English

A Kuratowski closure-complement variant whose solution is independent of ZF

General Topology 2020-05-28 v1

Abstract

We pose the following new variant of the Kuratowski closure-complement problem: How many distinct sets may be obtained by starting with a set AA of a Polish space XX, and applying only closure, complementation, and the dd operator, as often as desired, in any order? The set operator dd was studied by Kuratowski in his foundational text \textit{Topology: Volume I}; it assigns to AA the collection dAdA of all points of second category for AA. We show that in ZFC set theory, the answer to this variant problem is 2222. In a distinct system equiconsistent with ZFC, namely ZF+DC+PB, the answer is only 1818.

Cite

@article{arxiv.2005.13009,
  title  = {A Kuratowski closure-complement variant whose solution is independent of ZF},
  author = {Michael P. Cohen and Todd Johnson and Adam Kral and Aaron Li and Justin Soll},
  journal= {arXiv preprint arXiv:2005.13009},
  year   = {2020}
}

Comments

9 pages, 3 figures

R2 v1 2026-06-23T15:50:06.253Z