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Assuming countable choice, every countably compact paracompact Hausdorff space is compact
Statement
Assume the Axiom of Countable Choice. Every countably compact paracompact Hausdorff space is compact.
Facts & Assumptions
Given: Countable choice and a countably compact paracompact Hausdorff space .
Countable choice supplies a choice function for every sequence of nonempty sets (The Axiom of Countable Choice ()).
Countable compactness tests at most countable open covers, while compactness tests all open covers (Countably compact, Lindel"of, sequentially compact, limit point compact and -compact spaces, and relatively compact subsets).
Paracompactness supplies a locally finite open refining cover (Paracompactness: every open cover has a locally finite open refinement, with no separation axiom built into the word).
Hausdorff spaces separate distinct points by disjoint open neighbourhoods (Hausdorff space: distinct points have disjoint open neighbourhoods; every metrizable space is Hausdorff and the indiscrete topology on two points is not).
A locally finite union of closed sets is closed (Locally finite families remain locally finite after taking closures, closure commutes with their union, and a locally finite union of closed sets is closed).
Under countable choice, a countable union of at most countable sets is at most countable (Countable unions of at most countable sets, assuming , Finite, countably infinite, countable, uncountable).
Choice from a finite listed family is a theorem of ZF (Every natural-number-indexed list of nonempty sets has a choice function on its family of values).
Proof
Let be an arbitrary open cover, and take a locally finite open refining cover by [F2].
Suppose were infinite. For every the family of -element subsets of is nonempty; [A1] chooses one . Then is at most countable by [L2] and infinite because it has finite subsets of arbitrarily large size. Hence, by the definition of at-most-countable, is countably infinite; enumerate its distinct members as .
By [A1] choose for every . The range is infinite: if it were finite, local finiteness would make only finitely many distinct meet , but every contains . Moreover the singleton family is locally finite, since a neighbourhood meeting only finitely many can contain only finitely many points .
Hausdorffness makes points closed, so [L1] makes closed. For each , local finiteness gives a neighbourhood meeting only finitely many points of ; pass to an open subneighbourhood and remove those finitely many other closed points. Using [A1] along an enumeration of yields open sets with .
The open set , together with the at most countable family , is an open cover with no finite subcover, contradicting countable compactness in [F1].
Hence is finite. By [L3], select for each member of this finite refining family one containing member of ; the selected members form a finite subcover of .
Since was arbitrary, [F1] proves that is compact.
Remarks
Countable choice is spent twice: first to extract a countably infinite subfamily from a putatively infinite locally finite cover, and then to choose one point from each member of that subfamily. The final selection is only finite choice, which is available in ZF.
Depends on
- Locally finite families remain locally finite after taking closures, closure commutes with their union, and a locally finite union of closed sets is closed
- Countably compact, Lindel\"of, sequentially compact, limit point compact and $\sigma$-compact spaces, and relatively compact subsets
- Paracompactness: every open cover has a locally finite open refinement, with no separation axiom built into the word
- Hausdorff space: distinct points have disjoint open neighbourhoods; every metrizable space is Hausdorff and the indiscrete topology on two points is not
- The Axiom of Countable Choice ($\mathrm{AC}_\omega$)
- Finite, countably infinite, countable, uncountable
- Countable unions of at most countable sets, assuming $\mathrm{AC}_\omega$
- Every natural-number-indexed list of nonempty sets has a choice function on its family of values
Used by
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Sources
- Dartmouth Point-Set Topology, Lecture 25 (standard reference, not scraped)
- P. Bacon, Pacific Journal of Mathematics 32 (1970), countably compact paracompact spaces (standard reference, not scraped)