Quantitative structure of stable sets in arbitrary finite groups

Abstract

We show that a k k -stable set in a finite group can be approximated, up to given error ϵ > 0 \epsilon >0 , by left cosets of a subgroup of index ϵ - O k ( 1 ) \epsilon ^{\text {-} O_k(1)} . This improves the bound in a similar result of Terry and Wolf on stable arithmetic regularity in finite abelian groups, and leads to a quantitative account of work of the author, Pillay, and Terry on stable sets in arbitrary finite groups. We also prove an analogous result for finite stable sets of small tripling in arbitrary groups, which provides a quantitative version of recent work by Martin-Pizarro, Palacín, and Wolf. Our proofs use results on VC-dimension, and a finitization of model-theoretic techniques from stable group theory.