English

On Loeb and sequential spaces in $\mathbf{ZF}$

General Topology 2019-04-16 v1 Logic

Abstract

A topological space is called Loeb if the collection of all its non-empty closed sets has a choice function. In this article, in the absence of the axiom of choice, connections between Loeb and sequential spaces are investigated. Among other results, it is proved in ZF\mathbf{ZF} that if X\mathbf{X} is a Cantor completely metrizable second-countable space, then Xω\mathbf{X}^{\omega} is Loeb. If a sequential, sequentially locally compact space X\mathbf{X} has the property that every infinitely countable family of non-empty closed subsets of X\mathbf{X} has a choice function, then the Cartesian product X×Y\mathbf{X}\times\mathbf{Y} of X\mathbf{X} with any sequential space Y\mathbf{Y} is sequential. In consequence, it holds true in ZF\mathbf{ZF} that the Cartesian product of a sequential locally countably compact space with any sequential space is sequential. If R\mathbb{R} is sequential, then every second-countable compact Hausdorff space is sequential. It is also proved that, in some models of ZF\mathbf{ZF}, a countable product of Cantor completely metrizable second-countable spaces can fail to be Loeb and it is independent of ZF\mathbf{ZF} that every sequential subspace of % \mathbb{R} is Loeb. Some other sentences are shown to be independent of % \mathbf{ZF}. Several open problems are posed, among them, the following question: is Rω\mathbb{R}^{\omega} sequential if R\mathbb{R} is sequential?

Keywords

Cite

@article{arxiv.1904.06774,
  title  = {On Loeb and sequential spaces in $\mathbf{ZF}$},
  author = {Kyriakos Keremedis and Eliza Wajch},
  journal= {arXiv preprint arXiv:1904.06774},
  year   = {2019}
}
R2 v1 2026-06-23T08:39:11.182Z