arXiv subjects

Danielle S. Ulrich

Publications and source records attributed to Danielle S. Ulrich.

4 recordsLinked to original sources

arXiv2026

Borel completeness of $R$-modules when $R$ fails the DCC on pp-definable subgroups

We prove that for any countable ring $R$ (not necessarily commutative), if the associated left $R$-module ${}_R R$ has a strictly descending sequence of pp-definable subgroups, then the theory $Th(R^{(ω)})$ of the infinite dimensional direct sum is Borel complete. From this, we conclude that if $R$ is countable and not left perfect, then the theory of $R$-modules is Borel complete, and we give a full characterization of which countable simple rings have Borel complete theories. One special case is that the complete theory $Th({\mathbb Z}^{(ω)})$ is Borel complete, which strengthens the existing proofs of the Borel completeness of TFAB, the theory of torsion free abelian groups. The proof also introduces, relative to the chosen pp-chain, a proper two-sided ideal $L^R$, and a notion of f.g. hulls which, for countable rings and countable parameter sets in theories satisfying $T=T^{\aleph_0}$ exist and are unique up to isomorphism. These constructions may be of independent interest in the model theory of modules.

math.LO↗

arXiv2025

Equivalents of NOTOP

Working within the context of countable, superstable theories, we give many equivalents of a theory having NOTOP. In particular, NOTOP is equivalent to V-DI, the assertion that any type $V$-dominated by an independent triple is isolated over the triple. If $T$ has NOTOP, then every model $N$ is atomic over an independent tree of countable, elementary substructures, and hence is determined up to back-and-forth equivalence over such a tree. We also verify Shelah's assertion from Chapter XII of \cite{Shc} that NOTOP implies PMOP (without using NDOP).

math.LO↗

arXiv2024

Borel complexity of families of finite equivalence relations via large cardinals

We consider a large family of theories of equivalence relations, each with finitely many classes, and assuming the existence of an $ω$-Erdos cardinal, we determine which of these theories are Borel complete. We develop machinery, including {\em forbidding nested sequences} which implies a tight upper bound on Borel complexity, and {\em admitting cross-cutting absolutely indiscernible sets} which in our context implies Borel completeness. In the Appendix we classify the reducts of theories of refining equivalence relations, possibly with infinite splitting.

math.LO↗

arXiv2022

Borel complexity of modules

We prove that for a countable, commutative ring $R$, the class of countable $R$-modules either has only countably many isomorphism types, or else it is Borel complete. The machinery gives a succinct proof of the Borel completeness of TFAB, the class of torsion-free abelian groups. We also prove that for any countable ring $R$, both the class of left $R$-modules endowed with an endomorphism and the class of left $R$-modules with four named submodules are Borel complete.

math.LO↗