Finding any given 2-factor in sparse pseudorandom graphs efficiently

Abstract: Given an $n$-vertex pseudorandom graph $G$ and an $n$-vertex graph $H$ with maximum degree at most two, we wish to find a copy of $H$ in $G$, i.e.\ an embedding $\varphi\colon V(H)\to V(G)$ so that $\varphi(u)\varphi(v)\in E(G)$ for all $uv\in E(H)$. Particular instances of this problem include finding a triangle-factor and finding a Hamilton cycle in $G$. Here, we provide a deterministic polynomial time algorithm that finds a given $H$ in any suitably pseudorandom graph $G$. The pseudorandom graphs we consider are $(p,\lambda)$-bijumbled graphs of minimum degree which is a constant proportion of the average degree, i.e.\ $\Omega(pn)$. A $(p,\lambda)$-bijumbled graph is characterised through the discrepancy property: $\left|e(A,B)-p|A||B|\right |<\lambda\sqrt{|A||B|}$ for any two sets of vertices $A$ and $B$. Our condition $\lambda=O(p^2n/\log n)$ on bijumbledness is within a log factor from being tight and provides a positive answer to a recent question of Nenadov. We combine novel variants of the absorption-reservoir method, a powerful tool from extremal graph theory and random graphs. Our approach is based on that of Nenadov (\emph{Bulletin of the London Mathematical Society}, to appear) and on ours (arXiv:1806.01676), together with additional ideas and simplifications.