arXiv subjects

Michael D. Walton

Publications and source records attributed to Michael D. Walton.

2 recordsLinked to original sources

arXiv2026

Examples of non-tame abstract elementary classes of abelian groups

We construct an abstract elementary class $K_1$ of torsion-free abelian groups such that $K_1$ is not $(<\aleph_0)$-tame but is $\aleph_0$-tame. This answers a question of [BoVa17]. Furthermore, for every regular uncountable cardinal $μ$ less than the first measurable cardinal, we construct an abstract elementary class $K_2(2^μ)$ of torsion-free abelian groups such that $K_2(2^μ)$ is not $(<μ)$-tame. $K_1$ and $K_2(2^μ)$ are non-tame for algebraic reasons. Furthermore, they constitute the first examples of non-tame abstract elementary classes in a natural language.

math.LO↗

arXiv2026

An unstable abstract elementary class of modules: A variation of Paolini-Shelah's example

We construct a class $\hat{K}$ of torsion-free abelian groups such that $\hat{\mathbf{K}}=(\hat{K}, \leq_p)$ is an abstract elementary class with $\operatorname{LS}(\hat{\mathbf{K}})=\aleph_0$ such that: $(\cdot)$ $\hat{\mathbf{K}}$ is not stable; $(\cdot)$ $\hat{\mathbf{K}}$ has the joint embedding property and no maximal models, but does not have the amalgamation property; $(\cdot)$ $\hat{\mathbf{K}}$ is $(<\aleph_0)$-tame. The class we construct is a variation of [PaSh, Section 4] which isolates the core mechanism of the Paolini-Shelah construction.

math.LO↗