On non-degenerate Berge-Tur\'an problems

TL;DR

This paper investigates the maximum size of hypergraphs avoiding certain Berge subgraphs, establishing bounds and conjectures that relate hypergraph extremal problems to classical graph extremal functions.

Contribution

The authors propose a conjecture linking Berge hypergraph extremal numbers to classical graph extremal numbers and prove it in specific cases for k=3 and k=4.

Findings

Established bounds for Berge-F-free hypergraphs

Proved the conjecture for k=3 and k=4 cases

Derived a general bound with an additive constant

Abstract

Given a hypergraph $\mathcal{H}$ and a graph $G$, we say that $\mathcal{H}$ is a \textit{Berge}-$G$ if there is a bijection between the hyperedges of $\mathcal{H}$ and the edges of $G$ such that each hyperedge contains its image. We denote by $ex_k(n,\text{Berge-}F)$ the largest number of hyperedges in a $k$-uniform Berge-$F$-free graph. Let $ex(n,H,F)$ denote the largest number of copies of $H$ in $n$-vertex $F$-free graphs. It is known that $ex(n,K_k,F)\le ex_k(n,\text{Berge-}F)\le ex(n,K_k,F)+ex(n,F)$, thus if $\chi(F)>r$, then $ex_k(n,\text{Berge-}F)=(1+o(1)) ex(n,K_k,F)$. We conjecture that $ex_k(n,\text{Berge-}F)=ex(n,K_k,F)$ in this case. We prove this conjecture in several instances, including the cases $k=3$ and $k=4$. We prove the general bound $ex_k(n,\text{Berge-}F)= ex(n,K_k,F)+O(1)$.