The bondage number of graphs on topological surfaces: degree-S vertices and the average degree

Abstract

The bondage number $b(G)$ of a graph $G$ is the smallest number of edges whose removal from $G$ results in a graph with larger domination number. An orientable surface $\mathbb{S}_h$ of genus $h$, $h \geq 0$, is obtained from the sphere $\mathbb{S}_0$ by adding $h$ handles. A non-orientable surface $\mathbb{N}_q$ of genus $q$, $q \geq 1$, is obtained from the sphere by adding $q$ crosscaps. The Euler characteristic of a surface is defined by $\chi(\mathbb{S}_h) = 2 - 2h$ and $\chi(\mathbb{S}_q)= 2-q$. Let $G$ be a connected graph of order $n$ which is 2-cell embedded on a surface $\mathbb{M}$ with $\chi(\mathbb{M})= \chi$. We prove that $b(G) \leq 7+i$ when $\mathbb{M} = \mathbb{N}_i$, $i=1,2,3$, and $b(G) \leq 12$ when $\mathbb{M} \in \{\mathbb{N}_4, \mathbb{S}_2\}$. We give new arguments that improve the known upper bounds on the bondage number at least when $-7\chi/(\delta(G) - 5) < n \leq -12\chi$, $\delta(G) \geq 6$, where $\delta(G)$ is the minimum degree of $G$. We obtain sufficient conditions for the validity of the inequality $b(G) \leq 2s-2$, provided $G$ has degree $s$ vertices. In particular, we prove that if $\delta (G) = \delta \geq 6$, $\chi \leq -1$ and $-14\chi < \delta - 4 + 2(\delta -5)n$ then $b(G) \leq 2\delta -2$. We show that if $\gamma (G) = \gamma \not = 2$, where $\gamma (G)$ is the domination number of $G$, then $n \geq \gamma + (1 + \sqrt{9+8\gamma-8\chi})/2$; the bound is tight. We also present upper bounds for the bondage number of graphs in terms of the girth, domination number and Euler characteristic. As a corollary we prove that if $\gamma(G) \geq 4$ and $\chi \leq -1$, then $b(G) \leq 11 - 24\chi/(9 + \sqrt{41 - 8\chi})$. Several unanswered questions are posed.