The optimal chromatic bound for even-hole-free graphs without induced seven-vertex paths

TL;DR

This paper proves an optimal chromatic bound for even-hole-free graphs without induced seven-vertex paths, extending previous results and supporting Reed's Conjecture for this class of graphs.

Contribution

It establishes a tight chromatic bound for this class, extending prior bounds and confirming Reed's Conjecture in this context.

Findings

Proves $oxed{ ext{chromatic number} \, ext{bounded by } rac{5}{4} imes ext{clique number}}$ for the class.

Extends previous bounds from six-vertex to seven-vertex path restrictions.

Supports Reed's Conjecture for these graphs.

Abstract

The class of even-hole-free graphs has been extensively studied on its own and on its relation to perfect graphs. In this paper, we study the $\chi$-boundedness of even-hole-free graphs which itself is an important topic in graph theory. In particular, we prove that every even-hole-free graph $G$ without induced 7-vertex paths satisfies $\chi(G)\le \lceil\frac{5}{4}\omega(G)\rceil$, where $\chi(G)$ and $\omega(G)$ denote the chromatic number and clique number of $G$, respectively. This bound is optimal. Our result strictly extends the result of Karthick and Maffary \cite{KM19} on even-hole-free graphs without induced 6-vertex paths, and implies that even-hole-free graphs without induced 7-vertex paths satisfy Reed's Conjecture. Our proof relies on a heavy structural analysis on a maximal substructure called a nice blowup of a five-cycle and can be viewed for graphs in which all holes are of length five (graphs with all holes having the same length gain increasing interest in recent years \cite{COOK202496}). Our result gives a partial answer to a conjecture of Wang and Wu \cite{WW25} on graphs in which all holes are of length 5. One of the key technical ingredients is a technical lemma proved via clique cutset argument combined with the idea of Infinite Descent Method (often used in number theory).