English

Capturing sets of ordinals by normal ultrapowers

Logic 2023-02-28 v5

Abstract

We investigate the extent to which ultrapowers by normal measures on κ\kappa can be correct about powersets P(λ)\mathcal{P}(\lambda) for λ>κ\lambda>\kappa. We consider two versions of this questions, the capturing property CP(κ,λ)\mathrm{CP}(\kappa,\lambda) and the local capturing property LCP(κ,λ)\mathrm{LCP}(\kappa,\lambda). CP(κ,λ)\mathrm{CP}(\kappa,\lambda) holds if there is an ultrapower by a normal measure on κ\kappa which correctly computes P(λ)\mathcal{P}(\lambda). LCP(κ,λ)\mathrm{LCP}(\kappa,\lambda) is a weakening of CP(κ,λ)\mathrm{CP}(\kappa,\lambda) which holds if every subset of λ\lambda is contained in some ultrapower by a normal measure on κ\kappa. After examining the basic properties of these two notions, we identify the exact consistency strength of LCP(κ,κ+)\mathrm{LCP}(\kappa,\kappa^+). Building on results of Cummings, who determined the exact consistency strength of CP(κ,κ+)\mathrm{CP}(\kappa,\kappa^+), and using a forcing due to Apter and Shelah, we show that CP(κ,λ)\mathrm{CP}(\kappa,\lambda) can hold at the least measurable cardinal.

Keywords

Cite

@article{arxiv.1902.10638,
  title  = {Capturing sets of ordinals by normal ultrapowers},
  author = {Miha E. Habič and Radek Honzík},
  journal= {arXiv preprint arXiv:1902.10638},
  year   = {2023}
}

Comments

31 pages; corrected typos