Coloring directed hypergraphs

Abstract

Inspired by earlier results about proper and polychromatic coloring of hypergraphs, we investigate such colorings of directed hypergraphs, that is, hypergraphs in which the vertex set of each hyperedge is partitioned into two parts, a tail and a head. We present a conjecture of D. Pálvölgyi and the author, which states that directed hypergraphs with a certain restriction on their pairwise intersections can be properly colored with two colors. Besides other contributions, our main result is a proof of this conjecture for 3-uniform directed hypergraphs. This result can be phrased equivalently such that if a 3-uniform directed hypergraph avoids a certain directed hypergraph with two hyperedges, then it admits a proper 2-coloring. Previously, only extremal problems regarding the maximum number of edges of directed hypergraphs that avoid a certain hyperedge were studied.