Uniquely colorable hypergraphs

TL;DR

This paper determines the exact threshold for unique colorability in hypergraphs, revealing a phase transition phenomenon and extending classical graph results to hypergraphs with applications to degree bounds.

Contribution

It precisely computes the threshold  for all k  r  hypergraphs, uncovering a phase transition not seen in graphs, and extends classical theorems to hypergraph degree problems.

Findings

Determined  for all k  r  hypergraphs.

Identified a phase transition at approximately k = (4r-2)/3.

Derived tight bounds for minimum positive i-degrees in hypergraphs.

Abstract

An $r$-uniform hypergraph is uniquely $k$-colorable if there exists exactly one partition of its vertex set into $k$ parts such that every edge contains at most one vertex from each part. For integers $k \ge r \ge 2$, let $\Phi_{k,r}$ denote the minimum real number such that every $n$-vertex $k$-partite $r$-uniform hypergraph with positive codegree greater than $\Phi_{k,r} \cdot n$ and no isolated vertices is uniquely $k$-colorable. A classic result by of Bollob\'{a}s\cite{Bol78} established that $\Phi_{k,2} = \frac{3k-5}{3k-2}$ for every $k \ge 2$. We consider the uniquely colorable problem for hypergraphs. Our main result determines the precise value of $\Phi_{k,r}$ for all $k \ge r \ge 3$. In particular, we show that $\Phi_{k,r}$ exhibits a phase transition at approximately $k = \frac{4r-2}{3}$, a phenomenon not seen in the graph case. As an application of the main result, combined with a classic theorem by Frankl--F\"{u}redi--Kalai, we derive general bounds for the analogous problem on minimum positive $i$-degrees for all $1\leq i<r$, which are tight for infinitely many cases.