Relating Pairs of Distance Domination Parameters

Michael A. Henning1, Ortrud R. Oellermann 2, Henda C. Swart2
1Department of Mathematics University of Natal Pietermaritzburg, South Africa
2Department of Mathematics University of Natal Durban, South Africa

Abstract

Let \(n \geq 1\) be an integer and let \(G\) be a graph of order \(p\). A set \(\mathcal{D}\) of vertices of \(G\) is an \(n\)-dominating set (total \(n\)-dominating) set of \(G\) if every vertex of \(V(G) – \mathcal{D}\) (\(V(G)\), respectively) is within distance \(n\) from some vertex of \(\mathcal{D}\) other than itself. The minimum cardinality among all \(n\)-dominating sets (respectively, total \(n\)-dominating sets) of \(G\) is called the \(n\)-domination number (respectively, total \(n\)-domination number) and is denoted by \(\gamma_n(G)\) (respectively, \(\gamma_n^t(G)\)). A set \(\mathcal{I}\) of vertices of \(G\) is \(n\)-independent if the distance (in \(G\)) between every pair of distinct vertices of \(\mathcal{I}\) is at least \(n+1\). The minimum cardinality among all maximal \(n\)-independent sets of \(G\) is called the \(n\)-independence number of \(G\) and is denoted by \(i_n(G)\). Suppose \(\mathcal{I}_k\) is an \(n\)-independent set of \(k\) vertices of \(G\) for which there exists a vertex \(v\) of \(G\) that is within distance \(n\) from every vertex of \(\mathcal{I}_k\). Then a connected subgraph of minimum size that contains the vertices of \(\mathcal{I}_k \cup \{v\}\) is called an \(n\)-generalized \(K_{1,k}\) in \(G\). It is shown that if \(G\) contains no \(n\)-generalized \(K_{1,3}\), then \(\gamma_n(G) = i_n(G)\). Further, it is shown if \(G\) contains no \(n\)-generalized \(K_{1,{k+1}}\), \(k \geq 2\), then \(i_n(G) \leq (k-1)\gamma_n(G) – (k-2)\). It is shown that if \(G\) is a connected graph with at least \(n + 1\) vertices, then there exists a minimum \(n\)-dominating set \(\mathcal{D}\) of \(G\) such that for each \(d \in \mathcal{D}\), there exists a vertex \(v \in V(G) – \mathcal{D}\) at distance \(n\) from \(d\) and distance at least \(n+1\) from every vertex of \(\mathcal{D} – \{d\}\). Using this result, it is shown if \(G\) is a connected graph on \(p \geq 2n+1\) vertices, then \(\gamma_n(G) \leq p/(n + 1)\) and that \(i_n(G) + n\gamma_n(G) \leq p\). Finally, it is shown that if \(T\) is a tree on \(p \geq 2n + 1\) vertices, then \(i_n(G) + n\gamma_n^t(G) \leq p\).