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On the extent of star countable spaces. (English) Zbl 1246.54017

For a topological property \(\mathcal P\), a space \(X\) is star \(\mathcal P\) if for every open cover \(\mathcal U\) of \(X\) there exists \(Y \subset X\) such that \(X=\text{St}(Y,\mathcal U) = \bigcup\{U\in\mathcal U: U\cap Y\neq\emptyset\}\), and \(Y\) has property \(\mathcal P\). Continuing the research done by O. T. Alas, L. R. Junqueira and R. G. Wilson in [Topology Appl. 158, No. 4, 620–626 (2011; Zbl 1226.54023)], the authors study star countable and star Lindelöf spaces establishing, among other things, that there exist first countable pseudocompact spaces which are not star Lindelöf. Also, some classes of spaces are described where star countability is equivalent to countable extent, and it is shown that a star countable space with a dense \(\sigma \)-compact subspace can have arbitrary extent. It is proved that for any \(\omega _{1}\)-monolithic compact space \(X\), if \(C _{p }(X)\) is star countable, then it is Lindelöf. The paper concludes with a list of related open problems.

MSC:

54D20 Noncompact covering properties (paracompact, Lindelöf, etc.)
54A25 Cardinality properties (cardinal functions and inequalities, discrete subsets)
54D60 Realcompactness and realcompactification
54C35 Function spaces in general topology
54H11 Topological groups (topological aspects)
54C25 Embedding

Citations:

Zbl 1226.54023
Full Text: DOI

References:

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