On the global offensive alliance in unicycle graphs

Abstract

For a graph $G=(V,E)$, a set $S\subseteq V$ is a dominating set if every vertex in $V-S$ has at least a neighbor in $S$. A dominating set $S$ is a global offensive alliance if for each vertex $v$ in $V-S$ at least half the vertices from the closed neighborhood of $v$ are in $S.$ The domination number $\gamma(G)$ is the minimum cardinality of a dominating set of $G$, and the global offensive alliance number $\gamma_{o}(G)$ is the minimum cardinality of a global offensive alliance of $G$. We show that if $G$ is a connected unicycle graph of order $n$ with $l(G)$ leaves and $s(G)$ support vertices then $\gamma_{o}(G)\geq\frac{n-l(G)+s(G)}{3}$. Moreover, we characterize all extremal unicycle graphs attaining this bound.