English

The set-theoretic Kaufmann-Clote question

Logic 2025-07-18 v2

Abstract

Let M\mathsf{M} be the set theory obtained from ZF\mathsf{ZF} by removing the collection scheme, restricting separation to Δ0\Delta_0-formulae and adding an axiom asserting that every set is contained in a transitive set. Let Πn-Collection\Pi_n\textsf{-Collection} denote the restriction of the collection scheme to Πn\Pi_n-formulae. In this paper we prove that for n1n \geq 1, if M\mathcal{M} is a model of M+Πn-Collection+V=L\mathsf{M}+\Pi_n\textsf{-Collection}+\mathsf{V=L} and N\mathcal{N} is a Σn+1\Sigma_{n+1}-elementary end extension of M\mathcal{M} that satisfies Πn1-Colelction\Pi_{n-1}\textsf{-Colelction} and that contains a new ordinal but no least new ordinal, then Πn+1-Collection\Pi_{n+1}\textsf{-Collection} holds in M\mathcal{M}. This result is used to show that for n1n \geq 1, the minimum model of M+Πn-Collection\mathsf{M}+\Pi_n\textsf{-Collection} has no Σn+1\Sigma_{n+1}-elementary end extension that satisfies Πn1-Collection\Pi_{n-1}\textsf{-Collection}, providing a negative answer to the generalisation of a question posed by Kaufmann.

Keywords

Cite

@article{arxiv.2507.01176,
  title  = {The set-theoretic Kaufmann-Clote question},
  author = {Zachiri McKenzie},
  journal= {arXiv preprint arXiv:2507.01176},
  year   = {2025}
}

Comments

15 pages

R2 v1 2026-07-01T03:42:20.605Z