The set-theoretic Kaufmann-Clote question (2507.01176v1)

Abstract: Let $\mathsf{M}$ be the set theory obtained from $\mathsf{ZF}$ by removing the collection scheme, restricting separation to $\Delta_0$-formulae and adding an axiom asserting that every set is contained in a transitive set. Let $\Pi_n\textsf{-Collection}$ denote the restriction of the collection scheme to $\Pi_n$-formulae. In this paper we prove that for $n \geq 1$, if $\mathcal{M}$ is a model of $\mathsf{M}+\Pi_n\textsf{-Collection}+\mathsf{V=L}$ and $\mathcal{N}$ is a $\Sigma_{n+1}$-elementary end extension of $\mathcal{M}$ that satisfies $\Pi_{n-1}\textsf{-Colelction}$ and that contains a new ordinal but no least new ordinal, then $\Pi_{n+1}\textsf{-Collection}$ holds in $\mathcal{M}$. This result is used to show that for $n \geq 1$, the minimum model of $\mathsf{M}+\Pi_n\textsf{-Collection}$ has no $\Sigma_{n+1}$-elementary end extension that satisfies $\Pi_{n-1}\textsf{-Collection}$, providing a negative answer to the generalisation of a question posed by Kaufmann.