Chromatic Roots and Minor-Closed Families of Graphs

Abstract

Given a minor-closed class of graphs $\mathcal{G}$, what is the infimum of the nontrivial roots of the chromatic polynomial of $G \in \mathcal{G}$? When $\mathcal{G}$ is the class of all graphs, the answer is known to be $32/27$. We answer this question exactly for three minor-closed classes of graphs. Furthermore, we conjecture precisely when the value is larger than $32/27$.