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Article . 1994
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Bases, $pi$-bases and quasi-developments.

Bases, \(\pi\)-bases and quasi-developments
Authors: COSTANTINI, Camillo; A. FEDELI; J. PELANT;

Bases, $pi$-bases and quasi-developments.

Abstract

A sequence \(\{{\mathcal U}_n : n < \omega\}\) of families of open sets of a topological space \(X\) is called a strong quasi-development for \(X\) if for every \(x \in X\) and for every open neighbourhood \(V\) of \(x\) there exist an open neighbourhood \(W\) of \(x\) and an \(n < \omega\) such that \(x \in \bigcup \{U : U \in {\mathcal U}_n\}\) and \(x \in \text{St} (W, {\mathcal U}_n) \subset V\). It is called a quasi-development of order 2 for \(X\) if for every \(x \in X\) and for every open neighbourhood \(V\) of \(x\) there exists an \(n < \omega\) such that \(x \in \bigcup \{U : U \in {\mathcal U}_n\}\) and \(\text{St(St} (x, {\mathcal U}_n), {\mathcal U}_n) \subset V\). In 1974 it was shown by \textit{C. E. Aull} that a topological space \(X\) has a \(\sigma\)-disjoint base if and only if it is quasi-developable and hereditarily screenable [J. Lond. Math. Soc., II. Ser. 9, 197-204 (1974; Zbl 0295.54023)]. In this paper it is shwon that the following conditions are also equivalent: (1) \(X\) has a \(\sigma\)-disjoint base. (2) \(X\) has a strong quasi-development. (3) \(X\) has a quasi-development of order 2. Additionally, spaces with a \(\sigma\)-disjoint \(\pi\)-base and almost countably subcompact spaces are characterized.

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Italy
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Keywords

almost countably subcompact spaces, spaces with a \(\sigma\)-disjoint base, Base properties of topological spaces, Moore spaces, spaces with a \(\sigma\)-disjoint \(\pi\)-base, Base; $pi$-base; σ-disjoint ($pi$)-base; countably complete $pi$-base; quasi-development; hereditarily screenable space; almost countably subcompact space., quasi-developable spaces

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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!
0
Average
Average
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