Expanders Satisfy the Weak Meyniel Conjecture

Abstract

We show that if $\{G_n\}_{n\geq 1}$ is a sequence of graphs of order $n$ with bounded maximum degree and isoperimetric function $Φ(G_n,n^{1-α})$ bounded away from $0$ as $n\rightarrow \infty$, then the cop number of $G_n$ is at most $O(n^{\frac{1+α}{2}+o(1)})$. It is unclear if this bound is tight given our assumption that the maximum degree of our sequence is bounded. All the same, recent work by Hosseini, Mohar, and Gonzalez Hermosillo de la Maza strongly motivates considering the bounded degree case, as they show that proving that the cop number of graphs of bounded max degree is $O(n^{1-ε})$ would also prove that for all graphs the cop number is $O(n^{1-α})$ for some $α>0$. This would resolve a weak version of a notable conjecture by Meyniel. Next, we prove that certain unintuitive graphs necessarily exist and have large cop number if the weak version Meyniel's conjecture is false. We concluding by conjecturing that any such unintuitive graphs must be expanders and would therefore not have large cop number by the upper bound we show. Thus, our conjecture would imply a weak version of Meyniel's conjecture.