Given a set \( S \subseteq V \) in a graph \( G = (V, E) \), we say that a vertex \( v \in V \) is perfect if \( |N[v] \cap S| = 1 \), that is, the closed neighborhood \( N[v] = \{v\} \cup \{u \mid uv \in E\} \) of \( v \) contains exactly one vertex in \( S \). A vertex \( v \) is almost perfect if it is either perfect or is adjacent to a perfect vertex. Similarly, we can say that a set \( S \subset V \) is (almost) perfect if every vertex \( v \in S \) is (almost) perfect; \( S \) is externally (almost) perfect if every vertex \( u \in V – S \) is (almost) perfect; and \( S \) is completely (almost) perfect if every vertex \( v \in V \) is (almost) perfect. In this paper, we relate these concepts of perfection to independent sets, dominating sets, efficient and perfect dominating sets, distance-2 dominating sets, and to perfect neighborhood sets in graphs. The concept of a set being almost perfect also provides an equivalent definition of irredundance in graphs.

A graph \( G \) is \( k \)-edge-\( i \)-critical if it has independent domination number \( i(G) = k \), and \( i(G + xy) < i(G) \) whenever \( xy \notin E(G) \). The following results are obtained for \( 3 \)-edge-\( i \)-critical graphs \( G \):

1. If \( \delta \geq 3 \), then \( G \) is hamiltonian.
2. If \( \delta = 2 \), then there is exactly one family of non-hamiltonian graphs.
3. If \( |V(G)| > 6 \), then \( G \) has a Hamilton path.

The proofs of these results rely on a closure operation, a characterization of the \( 2 \)-connected, \( 3 \)-edge-\( i \)-critical graphs with \( \delta = 2 \), and a characterization of the \( 3 \)-edge-\( i \)-critical graphs with a cut vertex.

An identifying code in a graph \( G \) is a set \( D \) of vertices such that the closed neighborhood of each vertex of the graph has a nonempty, distinct intersection with \( D \). The minimum cardinality of an identifying code is denoted \( \gamma^{ID}(G) \). Building upon recent results of Gravier, Moncel, and Semri, we show for \( n \leq m \) that \( \gamma^{ID} (K_n \Box K_m) = \max\{2m – n, m + \lfloor n/2 \rfloor\} \). Furthermore, we improve upon the bounds for \( \gamma^{ID}(G \Box K_m) \) and explore the specific case when \( G \) is the Cartesian product of multiple cliques.

Mobile guards on the vertices of a graph are used to defend it against an infinite sequence of attacks on its vertices. The locations of the guards must induce a vertex cover at all times. We compare this new model of graph protection with other previously studied parameters, including such as the eternal domination number and the variation of the eternal vertex cover problem in which attacks occur at edges

Suppose each vertex in a graph \( G \) has a unit of information and that all the units must be collected at a vertex \( u \) in \( G \). Assuming that a vertex can receive (from its neighbors) an unlimited number of units at each discrete moment but can only send one at a time, find the shortest collection time, \( \operatorname{col}_u(G) \), needed to collect all the information at \( u \) and an optimal protocol that achieves this.

We derive lower and upper bounds for the problem, give a polynomial time algorithm in the general case, and a linear time algorithm for hypercubes.

A (di)graph \( G \) is \({homomorphically; full}\) if every homomorphic image of \( G \) is a sub(di)graph of \( G \). This class of (di)graphs arose in the study of whether a homomorphism from a given graph \( G \) to a fixed graph \( H \) can be factored through a fixed graph \( Y \). Brewster and MacGillivray proved that the homomorphically full irreflexive graphs are precisely the graphs that contain neither \( P_4 \) nor \( 2K_2 \) as an induced subgraph. In this paper, we show that the homomorphically full reflexive graphs are precisely threshold graphs, i.e., the graphs that contain none of \( P_4 \), \( 2K_2 \), and \( C_4 \) as an induced subgraph. We also characterize the reflexive semicomplete digraphs that are homomorphically full, and discuss the relationship of these digraphs and Ferrers digraphs.

The eternal domination number of graph \( G \) is the smallest set of mobile guards which can defend \( G \) against an infinite sequence of attacks on its vertices. In this paper we give results for the eternal domination numbers of \( P_4 \Box P_n \).

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 \( G \). It has been shown that these concepts generalize both edge domination and matchings in graphs. In this paper, we consider the two color frames \( Y_1 \) and \( Y_2 \) that result from the claw \( K_{1,3} \), where \( Y_1 \) has exactly one red edge and \( Y_2 \) has exactly two red edges. An edge \( e \) in a graph \( G \) is a non-claw edge if \( e \) belongs to no claw in \( G \). It is shown that if \( G \) is a connected graph containing \( \ell \) non-claw edges, then \( \chi’_{Y_1}(G) \leq \chi’_{Y_2}(G) \leq 3\chi’_{Y_1}(G) – 2\ell \) and \( \chi’_{Y_1}(G) = \chi’_{Y_2}(G) \) if and only if \( G \) is a path or cycle. Furthermore, a pair \( a, b \) of positive integers can be realized as the \( Y_1 \)-chromatic index and \( Y_2 \)-chromatic index for some connected graph of order at least 4 if and only if \( a \leq b \leq 3a \) and \( b \geq 2 \).

We prove nonexistence of circulant weighing matrices with parameters from seven previously open entries of the updated Strassler’s table. The method of proof utilizes some modular constraints on circulant weighing matrices with multipliers.

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 monophonic set of \( G \) if each vertex \( v \) of \( G \) lies on an \( x-y \) monophonic path for some elements \( x \) and \( y \) in \( S \). The minimum cardinality of a monophonic set of \( G \) is the monophonic number of \( G \), denoted by \( m(G) \). 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 \( dm(G) \). We determine bounds for it and characterize graphs which realize these bounds. Also, for each pair \( a, b \) of integers with \( 2 \leq a \leq b \), we prove that there is a connected graph \( G \) with \( m(G) = a \) and \( dm(G) = b \).