English

Determinacy from strong compactness of $\omega_1$

Logic 2016-09-20 v1

Abstract

In the absence of the Axiom of Choice, the "small" cardinal ω1\omega_1 can exhibit properties more usually associated with large cardinals, such as strong compactness and supercompactness. For a local version of strong compactness, we say that ω1\omega_1 is XX-strongly compact (where XX is any set) if there is a fine, countably complete measure on Pω1(X)\mathcal{P}_{\omega_1}(X). Working in ZF+DC\mathsf{ZF} + \mathsf{DC}, we prove that the P(ω1)\mathcal{P}(\omega_1)-strong compactness and P(R)\mathcal{P}(\mathbb{R})-strong compactness of ω1\omega_1 are equiconsistent with AD\mathsf{AD} and ADR+DC\mathsf{AD}_\mathbb{R} + \mathsf{DC} respectively, where AD\mathsf{AD} denotes the Axiom of Determinacy and ADR\mathsf{AD}_\mathbb{R} denotes the Axiom of Real Determinacy. The P(R)\mathcal{P}(\mathbb{R})-supercompactness of ω1\omega_1 is shown to be slightly stronger than ADR+DC\mathsf{AD}_\mathbb{R} + \mathsf{DC}, but its consistency strength is not computed precisely. An equiconsistency result at the level of ADR\mathsf{AD}_\mathbb{R} without DC\mathsf{DC} is also obtained.

Keywords

Cite

@article{arxiv.1609.05411,
  title  = {Determinacy from strong compactness of $\omega_1$},
  author = {Nam Trang and Trevor Wilson},
  journal= {arXiv preprint arXiv:1609.05411},
  year   = {2016}
}