  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.
