A note on Hadwiger's Conjecture for $W_5$-free graphs with independence number two

Abstract

The Hadwiger number of a graph $G$, denoted $h(G)$, is the largest integer $t$ such that $G$ contains $K_t$ as a minor. A famous conjecture due to Hadwiger in 1943 states that for every graph $G$, $h(G)\ge \chi (G)$, where $\chi (G)$ denotes the chromatic number of $G$. Let $\alpha (G)$ denote the independence number of $G$. A graph is $H$-free if it does not contain the graph $H$ as an induced subgraph. In 2003, Plummer, Stiebitz and Toft proved that $h(G)\ge \chi (G)$ for all $H$-free graphs $G$ with $\alpha (G)\le 2$, where $H$ is any graph on four vertices with $\alpha (H)\le 2$, $H=C_5$, or $H$ is a particular graph on seven vertices. In 2010, Kriesell considered a particular strengthening of Hadwiger's conjecture due to Seymour and subsequently generalized the statement to include all forbidden subgraphs $H$ on five vertices with $\alpha (H)\le 2$. In this note, we prove that $h(G)\ge \chi (G)$ for all $W_5$-free graphs $G$ with $\alpha (G)\le 2$, where $W_5$ denotes the wheel on six vertices.

7 pages, 1 figure