Coloring Graphs with Forbidden Minors

TL;DR

This paper advances the understanding of graph coloring related to forbidden minors, providing new bounds and proofs for graphs excluding certain complete minors, and introduces a novel Kempe-chain method.

Contribution

It offers a short, computer-free proof for graphs with no $K_t$ minor being $(2t-6)$-colorable for specific $t$, and generalizes this result with new bounds and methods.

Findings

Graphs with no $K_7$ minor are $6$-colorable.

Graphs with no $K_8^-$ minor are $9$-colorable.

Kempe-chain method developed is of independent interest.

Abstract

Hadwiger's conjecture from 1943 states that for every integer $t\ge1$, every graph either can be $t$-colored or has a subgraph that can be contracted to the complete graph on $t+1$ vertices. As pointed out by Paul Seymour in his recent survey on Hadwiger's conjecture, proving that graphs with no $K_7$ minor are $6$-colorable is the first case of Hadwiger's conjecture that is still open. It is not known yet whether graphs with no $K_7$ minor are $7$-colorable. Using a Kempe-chain argument along with the fact that an induced path on three vertices is dominating in a graph with independence number two, we first give a very short and computer-free proof of a recent result of Albar and Gon\c{c}alves and generalize it to the next step by showing that every graph with no $K_t$ minor is $(2t-6)$-colorable, where $t\in\{7,8,9\}$. We then prove that graphs with no $K_8^-$ minor are $9$-colorable and graphs with no $K_8^=$ minor are $8$-colorable. Finally we prove that if Mader's bound for the extremal function for $K_p$ minors is true, then every graph with no $K_p$ minor is $(2t-6)$-colorable for all $p\ge5$. This implies our first result. We believe that the Kempe-chain method we have developed in this paper is of independent interest.