Exponentially Many Correspondence Colourings of Planar and Locally Planar Graphs

TL;DR

This paper proves that planar graphs with 5-correspondence assignments have exponentially many valid colourings, confirming a conjecture, and introduces a general method linking hyperbolicity theorems to lower bounds on colourings.

Contribution

It introduces a new method connecting hyperbolicity theorems to counting colourings, confirming a conjecture on exponential colourings of planar graphs with 5-correspondence assignments.

Findings

Planar graphs with 5-correspondence assignments have at least 2^{c·v(G)} colourings.

The method applies to counting 3-correspondence colourings of planar graphs with girth ≥ 5.

Analogous results hold for locally planar graphs and specific girth conditions.

Abstract

We show that there exists a constant $c > 0$ such that if $G$ is a planar graph with 5-correspondence assignment $(L,M)$, then $G$ has at least $2^{c\cdot v(G)}$ distinct $(L,M)$-colourings. This confirms a conjecture of Langhede and Thomassen. More broadly, we introduce a general method showing how hyperbolicity theorems for certain families of critical graphs can be used to derive lower bounds on the number of colourings of the associated class of planar graphs. Hence our main result follows from this method plus a technical theorem (that we proved in a previous paper) involving the hyperbolicity of graphs critical for $5$-correspondence colouring. We further demonstrate our method in the case of counting 3-correspondence colourings of planar graphs of girth at least five. Finally, we use these theorems to show analogous results hold in the case of counting 5-correspondence colourings of locally planar graphs, and counting 3-correspondence colourings of locally planar graphs of girth at least five.