Some Notions of Separability of Metric Spaces in $\mathbf {ZF}$ and Their Relation to Compactness

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Dane publikacji

  • DOI: 10.4064/ba8087-12-2016

  • Tom 64

  • Zeszyt 2

  • Czasopismo: Bulletin of the Polish Academy of Sciences Mathematics

  • Strony: 109-136

  • Data publikacji online: 19.12.2016

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Abstrakt

In the realm of metric spaces we show in $\mathbf {ZF}$ that:

(1) Quasi separability (a metric space $\mathbf {X}=(X,d)$ is quasi separable iff $\mathbf {X}$ has a dense subset which is expressible as a countable union of finite sets) is the weakest property under which a limit point compact metric space is compact.

(2) $\omega $-quasi separability (a metric space $\mathbf {X}=(X,d)$ is $\omega $-quasi separable iff $\mathbf {X}$ has a dense subset which is expressible as a countable union of countable sets) is a property under which a countably compact metric space is compact.

(3) The statement “Every totally bounded metric space is separable” does not imply the countable choice axiom $\mathbf {CAC}$.