The complexity of open k-monopolies in graphs for negative k

Abstract

Let \(G\) be a graph with vertex set \(V(G)\), \(\delta(G)\) minimum degree of \(G\) and \(k\in\left\{1-\left\lceil\frac{\delta(G)}{2}\right\rceil,\ldots ,\left\lfloor \frac{\delta(G)}{2}\right\rfloor\right\}\). Given a nonempty set \(M\subseteq V(G)\) a vertex \(v\) of \(G\) is said to be \(k\)-controlled by \(M\) if \(\delta_M(v)\ge\frac{\delta_{V(G)}(v)}{2}+k\) where \(\delta_M(v)\) represents the number of neighbors of \(v\) in \(M\). The set \(M\) is called an open \(k\)-monopoly for \(G\) if it \(k\)-controls every vertex \(v\) of \(G\). In this short note we prove that the problem of computing the minimum cardinality of an open \(k\)-monopoly in a graph for a negative integer \(k\) is NP-complete even restricted to chordal graphs.