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arXiv · 2608.19548

The lattice of abstract elementary classes of modules

Tapani Hyttinen·Marcos Mazari-Armida·Gianluca Paolini
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Abstract

Let $R$ be a ring. We organize the abstract elementary classes whose underlying class is the class of all $R$-modules and whose strong submodel relation lies between the submodule and direct summand relations into a lattice $\mathscr{L}_{R}$, ordered by reverse inclusion. We establish the basic lattice-theoretic properties of $\mathscr{L}_{R}$ and investigate its two natural sublattices, below and above purity. Below purity, we isolate relations defined by first-order pp-formulas for which amalgamation, tameness, and stability hold. Above purity, we introduce relations defined by infinitary pp-formulas and prove a broad stability result. Specializing to abelian groups, we show that the lattice $\mathscr{L}_{\mathbb{Z}}$ has the following properties: it has a strong submodel relation that is not positive syntactic, it contains an uncountable antichain and a strictly increasing proper-class-sized chain, and it has a broad region above purity where amalgamation fails.

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Tapani Hyttinen, Marcos Mazari-Armida, Gianluca Paolini. 2026-08-20. The lattice of abstract elementary classes of modules. https://arxiv.org/abs/2608.19548

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