Degree stability of graphs forbidding odd cycles (2305.02762v1)

Abstract: Erd\H{o}s and Simonovits asked the following question: For an integer $r\geq 2$ and a family of non-bipartite graphs $\mathcal{H}$, what is the tight bound of $\alpha$ such that any $\mathcal{H}$-free $n$-vertex graph with minimum degree at least $\alpha n$ has chromatic number at most $r$? We answer this question for $r=2$ and any family of odd cycles. Let $l\le k$ and $n\ge 1000k^{8}$ be positive integers. Let ${\mathcal C}$ be a family of odd cycles, $C_{2l+1}$ be the shortest odd cycle not in ${\mathcal C}$, and $C_{2k+1}$ be the longest odd cycle in ${\mathcal C}$. Let $BC_{2l+1}(n)$ denote the graph obtained by taking $2l+1$ vertex-disjoint copies of $K_{\frac{n}{2(2l+1)},\frac{n}{2(2l+1)}}$ and selecting a vertex in each of them such that these vertices form a cycle of length $2l+1$. Let $C_{2k+3}({n \over 2k+3})$ denote the balanced blow up of $C_{2k+3}$ with $n$ vertices. Note that both $BC_{2l+1}(n)$ and $C_{2k+3}({n \over 2k+3})$ are $n$-vertex ${\mathcal C}$-free non-bipartite graphs. We show that if $G$ is an $n$-vertex ${\mathcal C}$-free graph with $\delta(G)>\max{ \frac{n}{2(2l+1)}, \frac{2}{2k+3}n}$, then $G$ is bipartite. The bound is tight evident by $BC_{2l+1}(n)$ and $C_{2k+3}({n \over 2k+3})$. Moreover, the only $n$-vertex ${\mathcal C}$-free non-bipartite graph with minimum degree $\max{ \frac{n}{2(2l+1)}, \frac{2}{2k+3}n}=\frac{n}{2(2l+1)}$ is $BC_{2l+1}(n)$, and the the only $n$-vertex ${\mathcal C}$-free non-bipartite graph with minimum degree $\max{ \frac{n}{2(2l+1)}, \frac{2}{2k+3}n}=\frac{2}{2k+3}n$ is $C_{2k+3}({n \over 2k+3})$. Our result also unifies stability results of Andr\'{a}sfai, Erd\H{o}s and S\'{o}s, H\"{a}ggkvist and Yuan and Peng for large $n$.