On subgraphs of $C_{2k}$-free graphs and a problem of K\"uhn and Osthus

TL;DR

This paper determines the maximum edge fraction in bipartite, $C_4$-free subgraphs of $C_6$-free graphs, generalizes probabilistic hypergraph girth results, and provides new proofs and answers related to $C_{2k}$-free graphs.

Contribution

It proves the exact maximum edge fraction for bipartite, $C_4$-free subgraphs in $C_6$-free graphs and extends probabilistic girth results for hypergraphs, also offering new proofs and answers to longstanding questions.

Findings

The maximum edge fraction is exactly 3/8 for bipartite, $C_4$-free subgraphs in $C_6$-free graphs.

Constructs $C_{2k}$-free graphs with specific edge fraction bounds avoiding certain bipartite subgraphs.

Provides a new short proof of a known result and answers a question of K"uhn and Osthus.

Abstract

Let $c$ denote the largest constant such that every $C_{6}$-free graph $G$ contains a bipartite and $C_4$-free subgraph having $c$ fraction of edges of $G$. Gy\H{o}ri et al. showed that $\frac{3}{8} \le c \le \frac{2}{5}$. We prove that $c=\frac{3}{8}$. More generally, we show that for any $\varepsilon>0$, and any integer $k \ge 2$, there is a $C_{2k}$-free graph $G_1$ which does not contain a bipartite subgraph of girth greater than $2k$ with more than $\left(1-\frac{1}{2^{2k-2}}\right)\frac{2}{2k-1}(1+\varepsilon)$ fraction of the edges of $G_1$. There also exists a $C_{2k}$-free graph $G_2$ which does not contain a bipartite and $C_4$-free subgraph with more than $\left(1-\frac{1}{2^{k-1}}\right)\frac{1}{k-1}(1+\varepsilon)$ fraction of the edges of $G_2$. One of our proofs uses the following statement, which we prove using probabilistic ideas, generalizing a theorem of Erd\H{o}s: For any $\varepsilon>0$, and any integers $a$, $b$, $k \ge 2$, there exists an $a$-uniform hypergraph $H$ of girth greater than $k$ which does not contain any $b$-colorable subhypergraph with more than $\left(1-\frac{1}{b^{a-1}}\right)\left(1+\varepsilon\right)$ fraction of the hyperedges of $H$. We also prove further generalizations of this theorem. In addition, we give a new and very short proof of a result of K\"uhn and Osthus, which states that every bipartite $C_{2k}$-free graph $G$ contains a $C_{4}$-free subgraph with at least $1/(k-1)$ fraction of the edges of $G$. We also answer a question of K\"uhn and Osthus about $C_{2k}$-free graphs obtained by pasting together $C_{2l}$'s (with $k>l\ge3$).