On Gy\'arf\'as' Path-Colour Problem

Abstract

In their 1997 paper titled ``Fruit Salad", Gy\'{a}rf\'{a}s posed the following conjecture: there exists a constant $k$ such that if each path of a graph spans a $3$-colourable subgraph, then the graph is $k$-colourable. It is noted that $k=4$ might suffice. Let $r(G)$ be the maximum chromatic number of any subgraph $H$ of $G$ where $H$ is spanned by a path. The only progress on this conjecture comes from Randerath and Schiermeyer in 2002, who proved that if $G$ is an $n$ vertex graph, then $\chi(G) \leq r(G)\log_{\frac{8}{7}}(n)$. We prove that for all natural numbers $r$, there exists a graph $G$ with $r(G)\leq r$ and $\chi(G)\geq \lfloor\frac{3r}{2}\rfloor -1$. Hence, for all constants $k$ there exists a graph with $\chi - r > k$. Our proof is constructive. We also study this problem in graphs with a forbidden induced subgraph. We show that if $G$ is $K_{1,t}$-free, for $t\geq 4$, then $\chi(G) \leq (t-1)(r(G)+\binom{t-1}{2}-3)$. If $G$ is claw-free, then we prove $\chi(G) \leq 2r(G)$. Additionally, the graphs $G$ where every induced subgraph $G'$ of $G$ satisfy $\chi(G') = r(G')$ are considered. We call such graphs path-perfect, as this class generalizes perfect graphs. We prove that if $H$ is a forest with at most $4$ vertices other than the claw, then every $H$-free graph $G$ has $\chi(G) \leq r(G)+1$. We also prove that if $H$ is additionally not isomorphic to $2K_2$ or $K_2+2K_1$, then all $H$-free graphs are path-perfect.