# A first-order theory of Ulm type

## TL;DR

This paper introduces a first-order theory, $T_p$, that characterizes abelian $p$-groups up to bi-interpretability, providing a new example of a theory of 'Ulm type' with specific computability and classification properties.

## Contribution

It constructs an elementary first-order theory of Ulm type that captures abelian $p$-groups and addresses a question by Knight about such theories.

## Key findings

- Models are bi-interpretable with abelian $p$-groups.
- Any two low models with same infinitary theory are isomorphic.
- The isomorphism problem is $oldsymbol{ ext{Σ}}^1_1$-complete but not Borel.

## Abstract

The class of abelian $p$-groups are an example of some very interesting phenomena in computable structure theory. We will give an elementary first-order theory $T_p$ whose models are each bi-interpretable with the disjoint union of an abelian $p$-group and a pure set (and so that every abelian $p$-group is bi-interpretable with a model of $T_p$) using computable infinitary formulas. This answers a question of Knight by giving an example of an elementary first-order theory of "Ulm type": Any two models, low for $\omega_1^{CK}$, and with the same computable infinitary theory, are isomorphic. It also gives a new example of an elementary first-order theory whose isomorphism problem is $\mathbf{\Sigma}^1_1$-complete but not Borel complete.