On various notions of universally Baire sets and reflection of non-Baire property in Cantor sets

Abstract

Universally Baire sets are a standard tool in modern set theory, where they are closely connected with forcing and absoluteness. In this paper we study universal Baireness instead as purely a regularity notion for subsets of Polish spaces. Our aim is not to develop the set-theoretic theory, but to place the notion on a systematic descriptive-set-theoretic footing and to clarify the several nonequivalent definitions that occur in the literature. A subset of a Polish space is called \emph{universally Baire} if all its continuous preimages possess the property of Baire. However, it is important to specify what the domain of these continuous functions can be. In fact, a plethora of definitions are used by various authors in the literature, the class of possible domains is sometimes the class of Polish spaces, compact metrizable spaces, all topological spaces, compact topological spaces, etc. We clarify this situation by demonstrating that the above four options are pairwise nonequivalent under the Continuum Hypothesis, we determine all the implications between these four notions that are provable in ZFC, and also show that all the notions used in the literature are equivalent to one of these four. Furthermore, we use Miller forcing to show that actually the cases of Polish and compact metrizable spaces are consistently equivalent. We show that numerous definitions using Borel isomorphisms, changes of topologies and Boolean algebras are all equivalent to the case of Polish domains. We present an argument that the notion using Polish domains is the natural dual to the notion of universally measurable sets. Finally, as a byproduct, we prove that the following statement is independent of ZFC: if a subset of a Polish space has the property of Baire relative to every nowhere dense Cantor subspace, then the subset has the property of Baire in the whole space.

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BibTeX RIS

BibTeX

@misc{elekes2026various,
  title = {{On various notions of universally Baire sets and reflection of non-Baire property in Cantor sets}},
  author = {Márton Elekes and Máté Pálfy},
  howpublished = {arXiv},
  year = {2026},
  month = oct,
  doi = {10.48550/arxiv.2610.36265},
  eprint = {2610.36265},
  archivePrefix = {arXiv},
  url = {https://arxiv.org/abs/2610.36265},
}

RIS

TY  - UNPB
TI  - On various notions of universally Baire sets and reflection of non-Baire property in Cantor sets
AU  - Márton Elekes
AU  - Máté Pálfy
PY  - 2026
DA  - 2026/10/26
PB  - arXiv
DO  - 10.48550/arxiv.2610.36265
UR  - https://arxiv.org/abs/2610.36265
AB  - Universally Baire sets are a standard tool in modern set theory, where they are closely connected with forcing and absoluteness. In this paper we study universal Baireness instead as purely a regularity notion for subsets of Polish spaces. Our aim is not to develop the set-theoretic theory, but to place the notion on a systematic descriptive-set-theoretic footing and to clarify the several nonequivalent definitions that occur in the literature. A subset of a Polish space is called \emph{universally Baire} if all its continuous preimages possess the property of Baire. However, it is important to specify what the domain of these continuous functions can be. In fact, a plethora of definitions are used by various authors in the literature, the class of possible domains is sometimes the class of Polish spaces, compact metrizable spaces, all topological spaces, compact topological spaces, etc. We clarify this situation by demonstrating that the above four options are pairwise nonequivalent under the Continuum Hypothesis, we determine all the implications between these four notions that are provable in ZFC, and also show that all the notions used in the literature are equivalent to one of these four. Furthermore, we use Miller forcing to show that actually the cases of Polish and compact metrizable spaces are consistently equivalent. We show that numerous definitions using Borel isomorphisms, changes of topologies and Boolean algebras are all equivalent to the case of Polish domains. We present an argument that the notion using Polish domains is the natural dual to the notion of universally measurable sets. Finally, as a byproduct, we prove that the following statement is independent of ZFC: if a subset of a Polish space has the property of Baire relative to every nowhere dense Cantor subspace, then the subset has the property of Baire in the whole space.
ER  -

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