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image/svg+xml Jakob Voss, based on art designer at PLoS, modified by Wikipedia users Nina and Beao Closed Access logo, derived from PLoS Open Access logo. This version with transparent background. http://commons.wikimedia.org/wiki/File:Closed_Access_logo_transparent.svg Jakob Voss, based on art designer at PLoS, modified by Wikipedia users Nina and Beao Acta Mathematica Hun...arrow_drop_down
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Acta Mathematica Hungarica
Article . 2001 . Peer-reviewed
License: Springer Nature TDM
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image/svg+xml Jakob Voss, based on art designer at PLoS, modified by Wikipedia users Nina and Beao Closed Access logo, derived from PLoS Open Access logo. This version with transparent background. http://commons.wikimedia.org/wiki/File:Closed_Access_logo_transparent.svg Jakob Voss, based on art designer at PLoS, modified by Wikipedia users Nina and Beao
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On Finally Compact Spaces

On finally compact spaces
Authors: Ergun, N.; Noiri, T.;

On Finally Compact Spaces

Abstract

For an infinite cardinal \(\kappa\), \(\kappa^+\) denotes the smallest cardinal greater than \(\kappa\). A space \(X\) is called finally \(\kappa^+\)-compact if every open cover of \(X\) has a subcover with cardinality \(\leq\kappa\). The authors define weakly \(\kappa\overline{\theta}\)-refinable spaces and study conditions under which a countably compact space is finally \(\kappa^+\)-compact and under which a space in which discrete families are countable is finally \(\kappa^+\)-compact. A space \(X\) is weakly \(\kappa\overline{\theta}\)-refinable if every open cover of \(X\) has an open refinement \({\mathcal C}=\bigcup_{\alpha<\kappa}{\mathcal C}_\alpha\) such that for each \(x\in X\) there exists an \(\alpha(x)<\kappa\) with \(0<\text{ord}(x,{\mathcal C}_{\alpha(x)})<\kappa\) and the open cover \(\{\bigcup{\mathcal C}_\alpha:\alpha<\kappa\}\) is point-finite. In case \(\kappa=\omega\), this notion coincides with weak \(\overline{\theta}\)-refinability defined by J. C. Smith. Main results are: (1) A countably compact space \(X\) is finally \(\kappa^+\)-compact if and only if every open cover of \(X\) has an open refinement \({\mathcal C}=\bigcup_{n<\omega}{\mathcal C}_n\) such that for each \(x\in X\) there exists an \(n(x)<\omega\) such that \(0<\text{ord}(x,{\mathcal C}_{n(x)})\leq\kappa\). (2) A topological space in which discrete families are countable is finally \(\kappa^+\)-compact if it is weakly \(\kappa\overline{\theta}\)-refinable. The case \(\kappa=\omega\) of (1) is a theorem of \textit{H. H. Wicke} and \textit{J. M. Worrell jun.} [Proc. Am. Math. Soc. 55, 427-431 (1976; Zbl 0323.54013)] that a countably compact, weakly \(\delta\theta\)-refinable space is Lindelöf, and hence, compact. The case \(\kappa=\omega\) of (2) is a theorem of \textit{J. C. Smith} [ibid. 53, 511-517 (1975; Zbl 0338.54013)] that a space in which discrete families are countable is Lindelöf if it is weakly \(\overline{\theta}\)-refinable.

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Keywords

finally compact, Noncompact covering properties (paracompact, Lindelöf, etc.), weakly \(\delta\theta\)-refinable, \(\delta\theta\)-refinable, weakly \(\kappa\overline{\theta}\)-refinable, \(p\)-spaces, \(M\)-spaces, \(\sigma\)-spaces, etc.

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selected citations
These citations are derived from selected sources.
This is an alternative to the "Influence" indicator, which also reflects the overall/total impact of an article in the research community at large, based on the underlying citation network (diachronically).
BIP!Citations provided by BIP!
popularity
This indicator reflects the "current" impact/attention (the "hype") of an article in the research community at large, based on the underlying citation network.
BIP!Popularity provided by BIP!
influence
This indicator reflects the overall/total impact of an article in the research community at large, based on the underlying citation network (diachronically).
BIP!Influence provided by BIP!
impulse
This indicator reflects the initial momentum of an article directly after its publication, based on the underlying citation network.
BIP!Impulse provided by BIP!
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