Most Generalized Petersen graphs of girth 8 have cop number 4

A generalized Petersen graph $GP(n,k)$ is a regular cubic graph on $2n$ vertices (the parameter $k$ is used to define some of the edges). It was previously shown (Ball et al., 2015) that the cop number of $GP(n,k)$ is at most $4$, for all permissible values of $n$ and $k$. In this paper we prove that the cop number of "most" generalized Petersen graphs is exactly $4$. More precisely, we show that unless $n$ and $k$ fall into certain specified categories, then the cop number of $GP(n,k)$ is $4$. The graphs to which our result applies all have girth $8$. In fact, our argument is slightly more general: we show that in any cubic graph of girth at least $8$, unless there exist two cycles of length $8$ whose intersection is a path of length $2$, then the cop number of the graph is at least $4$. Even more generally, in a graph of girth at least $9$ and minimum valency $\delta$, the cop number is at least $\delta+1$.