Equitable list coloring of planar graphs with given maximum degree

TL;DR

This paper proves that planar graphs with maximum degree at most r are equitably r-choosable for r ≥ 9, confirming a conjecture for a broad class of graphs including all planar graphs.

Contribution

It establishes that for r ≥ 9, planar graphs with maximum degree at most r are equitably r-choosable, extending the conjecture to a wider class of graphs.

Findings

Proves equitable list coloring for planar graphs with max degree ≤ r for r ≥ 9.

Introduces a broader class of graphs where the conjecture holds.

Confirms the KPW conjecture for all graphs in the class ${ ext{B}}$, including planar graphs.

Abstract

If $L$ is a list assignment of $r$ colors to each vertex of an $n$-vertex graph $G$, then an equitable $L$-coloring of $G$ is a proper coloring of vertices of $G$ from their lists such that no color is used more than $\lceil n/r\rceil$ times. A graph is equitably $r$-choosable if it has an equitable $L$-coloring for every $r$-list assignment $L$. In 2003, Kostochka, Pelsmajer and West (KPW) conjectured that an analog of the famous Hajnal-Szemer\'edi Theorem on equitable coloring holds for equitable list coloring, namely, that for each positive integer $r$ every graph $G$ with maximum degree at most $r-1$ is equitably $r$-choosable. The main result of this paper is that for each $r\geq 9$ and each planar graph $G$, a stronger statement holds: if the maximum degree of $G$ is at most $r$, then $G$ is equitably $r$-choosable. In fact, we prove the result for a broader class of graphs -- the class ${\mathcal{B}}$ of the graphs in which each bipartite subgraph $B$ with $|V(B)|\ge3$ has at most $2|V(B)|-4$ edges. Together with some known results, this implies that the KPW Conjecture holds for all graphs in ${\mathcal{B}}$, in particular, for all planar graphs.