Excellent graphs with respect to domination: subgraphs induced by minimum dominating sets

Abstract

A graph $G=(V,E)$ is $\gamma$-excellent if $V$ is a union of all $\gamma$-sets of $G$, where $\gamma$ stands for the domination number. Let $\mathcal{I}$ be a set of all mutually nonisomorphic graphs and $\emptyset \not= \mathcal{H} \subsetneq \mathcal{I}$. In this paper we initiate the study of the $\mathcal{H}$-$\gamma$-excellent graphs, which we define as follows. A graph $G$ is $\mathcal{H}$-$\gamma$-excellent if the following hold: (i) for every $H \in \mathcal{H}$ and for each $x \in V(G)$ there exists an induced subgraph $H_x$ of $G$ such that $H$ and $H_x$ are isomorphic, $x \in V(H_x)$ and $V(H_x)$ is a subset of some $\gamma$-set of $G$, and (ii) the vertex set of every induced subgraph $H$ of $G$, which is isomorphic to some element of $\mathcal{H}$, is a subset of some $\gamma$-set of $G$. For each of some well known graphs, including cycles, trees and some cartesian products of two graphs, we describe its largest set $\mathcal{H} \subsetneq \mathcal{I}$ for which the graph is $\mathcal{H}$-$\gamma$-excellent. Results on $\gamma$-excellent regular graphs and a generalized lexicographic product of graphs are presented. Several open problems and questions are posed.