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We determine upper bounds for $\gamma \left(Q{n}^t\right)$ and $i\left(Q{ₙ}^t\right)$, the domination and independent domination numbers, respectively, of the graph $Q{ₙ}^t$ obtained from the moves of queens on the n×n chessboard drawn on the torus.

A secure (total) dominating set of a graph G = (V,E) is a (total) dominating set X ⊆ V with the property that for each u ∈ V-X, there exists x ∈ X adjacent to u such that $(X-x)\cup u$ is (total) dominating. The smallest cardinality of a secure (total) dominating set is the secure (total) domination number ${\gamma }_s\left(G\right)\left({\gamma }_{st}\left(G\right)\right)$. We characterize graphs with equal total and secure total domination numbers. We show that if G has minimum degree at least two, then ${\gamma }_{st}\left(G\right)\le {\gamma }_s\left(G\right)$. We also show that ${\gamma }_{st}\left(G\right)$ is at most twice the clique covering number of...

The paired domination number ${\gamma }_{pr}\left(G\right)$ of a graph G is the smallest cardinality of a dominating set S of G such that ⟨S⟩ has a perfect matching. The generalized prisms πG of G are the graphs obtained by joining the vertices of two disjoint copies of G by |V(G)| independent edges. We provide characterizations of the following three classes of graphs: ${\gamma }_{pr}\left(\pi G\right)=2{\gamma }_{pr}\left(G\right)$ for all πG; ${\gamma }_{pr}\left(K₂☐G\right)=2{\gamma }_{pr}\left(G\right)$; ${\gamma }_{pr}\left(K₂☐G\right)={\gamma }_{pr}\left(G\right)$.

Let G ☐ H denote the Cartesian product of the graphs G and H. In 2004, Hartnell and Rall [On dominating the Cartesian product of a graph and K₂, Discuss. Math. Graph Theory 24(3) (2004), 389-402] characterized prism fixers, i.e., graphs G for which γ(G ☐ K₂) = γ(G), and noted that γ(G ☐ Kₙ) ≥ min{|V(G)|, γ(G)+n-2}. We call a graph G a consistent fixer if γ(G ☐ Kₙ) = γ(G)+n-2 for each n such that 2 ≤ n < |V(G)|- γ(G)+2, and characterize this class of graphs. Also in 2004, Burger,...

A graph G is called a prism fixer if γ(G×K₂) = γ(G), where γ(G) denotes the domination number of G. A symmetric γ-set of G is a minimum dominating set D which admits a partition D = D₁∪ D₂ such that $V\left(G\right)-N\left[D_i\right]=D_j$, i,j = 1,2, i ≠ j. It is known that G is a prism fixer if and only if G has a symmetric γ-set. Hartnell and Rall [On dominating the Cartesian product of a graph and K₂, Discuss. Math. Graph Theory 24 (2004), 389-402] conjectured that if G is a connected, bipartite graph such that V(G) can be partitioned...

In this paper we continue the study of paired-domination in graphs introduced by Haynes and Slater (Networks (1998), 199–206). A paired-dominating set of a graph $G$ with no isolated vertex is a dominating set of vertices whose induced subgraph has a perfect matching. The paired-domination number of $G$, denoted by ${\gamma }_{\mathrm{pr}}\left(G\right)$, is the minimum cardinality of a paired-dominating set of $G$. The graph $G$ is paired-domination vertex critical if for every vertex $v$ of $G$ that is not adjacent to a vertex of degree one,...

For a permutation π of the vertex set of a graph G, the graph π G is obtained from two disjoint copies G₁ and G₂ of G by joining each v in G₁ to π(v) in G₂. Hence if π = 1, then πG = K₂×G, the prism of G. Clearly, γ(G) ≤ γ(πG) ≤ 2 γ(G). We study graphs for which γ(K₂×G) = 2γ(G), those for which γ(πG) = 2γ(G) for at least one permutation π of V(G) and those for which γ(πG) = 2γ(G) for each permutation π of V(G).

A multigraph G is triangle decomposable if its edge set can be partitioned into subsets, each of which induces a triangle of G, and rationally triangle decomposable if its triangles can be assigned rational weights such that for each edge e of G, the sum of the weights of the triangles that contain e equals 1. We present a necessary and sufficient condition for a planar multigraph to be triangle decomposable. We also show that if a simple planar graph is rationally triangle decomposable, then it...