Actions of trees on semigroups, and an infinitary Gowers--Hales--Jewett Ramsey theorem

TL;DR

This paper introduces a new framework of tree actions on semigroups to unify and generalize several infinitary Ramsey theorems, with applications to the structure of delta sets in amenable groups.

Contribution

It develops a general theory of tree actions on (filtered) semigroups that unifies multiple classical Ramsey theorems and extends them to a polynomial setting.

Findings

Unified several infinitary Ramsey theorems under a common framework.

Proved a polynomial version of the main theorem, generalizing the Milliken--Taylor theorem.

Applied results to the structure of delta sets in amenable groups.

Abstract

We introduce the notion of (Ramsey) action of a tree on a (filtered) semigroup. We then prove in this setting a general result providing a common generalization of the infinitary Gowers Ramsey theorem for multiple tetris operations, the infinitary Hales--Jewett theorems (for both located and nonlocated words), and the Farah--Hindman--McLeod Ramsey theorem for layered actions on partial semigroups. We also establish a polynomial version of our main result, recovering the polynomial Milliken--Taylor theorem of Bergelson--Hindman--Williams as a particular case. We present applications of our Ramsey-theoretic results to the structure of delta sets in amenable groups.