# Some stable non-elementary classes of modules

Research paper by Marcos Mazari-Armida

Indexed on: 07 Oct '20Published on: 06 Oct '20Published in: arXiv - Mathematics - Logic

#### Abstract

Fisher [Fis75] and Baur [Bau75] showed independently in the seventies that for any $T$ a complete first-order theory extending the theory of modules, $(Mod(T), \leq_p)$ is stable. In [Maz4, 2.12], it is asked if the same is true for any abstract elementary class $(K, \leq_p)$ such that $K$ is a class of modules and $\leq_p$ is the pure submodule relation. In this paper we give some instances where this is true: $\textbf{Theorem.}$ Assume $R$ is an associative ring with unity. Let $(K, \leq_p)$ be an AEC such that $K \subseteq R\text{-Mod}$ and $K$ is closed under finite direct sums, then: - If $K$ is closed under direct summands and pure-injective envelopes, then $(K, \leq_p)$ is $\lambda$-stable for every $\lambda$ such that $\lambda^{|R| + \aleph_0}= \lambda$. - If $K$ is closed under pure submodules and pure epimorphic images, then $(K, \leq_p)$ is $\lambda$-stable for every $\lambda$ such that $\lambda^{|R| + \aleph_0}= \lambda$. - Assume $R$ is left Von Neumann regular. If $K$ is closed under submodules and has arbitrarily large models, then $(K, \leq_p)$ is $\lambda$-stable for every $\lambda$ such that $\lambda^{|R| + \aleph_0}= \lambda$. As an application of these results we give new characterizations of noetherian rings, pure-semisimple rings, dedekind domains and fields via superstability. Moreover, we show how these results can be used to show a link between being good in the stability hierarchy and being good in the axiomatizability hierarchy. Another application is the existence of universal models with respect to pure embeddings in several classes of modules. Among them, the class of flat modules and the class of injective torsion modules.