The asymptotic behavior of the correspondence chromatic number

TL;DR

This paper extends classical graph coloring results to the more general correspondence coloring, establishing asymptotic bounds relating the correspondence chromatic number to average and maximum degrees, especially for triangle-free graphs.

Contribution

It proves an analogue of Alon's lower bound for correspondence coloring and generalizes Johansson's upper bound for triangle-free graphs to this new setting.

Findings

Correspondence chromatic number is at least proportional to degree over log degree.

For triangle-free graphs, the correspondence chromatic number is at most proportional to maximum degree over log maximum degree.

Regular triangle-free graphs have a correspondence chromatic number determined by their degree, up to a constant factor.

Abstract

Alon proved that for any graph $G$, $\chi_\ell(G) = \Omega(\ln d)$, where $\chi_\ell(G)$ is the list chromatic number of $G$ and $d$ is the average degree of $G$. Dvo\v{r}\'{a}k and Postle recently introduced a generalization of list coloring, which they called correspondence coloring. We establish an analogue of Alon's result for correspondence coloring; namely, we show that $\chi_c(G) = \Omega(d/\ln d)$, where $\chi_c(G)$ denotes the correspondence chromatic number of $G$. We also prove that for triangle-free $G$, $\chi_c(G) = O(\Delta/\ln \Delta)$, where $\Delta$ is the maximum degree of $G$ (this is a generalization of Johansson's result about list colorings). This implies that the correspondence chromatic number of a regular triangle-free graph is, up to a constant factor, determined by its degree.