Powered by OpenAIRE graph
Found an issue? Give us feedback
image/svg+xml art designer at PLoS, modified by Wikipedia users Nina, Beao, JakobVoss, and AnonMoos Open Access logo, converted into svg, designed by PLoS. This version with transparent background. http://commons.wikimedia.org/wiki/File:Open_Access_logo_PLoS_white.svg art designer at PLoS, modified by Wikipedia users Nina, Beao, JakobVoss, and AnonMoos http://www.plos.org/ OpenstarTsarrow_drop_down
image/svg+xml art designer at PLoS, modified by Wikipedia users Nina, Beao, JakobVoss, and AnonMoos Open Access logo, converted into svg, designed by PLoS. This version with transparent background. http://commons.wikimedia.org/wiki/File:Open_Access_logo_PLoS_white.svg art designer at PLoS, modified by Wikipedia users Nina, Beao, JakobVoss, and AnonMoos http://www.plos.org/
OpenstarTs
Article . 1998
Data sources: OpenstarTs
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
zbMATH Open
Article
Data sources: zbMATH Open
versions View all 2 versions
addClaim

The $\pi$-weights and $\pi$-characters of hyperspaces with the hit-and-miss topologies

The \(\pi\)-weights and \(\pi\)-characters of hyperspaces with the hit-and-miss topologies
Authors: Hou, Ji-Cheng;

The $\pi$-weights and $\pi$-characters of hyperspaces with the hit-and-miss topologies

Abstract

For a topological space \(X\), let \(2^X\) be the set of all closed subsets of \(A\) and \(\text{CL}(X)=2^X-\{\emptyset\}\). \(\Delta\) denotes a subset of \(\text{CL}(X)\) satisfying the following properties: (a) \(\Delta\) is closed with respect to the finite unions; (b) the intersection of every finite subset of \(\Delta\) belongs to \(\Delta\) if it contains an element of \(\Delta\). Let \((X,\tau)\) be a topological space and let \(\Delta\subset\text{CL}(X)\). The hit-and-miss topology \(\tau_\Delta\) on \(2^X\) has as a subbase all sets of the form \(V^-\), where \(V\in\tau\), and of the form \((A^C)^+\), where \(A\in \Delta\), and the space \((2^X, \tau_\Delta)\) is denoted by \(\Delta(X)\); for a subset \(A\) of \(X\), put \(A^-= \{F\in 2^X: F\cap A\neq \emptyset\}\), \(A^+=\{F\in 2^X: F\cap A^C\neq \emptyset\}\), where \(A^C\) denotes the complement of \(A\). Let \((X,\tau)\) be a topological space. For a subset \({\mathcal A}\) of \(\text{CL} (X)\), a subset \({\mathcal B}\) of \({\mathcal A}\) is said to be \(\tau\)-cofinal, if for any \(W\in\tau\) and \(A\in{\mathcal A}\) with \(W\cap A^C\neq \emptyset\), there exists \(B\in{\mathcal B}\) which contains \(A\) and \(W\cap B^C\neq \emptyset\) holds. The \(\tau\)-cofinality \(\tau k({\mathcal A})\) of \({\mathcal A}\) is defined as follows: \(\tau k ({\mathcal A})= \inf\{|{\mathcal B}|:{\mathcal B}\) is a \(\tau\)-cofinal subset of \({\mathcal A}\}+\omega\). The author discusses the cardinal invariant associated with \(\Delta\) and proves as main results the following theorems. Theorem 3.1. Let \((X,\tau)\) be either a quasi-regular and \(R_0\) or a \(T_1\) space. Then \[ \pi w(\Delta(X))= \pi\chi (\Delta(X))= \max\{\pi w(X), \tau k(\Delta)\}, \] where \(\pi w\) and \(\pi\chi\) denote the \(\pi\)-weight and \(\pi\)-character, respectively. Theorem 3.5. Let \(X\) be an \(R_0\) space. Then \(\pi w((2^X, \tau_v))= \pi w(X)\), where \(\tau_v\) denotes \(\tau_\Delta\) for \(\Delta= \text{CL} (X)\). A topological space \(X\) is said to be quasi-regular whenever each nonempty open subset of \(X\) contains a closed subset whose interior is nonempty, and \(X\) is an \(R_0\) space whenever each nonempty open subset of \(X\) contains the closure of each of its points.

Country
Italy
Related Organizations
Keywords

hit-and-miss topology, \(\tau\)-cofinality, $\pi$-weights, Hyperspace, Hyperspaces in general topology, 54B20, Cardinality properties (cardinal functions and inequalities, discrete subsets), \(\pi\)-character, 54A25, $\pi$-characters, \(\pi\)-weight

  • BIP!
    Impact byBIP!
    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).
    0
    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.
    Average
    influence
    This indicator reflects the overall/total impact of an article in the research community at large, based on the underlying citation network (diachronically).
    Average
    impulse
    This indicator reflects the initial momentum of an article directly after its publication, based on the underlying citation network.
    Average
Powered by OpenAIRE graph
Found an issue? Give us feedback
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
Average
Green