Bipartite Graphs whose Squares are not Chromatic-Choosable

Seog-Jin Kim,Boram Park

doi:10.37236/4343

Abstract

The square $G^2$ of a graph $G$ is the graph defined on $V(G)$ such that two vertices $u$ and $v$ are adjacent in $G^2$ if the distance between $u$ and $v$ in $G$ is at most 2. Let $\chi(H)$ and $\chi_{\ell}(H)$ be the chromatic number and the list chromatic number of $H$, respectively. A graph $H$ is called chromatic-choosable if $\chi_{\ell} (H) = \chi(H)$. It is an interesting problem to find graphs that are chromatic-choosable.Motivated by the List Total Coloring Conjecture, Kostochka and Woodall (2001) proposed the List Square Coloring Conjecture which states that $G^2$ is chromatic-choosable for every graph $G$. Recently, Kim and Park showed that the List Square Coloring Conjecture does not hold in general by finding a family of graphs whose squares are complete multipartite graphs and are not chromatic choosable. It is a well-known fact that the List Total Coloring Conjecture is true if the List Square Coloring Conjecture holds for special class of bipartite graphs. Hence a natural question is whether $G^2$ is chromatic-choosable or not for every bipartite graph $G$.In this paper, we give a bipartite graph $G$ such that $\chi_{\ell} (G^2) \neq \chi(G^2)$. Moreover, we show that the value $\chi_{\ell}(G^2) - \chi(G^2)$ can be arbitrarily large.

Highlights

A proper k-coloring φ : V (G) → {1, 2, . . . , k} of a graph G is an assignment of colors to the vertices of G so that any two adjacent vertices receive distinct colors
Kim and Park showed that the List Square Coloring Conjecture does not hold in general by finding a family of graphs whose squares are complete multipartite graphs and are not chromatic choosable. It is a well-known fact that the List Total Coloring Conjecture is true if the List Square Coloring Conjecture holds for special class of bipartite graphs
It was noted in [8] that the List Total Coloring Conjecture is true if the List Square Coloring Conjecture is true

Summary

Introduction

A proper k-coloring φ : V (G) → {1, 2, . . . , k} of a graph G is an assignment of colors to the vertices of G so that any two adjacent vertices receive distinct colors. A graph G is said to be k-choosable if for any list assignment L such that |L(v)| k for every vertex v, there exists a proper coloring φ such that φ(v) ∈ L(v) for every v ∈ V (G). The least k such that G is k-choosable is called the list chromatic number χl(G) of G. Motivated by the List Total Coloring Conjecture, Kostochka and Woodall [8] proposed the List Square Coloring Conjecture which states that G2 is chromatic-choosable for every graph G. We show that the gap between χl(G2) and χ(G2) can be arbitrarily large for bipartite graphs G

Construction

Further Discussion