English

Boolean-valued second-order logic revisited

Logic 2025-04-18 v1

Abstract

Following the paper~[3] by V\"{a}\"{a}n\"{a}nen and the author, we continue to investigate on the difference between Boolean-valued second-order logic and full second-order logic. We show that the compactness number of Boolean-valued second-order logic is equal to ω1\omega_1 if there are proper class many Woodin cardinals. This contrasts the result by Magidor~[10] that the compactness number of full second-order logic is the least extendible cardinal. We also introduce the inner model C2bC^{2b} constructed from Boolean-valued second-order logic using the construction of G\"{o}del's Constructible Universe L. We show that C2bC^{2b} is the least inner model of ZFC\mathsf{ZFC} closed under Mn#\mathrm{M}_n^{\#} operators for all n<ωn < \omega, and that C2bC^{2b} enjoys various nice properties as G\"{o}del's L does, assuming that Projective Determinacy holds in any set generic extension. This contrasts the result by Myhill and Scott~[14] that the inner model constructed from full second-order logic is equal to HOD, the class of all hereditarily ordinal definable sets.

Keywords

Cite

@article{arxiv.2504.12602,
  title  = {Boolean-valued second-order logic revisited},
  author = {Daisuke Ikegami},
  journal= {arXiv preprint arXiv:2504.12602},
  year   = {2025}
}
R2 v1 2026-06-28T23:01:26.361Z