An edge-coloring of a graph \(G\) with natural numbers \(1,2,\ldots\) is called a sum edge-coloring if the colors of edges incident to any vertex of \(G\) are distinct and the sum of the colors of the edges of \(G\) is minimum. The edge-chromatic sum of a graph \(G\) is the sum of the colors of edges in a sum edge-coloring of \(G\). In general, the problem of finding the edge-chromatic sum of an \(r\)-regular (\(r\geq 3\)) graph is \(NP\)-complete. In this paper we provide some bounds on the edge-chromatic sums of various products of graphs. In particular, we give tight upper bounds on the edge-chromatic sums of tensor, strong tensor, Cartesian, strong products and composition of graphs. We also determine the edge-chromatic sums and edge-strengths of the Cartesian products of regular graphs and paths (cycles) with an even number of vertices. Finally, we determine the edge-chromatic sums and edge-strengths of grids, cylinders, and tori.

Generalised nice sets are defined as subsets of edges of the extended Fano plane satisfying a certain absorbing property. The classification up to collineations, purely combinatorial in nature, provides 245 generalised nice sets. In turn, this gives rise to new Lie algebras obtained by modifying the bracket of homogeneous elements on an initial \(\mathbb Z_2^3\)-graded Lie algebra.

Let \(\alpha(n)\) denote the number of perfect square permutations in the symmetric group \(S_n\). The conjecture \(\alpha(2n+1) = (2n+1) \alpha(2n)\), provided by Stanley [4], was proved by Blum [1] using generating functions. However, several structural questions about these special permutations remained open. This paper presents an alternative and constructive proof for this conjecture, which highlights the deeper interplay between cycle structures and square properties. At the same time, we demonstrate that all permutations with an even number of even cycles in both \(S_{2n}\) and \(S_{2n+1}\) can be categorized into three disjoint types that correspond to each other.

In this paper, we generalize the \(k\)-Jacobsthal sequences and call them the generalized \((k,t)\)-Jacobsthal \(p\)-sequences. Also, we obtain combinatorial identities. Then, the generalized\((k,t)\)-Jacobsthal \(p\)-matrix is used to factorize the Pascal matrix. Finally, using the Riordan method, we obtain two factorizations of the Pascal matrix involving the generalized \((k,t)\)-Jacobsthal \(p\)-sequences.

An open-dominating set \(S\) for a graph \(G\) is a subset of vertices where every vertex has a neighbor in \(S\). An open-locating-dominating set \(S\) for a graph \(G\) is an open-dominating set such that each pair of distinct vertices in \(G\) have distinct set of open-neighbors in \(S\). We consider a type of a fault-tolerant open-locating dominating set called error-detecting open-locating-dominating sets. We present more results on the topic including its NP-completeness proof, extremal graphs, and a characterization of cubic graphs that permit an error-detecting open-locating-dominating set.

In this paper, we explore some interesting applications of the matrix tree theorem. In particular, we present a combinatorial interpretation of a distribution of \((n-1)^{n-1}\), in the context of uprooted spanning trees of the complete graph \(K_{n}\), which was previously obtained by Chauve–Dulucq–Guibert. Additionally, we establish a combinatorial explanation for the distribution of \(m^{n-1}n^{m-1}\), related to spanning trees of the complete bipartite graph \(K_{m,n}\), which seems new. Furthermore, we extend this study to the graph \(K_{n}\setminus \{e_{1,n}\}\), obtained by deleting an edge from \(K_n\), and derive a new identity for the number of its uprooted spanning trees.

The domatic number of a graph is the maximum number of pairwise disjoint dominating sets admitted by the graph. We introduce a game based around this graph invariant. The domatic number game is played on a graph \(G\) by two players, Alice and Bob, who take turns selecting a vertex and placing it into one of \(k\) sets. Alice is trying to make each of these sets into a dominating set of \(G\) while Bob’s goal is to prevent this from being accomplished. The maximum \(k\) for which Alice can achieve her goal when both players are playing optimal strategies, is called the game domatic number of \(G\). There are two versions of the game and two resulting invariants depending on whether Alice or Bob is the first to play. We prove several upper bounds on these game domatic numbers of arbitrary graphs and find the exact values for several classes of graphs including trees, complete bipartite graphs, cycles and some narrow grid graphs. We pose several open problems concerning the effect of standard graph operations on the game domatic number as well as a vexing question related to the monotonicity of the number of sets available to Alice.

Sparse magic squares are a generalization of magic squares and can be used to the magic labeling of graphs. An \(n\times n\) array based on \(\mathcal{X}\)\(=\{0,1,\cdots,nd\}\) is called a sparse magic square of order \(n\) with density \(d\) (\(d<n\)), denoted by SMS\((n,d)\), if each non-zero element of \(\mathcal{X}\) occurs exactly once in the array, and its row-sums, column-sums and two main diagonal sums is the same. An SMS\((n,d)\) is called pandiagonal (or perfect) denoted by PSMS\((n,d)\), if the sum of all elements in each broken diagonal is the same. A PSMS\((n,d)\) is called regular if there are eactly \(d\) positive entries in each row, each column and each main diagonal. In this paper, some construction of regular pandigonal sparse magic squares is provided and it is proved that there exists a regular PSMS\((n,6)\) for all positive integer \(n\equiv 5 \pmod{6}\), \(n>6\).

Two graphs are said to be \(Q\)-cospectral (respectively, \(A\)-cospectral) if they have the same signless Laplacian (respectively, adjacency) spectrum. A graph is said to be \(DQS\) (respectively, \(DAS\)) if there is no other non-isomorphic graphs \(Q\)-cospectral (respectively, \(A\)-cospectral) with it. A tree on \(n\) vertices with maximum degree \(d_1\) is called starlike and denoted by \(ST(n, d_1)\), if it has exactly one vertex with the degree greater than 2. A tree is called double starlike if it has exactly two vertices of degree greater than 2. If we attach \(p\) pendant vertices (vertices of degree 1) to each of pendant vertices of a path \(P_n\), the the resulting graph is called the double starlike tree \(H_n(p,p)\). In this article, we prove that all double starlike trees \(H_n(p,p)\) are \(DQS\), where \(p\geq 1, n\geq 2\) and \(p\) denotes . In addition, by a simple method, we show that all starlike trees are \(DQS\) excluding \(K_{1,3}=ST(4,3)\).