# Stability in the Erd\H{o}s--Gallai Theorem on cycles and paths, II

## TL;DR

This paper strengthens Kopylov's stability theorem for the Erd ext{o}s--Gallai cycle length problem, characterizing the structure of near-extremal 2-connected graphs with no long cycles.

## Contribution

It completes a stability theorem for the Erd ext{o}s--Gallai problem, identifying the structure of graphs close to extremal edge counts for odd cycle length constraints.

## Key findings

- Characterizes 2-connected graphs near extremal edge counts
- Identifies extremal graphs with maximum edges under cycle constraints
- Provides tight bounds for the number of edges in such graphs

## Abstract

The Erd\H{o}s--Gallai Theorem states that for $k \geq 3$, any $n$-vertex graph with no cycle of length at least $k$ has at most $\frac{1}{2}(k-1)(n-1)$ edges. A stronger version of the Erd\H{o}s--Gallai Theorem was given by Kopylov: If $G$ is a 2-connected $n$-vertex graph with no cycle of length at least $k$, then $e(G) \leq \max\{h(n,k,2),h(n,k,\lfloor \frac{k-1}{2}\rfloor)\}$, where $h(n,k,a) := {k - a \choose 2} + a(n - k + a)$. Furthermore, Kopylov presented the two possible extremal graphs, one with $h(n,k,2)$ edges and one with $h(n,k,\lfloor \frac{k-1}{2}\rfloor)$ edges. In this paper, we complete a stability theorem which strengthens Kopylov's result. In particular, we show that for $k \geq 3$ odd and all $n \geq k$, every $n$-vertex $2$-connected graph $G$ with no cycle of length at least $k$ is a subgraph of one of the two extremal graphs or $e(G) \leq \max\{h(n,k,3),h(n,k,\frac{k-3}{2})\}$. The upper bound for $e(G)$ here is tight.