Strengthened Brooks Theorem for digraphs of girth three

Abstract

Brooks' Theorem states that a connected graph $G$ of maximum degree $Δ$ has chromatic number at most $Δ$, unless $G$ is an odd cycle or a complete graph. A result of Johansson (1996) shows that if $G$ is triangle-free, then the chromatic number drops to $O(Δ/ \log Δ)$. In this paper, we derive a weak analog for the chromatic number of digraphs. We show that every (loopless) digraph $D$ without directed cycles of length two has chromatic number $χ(D) \leq (1-e^{-13}) \tildeΔ$, where $\tildeΔ$ is the maximum geometric mean of the out-degree and in-degree of a vertex in $D$, when $\tildeΔ$ is sufficiently large. As a corollary it is proved that there exists an absolute constant $α< 1$ such that $χ(D) \leq α(\tildeΔ + 1)$ for every $\tildeΔ > 2$.

12 pages