On the choosability with separation of planar graphs and its correspondence colouring analogue

TL;DR

This paper investigates a specialized list coloring problem for planar graphs with separation constraints, establishing conditions for colorability and providing a counterexample in the correspondence coloring context.

Contribution

It proves new conditions under which planar graphs are list colorable with separation constraints and presents a counterexample for the correspondence coloring analogue.

Findings

Planar graphs with certain list assignment conditions are colorable.

A counterexample shows the correspondence coloring analogue does not hold.

Conditions involving triangles and cliques determine colorability.

Abstract

A list assignment $L$ for a graph $G$ is an $(\ell,k)$-list assignment if $|L(v)|\geq \ell$ for each $v \in V(G)$ and $|L(u) \cap L(v)| \leq k$ for each $uv \in E(G)$. We say $G$ is $(\ell,k)$-choosable if it admits an $L$-colouring for every $(\ell, k)$-list assignment $L$. We prove that if $G$ is a planar graph with $(4,2)$-list assignment $L$ and for every triangle $T \subseteq G$ we have that $|\bigcap_{v \in V(T)} L(v)| \neq 2$, then $G$ is $L$-colourable. In fact, we prove a slightly stronger result: if $G$ contains a clique $H$ such that $V(H) \cap V(T) \neq \emptyset$ for every triangle $T \subseteq G$ with $|\bigcap_{v \in V(T)} L(v)| = 2$, then $G$ is $L$-colourable. Additionally, we give a counterexample to the correspondence colouring analogue of $(4,2)$-choosability for planar graphs.