English

Variations on $\Delta^1_1$ Determinacy and $\aleph_{\omega_1}$

Logic 2022-06-20 v3

Abstract

We consider a seemingly weaker form of Δ11\Delta^1_1 Turing determinacy. Let 2ρ<ω1CK2 \leq \rho < \omega_1^{\textrm{CK}}, Weak-Turing-Detρ(Δ11)\textrm{Weak-Turing-Det}_\rho (\Delta^1_1) is the statement: Every Δ11\Delta^1_1 set of reals cofinal in the Turing degrees contains two Turing distinct, Δρ0\Delta^0_\rho-equivalent reals. We show in ZF\textrm{ZF}^-: Weak-Turing-Detρ(Δ11)\textrm{Weak-Turing-Det}_\rho (\Delta^1_1) implies: for every ν<ω1CK\nu < \omega_1^{\textrm{CK}} there is a transitive model: MZF+ν existsM \models \textrm{ZF}^- + \aleph_\nu \textrm{ exists}. As a corollary: If every cofinal Δ11\Delta^1_1 set of Turing degrees contains both a degree and its jump, then for every ν<ω1CK\nu < \omega_1^{\textrm{CK}}, there is a transitive model: MZF+ν existsM \models \textrm{ZF}^- + \aleph_\nu \textrm{ exists}. -- With a simple proof, this improves upon a well-known result of Harvey Friedman on the strength of Borel determinacy (though not assessed level-by-level). -- Invoking Tony Martin's proof of Borel determinacy, Weak-Turing-Detρ(Δ11)\textrm{Weak-Turing-Det}_\rho (\Delta^1_1) implies Δ11\Delta^1_1 determinacy. We show further that Δ11\Delta^1_1 determinacy imparts weak determinacy properties to the class Σ11\Sigma^1_1.

Cite

@article{arxiv.1910.04481,
  title  = {Variations on $\Delta^1_1$ Determinacy and $\aleph_{\omega_1}$},
  author = {Ramez L. Sami},
  journal= {arXiv preprint arXiv:1910.04481},
  year   = {2022}
}

Comments

10 pages. Content identical to previous version: Corrected Mathjax Abstract which was a mess (author's mistake)

R2 v1 2026-06-23T11:39:37.088Z