New bounds for proper $h$-conflict-free colourings

Abstract

A proper $k$-colouring of a graph $G$ is called $h$-conflict-free if every vertex $v$ has at least $\min\, \{h, {\rm deg}(v)\}$ colours appearing exactly once in its neighbourhood. Let $\chi_{\rm pcf}^h(G)$ denote the minimum $k$ such that such a colouring exists. We show that for every fixed $h\ge 1$, every graph $G$ of maximum degree $\Delta$ satisfies $\chi_{\rm pcf}^h(G) \le h\Delta + \mathcal{O}(\log \Delta)$. This expands on the work of Cho et al., and improves a recent result of Liu and Reed in the case $h=1$. We conjecture that for every $h\ge 1$ and every graph $G$ of maximum degree $\Delta$ sufficiently large, the bound $\chi_{\rm pcf}^h(G) \le h\Delta + 1$ should hold, which would be tight. When the minimum degree $\delta$ of $G$ is sufficiently large, namely $\delta \ge \max\{100h, 2000\log \Delta\}$, we show that this upper bound can be further reduced to $\chi_{\rm{pcf}}^h(G) \le \Delta + \mathcal{O}(\sqrt{h\Delta})$. This improves a recent bound from Kamyczura and Przyby{\l}o when $\delta \le \sqrt{h\Delta}$.