Regularity lemmas for stable graphs

Abstract

Let G be a finite graph with the non-k-order property (essentially, a uniform finite bound on the size of an induced sub-half-graph). A major result of the paper applies model-theoretic arguments to obtain a stronger version of Szemer\'edi's regularity lemma for such graphs, Theorem 5.18, in which there are no irregular pairs, the bounds are significantly improved, and each component satisfies an indivisibility condition. Motivation for this work comes from a coincidence of model-theoretic and graph-theoretic ideas. Namely, it was known that the "irregular pairs" in the statement of Szemer\'edi's regularity lemma cannot be eliminated, due to the counterexample of half-graphs. The results of this paper show in what sense this counterexample is the only essential difficulty. The proof is largely model-theoretic (though written to be accessible to finite combinatorialists): arbitrarily large half-graphs coincide with model-theoretic instability, so in their absence, structure theorems and technology from stability theory apply. In addition to the theorem quoted, we give several other regularity lemmas with different advantages, in which the indivisibility condition on the components is improved (at the expense of letting the number of components grow with |G|) and extend some of these results to the larger class of graphs without the independence property.