Saturated free algebras and almost indiscernible theories

Abstract

We extend the concept of “almost indiscernible theory” introduced by Pillay and Sklinos in 2015 (which was itself a modernization and expansion of Baldwin and Shelah from 1983), to uncountable languages and uncountable parameter sequences. Roughly speaking a theory $$T$$ is almost indiscernible if some saturated model is in the algebraic closure of an indiscernible set of sequences. We show that such a theory $$T$$ is nonmultidimensional, superstable, and stable in all cardinals $$\ge |T|$$ . We prove a structure theorem for sufficiently large $$ a $$ -models $$M$$ , which states that over a suitable base, $$M$$ is in the algebraic closure of an independent set of realizations of weight one types (in possibly infinitely many variables). We also explore further the saturated free algebras of Baldwin and Shelah in both the countable and uncountable context. We study in particular theories and varieties of $$R$$ -modules, characterizing those rings $$ R $$ for which the free $$ R $$ -module on $$ \left| R\right| ^{+} $$ generators is saturated, and pointing out a counterexample to a conjecture by Pillay and Sklinos.