English

Intermediate models with deep failure of choice

Logic 2024-07-03 v1

Abstract

The following question was asked by Grigorieff: Suppose VV is a ZFC model and V[G]V[G] is a set-generic extension of VV. Can there be a ZF model NN so that VNV[G]V\subset N \subset V[G] yet NN is not equal to V(A)V(A) for any set AV[G]A\in V[G]? The first such model was constructed by Karagila. This is the so-called \emph{Bristol model}, an intermediate model between LL and L[c]L[c] where cc is a Cohen-generic real over LL. Karagila further proves that the Kinna-Wager degree is unbounded in this model. We prove that such an intermediate extension can be found in a Cohen-generic extension of \emph{any} ground model, fully resolving Grigorieff's question. That is, let VV be \emph{any} ZF model and cc a Cohen-generic real over VV. We prove that there is an intermediate ZF-model VNV[c]V\subset N \subset V[c] so that NN is not equal to V(A)V(A) for any set AV[c]A\in V[c], the Kinna-Wagner degree of NN is unbounded and, in particular, no set forcing in NN forces the axiom of choice. Therefore, there are class many different intermediate models of ZF between VV and V[c]V[c].

Cite

@article{arxiv.2407.02033,
  title  = {Intermediate models with deep failure of choice},
  author = {Yair Hayut and Assaf Shani},
  journal= {arXiv preprint arXiv:2407.02033},
  year   = {2024}
}
R2 v1 2026-06-28T17:26:07.323Z