)f(v)}{f(u) + f(v)}\right\rceil\) or \(\left\lfloor \frac{2f(u)f(v)}{f(u) + f(v)} \right\rfloor\), then the edge labels are distinct. In this case, \(f\) is called a Harmonic mean labeling of \(G\). In this paper, we investigate some new families of Harmonic mean graphs.
The clique sum \(\Sigma = G[G_1,G_2,\ldots,G_n]\) is the lexicographic sum over \(G\) where each fiber \(G_i\) is a clique. We show the reconstruction number of \(\Sigma\) is three unless \(\Sigma\) is vertex transitive and \(G\) has order at least two. In the latter case, it follows that \(\Sigma = G[K_m]\) is a lexicographic product, and the reconstruction number is \(m+2\). This complements the bounds of Brewster, Hahn, Lamont, and Lipka. It also extends the work of Myrvold and Molina.
We provide a new proof of a result of Hanson and Toft classifying the maximum-size \(K_r\)-free graphs on \(n\) vertices with chromatic number at least \(r\).
Much research has been done on the edge decomposition of \(\lambda\) copies of the complete graph \(G\) with respect to some specified subgraph \(H\) of \(G\). This is equivalent to the investigation of \((G, H)\)-designs of index \(\lambda\). In this paper, we present a fundamental theorem on the decomposition of \(\lambda\) copies of a complete bipartite graph. As an application of this result, we show that necessary conditions are sufficient for the decomposition of \(\lambda\) copies of a complete bipartite graph into several multi-subgraphs \(H\) with a number of vertices less than or equal to \(4\) and the number of edges less than or equal to \(4\), with some exceptions where decompositions do not exist. These decomposition problems are interesting to study as various decompositions do not exist even when necessary conditions are satisfied.
If the integer \(r \geq 2\), say that a composition of the natural number \(n\) is \(r\)-\({regular}\) if no part is divisible by \(r\). Let \(c_r(n)\) denote the number of \(r\)-regular compositions of \(n\) (with \(c_r(0) = 1\)). We show that \(c_r(n)\) satisfies a linear recurrence of order \(r\). We also obtain asymptotic estimates for \(c_r(n)\), and we evaluate \(c_r(n)\) for \(2 \leq r \leq 5\) and \(1 \leq n \leq 10\).
Using only the skein relation and some combinatorics, we find a closed form for the Conway polynomial of \((m,3)\) torus links and a trio of recurrence relations that define the Conway polynomial of any \((m,4)\) torus link.
For positive integers \(c\) and \(d\), let \(K_{c\times d}\) denote the complete multipartite graph with \(c\) parts, each containing \(d\) vertices. Let \(G\) with \(n\) edges be the union of two vertex-disjoint even cycles. We use graph labelings to show that there exists a cyclic \(G\)-decomposition of \(K_{(2n+1)\times t}\), \(K_{(n/2+1)\times 4t}\), \(K_{5\times (n/2)t}\), and of \(K_{2\times 2nt}\) for every positive integer \(t\). If \(n \equiv 0 \pmod{4}\), then there also exists a cyclic \(G\)-decomposition of \(K_{(n+1)\times 2t}\), \(K_{(n/4+1)\times 8t}\), \(K_{9\times (n/4)t}\), and of \(K_{3\times nt}\) for every positive integer \(t\).
For a Hamiltonian graph \(G\), the Hamiltonian cycle extension number of \(G\) is the maximum positive integer \(k\) for which every path of order \(k\) or less is a subpath of some Hamiltonian cycle of \(G\). The Hamiltonian cycle extension numbers of all Hamiltonian complete multipartite graphs are determined. Sharp lower bounds for the Hamiltonian cycle extension number of a Hamiltonian graph are presented in terms of its minimum degree and order, its size and the sum of the degrees of every two non-adjacent vertices. Hamiltonian cycle extension numbers are also determined for powers of cycles.
We give conditions on the numbers \(\{\varphi_{ij}\}\) under which a vertex-degree-based topological index \(TI\) of the form
\[
TI(G) = \sum_{1\leq i\leq j\leq n-1} m_{ij}\varphi_{ij},
\]
where \(G\) is a graph with \(n\) vertices and \(m_{ij}\) is the number of \(ij\)-edges, has the zigzag chain as an extreme value among all polyomino chains. As a consequence, we deduce that over the polyomino chains, the zigzag chain has the maximal value of the Randić index, the sum-connectivity index, the harmonic index, and the geometric-arithmetic index, and the minimal value of the first Zagreb index, second Zagreb index, and atom-bond-connectivity index.
A cluster of \( 2n+1 \) cubes comprising the central cube and reflections in all its faces is called the \( n \)-dimensional cube. If \( 2n+1 \) is not a prime, then there are infinitely many tilings of \( \mathbb{R}^n \) by crosses, but it has been conjectured that there is a unique tiling of \( \mathbb{R}^n \) by crosses otherwise. The conjecture has been proved for \( n=2,3 \), and in this paper, we prove it also for \( n=5 \). So there is a unique tiling of \( \mathbb{R}^3 \) by crosses, there are infinitely many tilings of \( \mathbb{R}^4 \), but for \( \mathbb{R}^5 \), there is again only one tiling by crosses. We consider this result to be a paradox as our intuition suggests that “the higher the dimension of the space, the more freedom we get.
“`
A graph \( G \) is collapsible if for every even subset \( R \subseteq V(G) \), there is a spanning connected subgraph \( H_R \) of \( G \) whose set of odd degree vertices is \( R \). A graph is reduced if it has no nontrivial collapsible subgraphs. Catlin [4] showed that the existence of spanning Eulerian subgraphs in a graph \( G \) can be determined by the reduced graph obtained from \( G \) by contracting all the collapsible subgraphs of \( G \). In this paper, we present a result on 3-edge-connected reduced graphs of small orders. Then, we prove that a 3-edge-connected graph \( G \) of order \( n \) either has a spanning Eulerian subgraph or can be contracted to the Petersen graph if \( G \) satisfies one of the following:
These are improvements of prior results in [16], [18], [24], and [25].
In this note, we consider the lexicographical ordering by spectral moments of trees with a given degree sequence. Such questions have been studied for a variety of different categories of trees. Particularly, the last tree in this ordering among trees with a given degree sequence was recently identified in two independent manuscripts. The characterization of the first such trees, however, remains open. We make some progress on this question in this note, by making use of the interpretation of the spectral moment in terms of numbers of paths and the product of adjacent vertex degrees, the first trees are characterized with the additional condition that the nonleaf vertex degrees are different from each other. We also comment on the case when there are repetitions in the vertex degrees.
We determine all 120 nonisomorphic systems obtainable from the projective Steiner triple system of order 31 by at most three Pasch trades. Exactly three of these, each corresponding to three Pasch trades, are rigid. Thus three Pasch trades suffice, and are required, in
order to convert the projective system of order 31 to a rigid system. This contrasts with the projective system of order 15 where four Pasch trades are required. We also show that four Pasch trades are required in order to convert the projective system of order 63 to a
rigid system.
In the paper “Eternal security in graphs” by Goddard, Hedetniemi and Hedetniemi (2005, [4]), the authors claimed that, for any Cayley graph, the eternal \(m\)-security number equals the minimum cardinality of a dominating set. However, the equality is false. In this note, we present a counterexample and comment on the eternal \(m\)-security number for Cayley graphs.
