# Odd cycles in subgraphs of sparse pseudorandom graphs

## TL;DR

This paper proves that in sparse pseudorandom graphs, any sufficiently dense subgraph contains odd cycles of a given length, and every 2-edge coloring contains many monochromatic odd cycles, confirming conjectures and extending known theorems.

## Contribution

It fully resolves a conjecture about the existence of odd cycles in dense subgraphs of pseudorandom graphs and establishes a new Ramsey multiplicity result for odd cycles.

## Key findings

- Every dense subgraph of a pseudorandom graph contains an odd cycle of specified length.
- Every 2-edge coloring of such graphs contains many monochromatic odd cycles.
- Results are asymptotically optimal, matching known extremal constructions.

## Abstract

We answer two extremal questions about odd cycles that naturally arise in the study of sparse pseudorandom graphs. Let $\Gamma$ be an $(n,d,\lambda)$-graph, i.e., $n$-vertex, $d$-regular graphs with all nontrivial eigenvalues in the interval $[-\lambda,\lambda]$. Krivelevich, Lee, and Sudakov conjectured that, whenever $\lambda^{2k-1}\ll d^{2k}/n$, every subgraph $G$ of $\Gamma$ with $(1/2+o(1))e(\Gamma)$ edges contains an odd cycle $C_{2k+1}$. Aigner-Horev, H\`{a}n, and the third author proved a weaker statement by allowing an extra polylogarithmic factor in the assumption $\lambda^{2k-1}\ll d^{2k}/n$, but we completely remove it and hence settle the conjecture. This also generalises Sudakov, Szabo, and Vu's Tur\'{a}n-type theorem for triangles. Secondly, we obtain a Ramsey multiplicity result for odd cycles. Namely, in the same range of parameters, we prove that every 2-edge-colouring of $\Gamma$ contains at least $(1-o(1))2^{-2k}d^{2k+1}$ monochromatic copies of $C_{2k+1}$. Both results are asymptotically best possible by Alon and Kahale's construction of $C_{2k+1}$-free pseudorandom graphs.