English

Between Polish and completely Baire

General Topology 2014-06-02 v1 Logic

Abstract

All spaces are assumed to be separable and metrizable. Consider the following properties of a space XX. (1) XX is Polish. (2) For every countable crowded QXQ\subseteq X there exists a crowded QQQ'\subseteq Q with compact closure. (3) Every closed subspace of XX is either scattered or it contains a homeomorphic copy of 2ω2^\omega. (4) Every closed subspace of XX is a Baire space. While (4) is the well-known property of being completely Baire, properties (2) and (3) have been recently introduced by Kunen, Medini and Zdomskyy, who named them the Miller property and the Cantor-Bendixson property respectively. It turns out that the implications (1)(2)(3)(4)(1)\rightarrow (2)\rightarrow (3)\rightarrow (4) hold for every space XX. Furthermore, it follows from a classical result of Hurewicz that all these implications are equivalences if XX is coanalytic. Under the axiom of Projective Determinacy, this equivalence result extends to all projective spaces. We will complete the picture by giving a ZFC\mathsf{ZFC} counterexample and a consistent definable counterexample of lowest possible complexity to the implication (i)(i+1)(i)\leftarrow (i+1) for i=1,2,3i=1,2,3. For one of these counterexamples we will need a classical theorem of Martin and Solovay, of which we give a new proof, based on a result of Baldwin and Beaudoin. Finally, using a method of Fischer and Friedman, we will investigate how changing the value of the continuum affects the definability of these counterexamples. Along the way, we will show that every uncountable completely Baire space has size continuum.

Keywords

Cite

@article{arxiv.1405.7899,
  title  = {Between Polish and completely Baire},
  author = {Andrea Medini and Lyubomyr Zdomskyy},
  journal= {arXiv preprint arXiv:1405.7899},
  year   = {2014}
}

Comments

14 pages

R2 v1 2026-06-22T04:27:06.595Z