Destructibility and Axiomatizability of Kaufmann Models

Abstract

A Kaufmann model is an $\omega_1$-like, recursively saturated, rather classless model of $\mathrm{PA}$ or $\mathrm{ZF}$. Such models were constructed by Kaufmann under the combinatorial principle $\diamondsuit_{\omega_1}$ and Shelah showed they exist in $\mathrm{ZFC}$ by an absoluteness argument. Kaufmann models are an important witness to the incompactness of $\omega_1$ similar to Aronszajn trees. In this paper we look at some set theoretic issues related to this motivated by the seemingly na\"{i}ve question of whether such a model can be "killed" by forcing without collapsing $\omega_1$. We show that the answer to this question is independent of $\mathrm{ZFC}$ and closely related to similar questions about Aronszajn trees. As an application of these methods we also show that it is independent of $\mathrm{ZFC}$ whether or not Kaufmann models can be axiomatized in the logic $L_{\omega_1, \omega} (Q)$ where $Q$ is the quantifier "there exists uncountably many".