Nearly tight bounds for MaxCut in hypergraphs

Abstract

An $r$-cut of a $k$-uniform hypergraph is a partition of its vertex set into $r$ parts, and the size of the cut is the number of edges which have at least one vertex in each part. The study of the possible size of the largest $r$-cut in a $k$-uniform hypergraph was initiated by Erd\H{o}s and Kleitman in 1968. For graphs, a celebrated result of Edwards states that every $m$-edge graph has a $2$-cut of size $m/2+\Omega(m^{1/2})$, which is sharp. In other words, there exists a cut which exceeds the expected size of a random cut by the order of $m^{1/2}$. Conlon, Fox, Kwan and Sudakov proved that any $k$-uniform hypergraph with $m$ edges has an $r$-cut whose size is $\Omega(m^{5/9})$ larger than the expected size of a random $r$-cut, provided that $k \geq 4$ or $r \geq 3$. They further conjectured that this can be improved to $\Omega(m^{2/3})$, which would be sharp. Recently, R\"aty and Tomon improved the bound $m^{5/9}$ to $m^{3/5-o(1)}$ when $r \in \{ k-1,k\}$. Using a novel approach, we prove the following approximate version of the Conlon-Fox-Kwan-Sudakov conjecture: for each $\varepsilon>0$, there is some $k_0=k_0(\varepsilon)$ such that for all $k>k_0$ and $2\leq r\leq k$, in every $k$-uniform hypergraph with $m$ edges there exists an $r$-cut exceeding the random one by $\Omega(m^{2/3-\varepsilon})$. Moreover, we show that (if $k\geq 4$ or $r\geq 3$) every $k$-uniform linear hypergraph has an $r$-cut exceeding the random one by $\Omega(m^{3/4})$, which is tight and proves a conjecture of R\"aty and Tomon.