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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 Israel Journal of Ma...arrow_drop_down
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
Israel Journal of Mathematics
Article . 2006 . Peer-reviewed
License: Springer TDM
Data sources: Crossref
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 . 2006
Data sources: zbMATH Open
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Ball-covering property of Banach spaces

Authors: Cheng, L. X.;

Ball-covering property of Banach spaces

Abstract

The paper deals with coverings of the unit sphere of a Banach space \(X\) by a collection of balls, not containing the origin. The main results are: (1) if \(\text{dim} X = n < \infty\), then the number of balls in such a covering cannot be smaller than \(n+1\), and if, moreover, \(X\) is smooth, then there is such a covering by exactly \(n+1\) balls. (2) If \(S_X\) admits such a covering by a countable number of balls \(B(x_n,r_n),\) \(n \in \mathbb N\), then \(X^*\) is weak-star separable, and if additionally \(\sup_n r_n< 1\), then \(X\) is separable. (3) The unit sphere of \(\ell_\infty\) can be covered by a countable collection of radius-one balls not containing the origin. Finally, (4) if \(X\) is a Gâteaux differentiability space with weak-star separable \(X^*\), then \(S_X\) admits such a covering by a countable number of balls (Theorem 4.3 of the paper). Also, in several places (Lemma 4.2, Theorem 4.3, Corollary 4.4, Theorem 4.5 and Theorem 4.7) the author confuses weak-star separability of \(X^*\) with weak-star separability of \(B_{X^*}\). Also there are some unclear places in the proof of Lemma 4.2, where the author confuses weak-star sequential compactness of \(B_{X^*}\) with the condition that for every \(A \subset B_{X^*}\) every weak-star cluster point of \(A\) is the weak-star limit of a sequence \((a_n) \subset A\) (i.e., that \((B_{X^*}, w^*)\) is angelic). This obstacle renders the validity of Theorem 4.3 questionable.

Country
China (People's Republic of)
Related Organizations
Keywords

separable Banach space, Geometry and structure of normed linear spaces, Gâteaux differentiability space, non-separable Banach space

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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!
20
Top 10%
Top 10%
Average
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