English

When does every definable nonempty set have a definable element?

Logic 2017-06-23 v1

Abstract

The assertion that every definable set has a definable element is equivalent over ZF to the principle V=HODV=\text{HOD}, and indeed, we prove, so is the assertion merely that every Π2\Pi_2-definable set has an ordinal-definable element. Meanwhile, every model of ZFC has a forcing extension satisfying VHODV\neq\text{HOD} in which every Σ2\Sigma_2-definable set has an ordinal-definable element. Similar results hold for HOD(R)\text{HOD}(\mathbb{R}) and HOD(Ordω)\text{HOD}(\text{Ord}^\omega) and other natural instances of HOD(X)\text{HOD}(X).

Keywords

Cite

@article{arxiv.1706.07285,
  title  = {When does every definable nonempty set have a definable element?},
  author = {François G. Dorais and Joel David Hamkins},
  journal= {arXiv preprint arXiv:1706.07285},
  year   = {2017}
}

Comments

9 pages. Commentary concerning this article can be made at http://jdh.hamkins.org/definable-sets-with-definable-elements

R2 v1 2026-06-22T20:26:33.554Z