Defensive k-alliances in graphs

Let $\Gamma=(V,E)$ be a simple graph. For a nonempty set $X\subseteq V$, and a vertex $v\in V$, $\delta_{X}(v)$ denotes the number of neighbors $v$ has in $X$. A nonempty set $S\subseteq V$ is a \emph{defensive $k$-alliance} in $\Gamma=(V,E)$ if $\delta_S(v)\ge \delta_{\bar{S}}(v)+k,$ $\forall v\in S.$ The defensive $k$-alliance number of $\Gamma$, denoted by $a_k(\Gamma)$, is defined as the minimum cardinality of a defensive $k$-alliance in $\Gamma$. We study the mathematical properties of $a_k(\Gamma)$.