Sequential approximations for types and Keisler measures

Abstract

This paper is a modified chapter of the author's Ph.D. thesis. We introduce the notions of sequentially approximated types and sequentially approximated Keisler measures. As the names imply, these are types which can be approximated by a sequence of realized types and measures which can be approximated by a sequence of "averaging measures" on tuples of realized types. We show that both generically stable types (in arbitrary theories) and Keisler measures which are finitely satisfiable over a countable model (in NIP theories) are sequentially approximated. We also introduce the notion of a smooth sequence in a measure over a model and give an equivalent characterization of generically stable measures (in NIP theories) via this definition.

Enriching a predicate and tame expansions of the integers

“…Unlike the situation with indiscernible sequences, there is always a plentiful supply of eventually indiscernible sequences inside a given structure in a countable language. In particular, we have the following fact, whose proof is a standard application of Ramsey's Theorem (see [Gan21,Section 4] and [Sim15, Section 2.1] for related discussion).…”