Let $G$ be a graph and $tau$ be an assignment of nonnegative integer thresholds to the vertices of $G$. A subset of vertices $D$ is said to be a $tau$-dynamic monopoly, if $V(G)$ can be partitioned into subsets $D_0, D_1, ldots, D_k$ such that $D_0=D$ and for any $iin {0, ldots, k-1}$, each vertex $v$ in $D_{i+1}$ has at least $tau(v)$ neighbors in $D_0cup ldots cup D_i$. Denote the size of smallest $tau$-dynamic monopoly by $dyn_{tau}(G)$ and the average of thresholds in $tau$ by $overline{tau}$. We show that the values of $dyn_{tau}(G)$ over all assignments $tau$ with the same average threshold is a continuous set of integers. For any positive number $t$, denote the maximum $dyn_{tau}(G)$ taken over all threshold assignments $tau$ with $overline{tau}leq t$, by $Ldyn_t(G)$. In fact, $Ldyn_t(G)$ shows the worst-case value of a dynamic monopoly when the average threshold is a given number $t$. We investigate under what conditions on $t$, there exists an upper bound for $Ldyn_{t}(G)$ of the form $c|G|$, where $c<1$. Next, we show that $Ldyn_t(G)$ is coNP-hard for planar graphs but has polynomial-time solution for forests.
تحميل البحث