Cops and Robbers on Graphs without Long or Short Holes

Abstract

In 2019, Sivaraman conjectured that every $P_k$-free graph has cop number at most $k-3$ for $k\geq 5$. Chudnovsky, Norin, Seymour, and Turcotte proved $P_5$-free graphs have cop number at most $2$. For $k\geq 6$ Sivaraman's conjecture remains widely open. There is little evidence to believe Sivaraman's conjecture is optimal, making the discovery of substantially better bounds interesting, even for subclasses of $P_k$-free graphs. We consider this problem for graphs with no long holes, which generalizes $P_k$-free graphs, that also have some forbidden shorter holes. Specifically, we show that for $t\geq 13$, $(C_4,\dots, C_{t}, C_k,C_{k+1},\dots)$-free graphs have cop number at most $\lceil \frac{k-2}{\lfloor \frac{t}{4}\rfloor }\rceil+1$. Taking $C_{k-1}$, which has cop number $2$ whenever $k\geq 5$, demonstrates that this bound is tight up to a small constant factor for infinity many pairs $k$ and $t$. The proof relies on showing that in graphs without short holes, the closed neighbourhood of a short induced path can be guarded by $1$ cop, even when these paths are not geodesics. Additionally, for $t\geq 2$ we show that $(C_{t+2},C_{t+3},C_k, C_{k+1},\dots, )$-free graphs have cop number at most $(t-1)\lceil \frac{(k-2)}{t}\rceil$. This show that $(C_4,C_5,C_k,C_{k+1},\dots)$-free graphs have cop number at most $\lceil \frac{k-2}{2}\rceil$, extending work by Clow and Meger on $(P_k,\text{claw}, \text{butterfly}, C_4,C_5)$-free graphs to a substantially more general class.