Exponentially Many 4-List-Colorings of Triangle-Free Graphs on Surfaces

Abstract

Thomassen proved that every planar graph $G$ on $n$ vertices has at least $2^{n/9}$ distinct $L$-colorings if $L$ is a 5-list-assignment for $G$ and at least $2^{n/10000}$ distinct $L$-colorings if $L$ is a 3-list-assignment for $G$ and $G$ has girth at least five. Postle and Thomas proved that if $G$ is a graph on $n$ vertices embedded on a surface $Σ$ of genus $g$, then there exist constants $ε,c_g > 0$ such that if $G$ has an $L$-coloring, then $G$ has at least $c_g2^{εn}$ distinct $L$-colorings if $L$ is a 5-list-assignment for $G$ or if $L$ is a 3-list-assignment for $G$ and $G$ has girth at least five. More generally, they proved that there exist constants $ε,α>0$ such that if $G$ is a graph on $n$ vertices embedded in a surface $Σ$ of fixed genus $g$, $H$ is a proper subgraph of $G$, and $ϕ$ is an $L$-coloring of $H$ that extends to an $L$-coloring of $G$, then $ϕ$ extends to at least $2^{ε(n - α(g + |V(H)|))}$ distinct $L$-colorings of $G$ if $L$ is a 5-list-assignment or if $L$ is a 3-list-assignment and $G$ has girth at least five. We prove the same result if $G$ is triangle-free and $L$ is a 4-list-assignment of $G$, where $ε=\frac{1}{8}$, and $α= 130$.

12 pages, 2 figures