Triangles in randomly perturbed graphs

TL;DR

This paper proves that in a graph combined with a random perturbation, the presence of many vertex-disjoint triangles is almost certain under certain conditions, extending classical results and solving open questions.

Contribution

It establishes the asymptotic existence of triangle packings in randomly perturbed graphs, answering a longstanding open problem and providing a stability version.

Findings

Almost sure existence of vertex-disjoint triangles in perturbed graphs

Optimal bounds on the probability parameter p

Extension of classical Dirac-type results to random perturbations

Abstract

We study the problem of finding pairwise vertex-disjoint triangles in the randomly perturbed graph model, which is the union of any $n$-vertex graph $G$ satisfying a given minimum degree condition and the binomial random graph $G(n,p)$. We prove that asymptotically almost surely $G \cup G(n,p)$ contains at least $\min\{\delta(G), \lfloor n/3 \rfloor\}$ pairwise vertex-disjoint triangles, provided $p \ge C \log n/n$, where $C$ is a large enough constant. This is a perturbed version of an old result of Dirac. Our result is asymptotically optimal and answers a question of Han, Morris, and Treglown [RSA, 2021, no. 3, 480--516] in a strong form. We also prove a stability version of our result, which in the case of pairwise vertex-disjoint triangles extends a result of Han, Morris, and Treglown [RSA, 2021, no. 3, 480--516]. Together with a result of Balogh, Treglown, and Wagner [CPC, 2019, no. 2, 159--176] this fully resolves the existence of triangle factors in randomly perturbed graphs. We believe that the methods introduced in this paper are useful for a variety of related problems: we discuss possible generalisations to clique factors, cycle factors, and $2$-universality.