Andrásfai--Erdős--Sós theorem for the generalized triangle

Abstract

The celebrated Andrásfai--Erdős--Sós Theorem from 1974 shows that every $n$-vertex triangle-free graph with minimum degree greater than $2n/5$ must be bipartite. Its extensions to $3$-uniform hypergraphs without the generalized triangle $F_5=\{abc,abd,cde\}$ have been explored in several previous works such as~\cite{LMR23unif,HLZ24}, demonstrating the existence of $\epsilon >0$ such that for large $n$, every $n$-vertex $F_5$-free $3$-graph with minimum degree greater than $(1/9-\epsilon )n^2$ must be $3$-partite. We determine the optimal value for $\epsilon$ by showing that for $n\ge 5000$, every $n$-vertex $F_5$-free $3$-graph with minimum degree greater than $4n^2/45$ must be $3$-partite, thus establishing the first tight Andrásfai--Erdős--Sós type theorem for hypergraphs. As a corollary, for all positive $n$, every $n$-vertex cancellative $3$-graph with minimum degree greater than $4n^2/45$ must be $3$-partite. This result is also optimal and considerably strengthens prior work, such as that by Bollobás~\cite{Bol74} and Keevash--Mubayi~\cite{KM04Cancel}.

proof of Proposition 3.1 is recovered to the previous version