English

Countably perfectly meager and countably perfectly null sets

Logic 2023-04-18 v1 General Topology

Abstract

We study a strengthening of the notion of a universally meager set and its dual counterpart that strengthens the notion of a universally null set. We say that a subset AA of a perfect Polish space XX is countably perfectly meager (respectively, countably perfectly null) in XX, if for every perfect Polish topology τ\tau on XX, giving the original Borel structure of XX, AA is covered by an FσF_\sigma-set FF in XX with the original Polish topology such that FF is meager with respect to τ\tau (respectively, for every finite, non-atomic, Borel measure μ\mu on XX, AA is covered by an FσF_\sigma-set FF in XX with μ(F)=0\mu(F)=0). We prove that if 2022^{\aleph_0}\leq \aleph_2, then there exists a universally meager set in 2N2^{\mathbb N} which is not countably perfectly meager in 2N2^{\mathbb N} (respectively, a universally null set in 2N2^{\mathbb N} which is not countably perfectly null in 2N2^{\mathbb N}).

Keywords

Cite

@article{arxiv.2304.07579,
  title  = {Countably perfectly meager and countably perfectly null sets},
  author = {Tomasz Weiss and Piotr Zakrzewski},
  journal= {arXiv preprint arXiv:2304.07579},
  year   = {2023}
}
R2 v1 2026-06-28T10:07:01.986Z