Chromatic Number of Random Kneser Hypergraphs

TL;DR

This paper extends the understanding of the chromatic number of random Kneser hypergraphs using topological and combinatorial methods, providing new bounds and almost sure convergence results.

Contribution

It generalizes Kupavskii's results to hypergraphs using the Z_p-Tucker lemma and offers new lower bounds for hypergraph colorings avoiding certain subhypergraphs.

Findings

Chromatic numbers of random Kneser hypergraphs are almost surely close to those of deterministic hypergraphs.

Introduces a lower bound for coloring hypergraphs avoiding monochromatic complete r-partite subhypergraphs.

Provides a combinatorial proof improving previous results for Kneser and Schrijver graphs.

Abstract

Recently, Kupavskii~[{\it On random subgraphs of {K}neser and {S}chrijver graphs. J. Combin. Theory Ser. A, {\rm 2016}.}] investigated the chromatic number of random Kneser graphs $\KG_{n,k}(\rho)$ and proved that, in many cases, the chromatic numbers of the random Kneser graph $\KG_{n,k}(\rho)$ and the Kneser graph $\KG_{n,k}$ are almost surely closed. He also marked the studying of the chromatic number of random Kneser hypergraphs $\KG^r_{n,k}(\rho)$ as a very interesting problem. With the help of $\Z_p$-Tucker lemma, a combinatorial generalization of the Borsuk-Ulam theorem, we generalize Kupavskii's result to random general Kneser hypergraphs by introducing an almost surely lower bound for the chromatic number of them. Roughly speaking, as a special case of our result, we show that the chromatic numbers of the random Kneser hypergraph $\KG^r_{n,k}(\rho)$ and the Kneser hypergraph $\KG^r_{n,k}$ are almost surely closed in many cases. Moreover, restricting to the Kneser and {S}chrijver graphs, we present a purely combinatorial proof for an improvement of Kupavskii's results. Also, for any hypergraph $\HH$, we present a lower bound for the minimum number of colors required in a coloring of $\KG^r(\mathcal{H})$ with no monochromatic $K_{t,\ldots,t}^r$ subhypergraph, where $K_{t,\ldots,t}^r$ is the complete $r$-uniform $r$-partite hypergraph with $t r$ vertices such that each of its parts has $t$ vertices. This result generalizes the lower bound for the chromatic number of $\KG^r(\mathcal{H})$ found by the present authors~[{\it On the chromatic number of general {K}neser hypergraphs. J. Combin. Theory, Ser. B, {\rm 2015}.}].