Asymmetric results about graph homomorphisms

Abstract

Many important results in extremal graph theory can be roughly summarised as "if a triangle-free graph $G$ has certain properties, then it has a homomorphism to a triangle-free graph $Γ$ of bounded size". For example, bounds on homomorphism thresholds give such a statement if $G$ has sufficiently high minimum degree, and the approximate homomorphism theorem gives such a statement for all $G$, if one weakens the notion of homomorphism appropriately. In this paper, we study asymmetric versions of these results, where the assumptions on $G$ and $Γ$ need not match. For example, we prove that if $G$ is a graph with odd girth at least $9$ and minimum degree at least $δ|G|$, then $G$ is homomorphic to a triangle-free graph whose size depends only on $δ$. Moreover, the odd girth assumption can be weakened to odd girth at least $7$ if $G$ has bounded VC dimension or bounded domination number. This gives a new and improved proof of a result of Huang et al. We also prove that in the asymmetric approximate homomorphism theorem, the bounds exhibit a rather surprising ``double phase transition'': the bounds are super-exponential if $G$ is only assumed to be triangle-free, they become exponential if $G$ is assumed to have odd girth $7$ or $9$, and become linear if $G$ has odd girth at least $11$. Our proofs use a wide variety of techniques, including entropy arguments, the Frieze--Kannan weak regularity lemma, properties of the generalised Mycielskian construction, and recent work on abundance and the asymmetric removal lemma.