Multiple list colouring of $3$-choice critical graphs

TL;DR

This paper proves that all bipartite 3-choice critical graphs, except specific known exceptions, are $(4m, 2m)$-choosable for every integer $m$, extending previous results and confirming a conjecture for a broader class.

Contribution

It generalizes prior findings by showing that all bipartite 3-choice critical graphs, aside from known counterexamples, are $(4m, 2m)$-choosable for all integers $m$, strengthening the understanding of list coloring.

Findings

All bipartite 3-choice critical graphs are $(4m, 2m)$-choosable for all integers $m$.

Confirmed conjecture for a broader class of bipartite 3-choice critical graphs.

Extended previous results by strengthening the conditions for $(4m, 2m)$-choosability.

Abstract

A graph $G$ is called $3$-choice critical if $G$ is not $2$-choosable but any proper subgraph is $2$-choosable. A characterization of $3$-choice critical graphs was given by Voigt in [On list Colourings and Choosability of Graphs, Habilitationsschrift, Tu Ilmenau(1998)]. Voigt conjectured that if $G$ is a bipartite $3$-choice critical graph, then $G$ is $(4m, 2m)$-choosable for every integer $m$. This conjecture was disproved by Meng, Puleo and Zhu in [On (4, 2)-Choosable Graphs, Journal of Graph Theory 85(2):412-428(2017)]. They showed that if $G=\Theta_{r,s,t}$ where $r,s,t$ have the same parity and $\min\{r,s,t\} \ge 3$, or $G=\Theta_{2,2,2,2p}$ with $p \ge 2$, then $G$ is bipartite $3$-choice critical, but not $(4,2)$-choosable. On the other hand, all the other bipartite 3-choice critical graphs are $(4,2)$-choosable. This paper strengthens the result of Meng, Puleo and Zhu and shows that all the other bipartite $3$-choice critical graphs are $(4m,2m)$-choosable for every integer $m$.