On first order amenability

Abstract

We introduce the notion of first order amenability, as a property of a first order theory $T$: every complete type over $\varnothing$, in possibly infinitely many variables, extends to an automorphism-invariant global Keisler measure in the same variables. Amenability of $T$ follows from amenability of the (topological) group $Aut(M)$ for all sufficiently large ${\aleph }_0$-homogeneous countable models $M$ of $T$ (assuming $T$ to be countable), but is radically less restrictive. First, we study basic properties of amenable theories, giving many equivalent conditions. Then, applying a version of the stabilizer theorem from [Amenability, connected components, and definable actions; E. Hrushovski, K. Krupiński, A. Pillay], we prove that if $T$ is amenable, then $T$ is G-compact, namely Lascar strong types and Kim-Pillay strong types over $\varnothing$ coincide. This extends and essentially generalizes a similar result proved via different methods for $\ddot{I}‰$-categorical theories in [Amenability, definable groups, and automorphism groups; K. Krupiński, A. Pillay] . In the special case when amenability is witnessed by $\varnothing$-definable global Keisler measures (which is for example the case for amenable $\ddot{I}‰$-categorical theories), we also give a different proof, based on stability in continuous logic. Parallel (but easier) results hold for the notion of extreme amenability.

This paper contains the material in Section 4 of our preprint "Amenability and definability" (arXiv:1901.02859v1). Following the advice of editors and referees we have divided that preprint into two papers, the current paper being the second