The Ramsey property for Banach spaces and Choquet simplices

TL;DR

This paper demonstrates the extreme amenability of automorphism groups of the Gurarij space and Poulsen simplex, computes their universal minimal flows, and establishes the approximate Ramsey property for classes of finite-dimensional Banach spaces, linking these results via Kechris-Pestov-Todorcevic correspondences.

Contribution

It provides the first direct application of Kechris-Pestov-Todorcevic correspondence in metric structures, connecting combinatorial principles with Banach space automorphism groups.

Findings

Gurarij space automorphism group is extremely amenable.

Universal minimal flow of Poulsen simplex automorphism group is its canonical action.

Pointwise stabilizers of faces and bifaces are extremely amenable.

Abstract

We show that the Gurarij space $\mathbb{G}$ has extremely amenable automorphism group. This answers a question of Melleray and Tsankov. We also compute the universal minimal flow of the automorphism group of the Poulsen simplex $\mathbb{P}$ and we prove that it consists of the canonical action on $\mathbb{P}$ itself. This answers a question of Conley and T\"{o}rnquist. We show that the pointwise stabilizer of any closed proper face of $\mathbb{P}$ is extremely amenable. Similarly, the pointwise stabilizer of any closed proper biface of the unit ball of the dual of the Gurarij space (the Lusky simplex) is extremely amenable. These results are obtained via several Kechris-Pestov-Todorcevic correspondences, by establishing the approximate Ramsey property for several classes of finite-dimensional Banach spaces and function systems and their versions with distinguished contractions. This is the first direct application of the Kechris-Pestov-Todorcevic correspondence in the setting of metric structures. The fundamental combinatorial principle that underpins the proofs is the Dual Ramsey Theorem of Graham and Rothschild.