On the largest chromatic number of $F$-free hypergraphs

TL;DR

This paper investigates the maximum chromatic number of hypergraphs that do not contain a specific subhypergraph, providing characterizations and bounds for both strong and weak colorings, including for Berge copies.

Contribution

It characterizes when $F$-free hypergraphs have bounded strong chromatic number and provides bounds for the case of the 3-uniform expansion of stars, extending to Berge copies.

Findings

Characterized hypergraphs with bounded strong chromatic number when $F$ is forbidden.

Provided asymptotically sharp bounds for the strong chromatic number of $S_k^+$-free hypergraphs.

Established tight bounds for the strong/weak chromatic numbers when forbidding Berge copies of specific graphs.

Abstract

Given a hypergraph $F$, what is the largest chromatic number that an $F$-free hypergraph can have? In the case of graphs, this question is easy to answer: the chromatic number is unbounded if $F$ contains a cycle, and the largest chromatic number of $F$-free graphs is $k-1$ if $F$ is a forest on $k$ vertices. The situation is more complicated for hypergraphs. The strong coloring of a hypergraph is a coloring of the vertices such that every hyperedge is rainbow. The weak coloring of a hypergraph is a coloring of the vertices such that no hyperedge is monochromatic. The strong/weak chromatic number of a hypergraph is the minimum number of colors in a strong/weak coloring of the hypergraph. Our question has been completely answered for the weak chromatic number, similarly to the graph case. We characterize the hypergraphs $F$ such that $F$-free hypergraphs have bounded strong chromatic number. The only remaining case is when $F$ is the 3-uniform expansion $S_k^+$ of a star with $k$ edges. Concerning the strong chromatic number of $S_k^+$-free hypergraphs, we give bounds that are asymptitically sharp as $k\rightarrow\infty$. We also consider the same problem when the Berge copies of a graph $F$ are forbidden. We characterize when the strong/weak chromatic numbers are bounded in this case, and obtain sharp results or bounds for specific trees. In particular, when $F$ is a path, we give a tight bound when $r=3$ and an asymptotically sharp bound when $r=4$.