Extension of Gyarfas-Sumner conjecture to digraphs

TL;DR

This paper explores extensions of the Gyárfás-Sumner conjecture to digraphs, proposing a characterization of digraph sets that force large dichromatic number and proving related coloring bounds for specific classes.

Contribution

It introduces a conjecture extending the Gyárfás-Sumner conjecture to digraphs and establishes new coloring bounds for triangle-free and $K_4$-free digraphs.

Findings

Oriented triangle-free graphs without a directed path of length 3 are 2-colorable.

$K_4$-free graphs have an upper bound of 414 on their dichromatic number.

Orientations of complete multipartite graphs without directed triangles are 2-colorable.

Abstract

The dichromatic number of a digraph $D$ is the minimum number of colors needed to color its vertices in such a way that each color class induces an acyclic digraph. As it generalizes the notion of the chromatic number of graphs, it has been a recent center of study. In this work we look at possible extensions of Gy\'arf\'as-Sumner conjecture. More precisely, we propose as a conjecture a simple characterization of finite sets $\mathcal F$ of digraphs such that every oriented graph with sufficiently large dichromatic number must contain a member of $\mathcal F$ as an induce subdigraph. Among notable results, we prove that oriented triangle-free graphs without a directed path of length $3$ are $2$-colorable. If condition of "triangle-free" is replaced with "$K_4$-free", then we have an upper bound of $414$. We also show that an orientation of complete multipartite graph with no directed triangle is 2-colorable. To prove these results we introduce the notion of \emph{nice sets} that might be of independent interest.