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Topology and its Applications
Article
License: Elsevier Non-Commercial
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Topology and its Applications
Article . 1994
License: Elsevier Non-Commercial
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Topology and its Applications
Article . 1994 . Peer-reviewed
License: Elsevier Non-Commercial
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Embeddings into c0

Embeddings into \(c_ 0\)
Authors: Jan Pelant;

Embeddings into c0

Abstract

\textit{I. Aharoni} [Israel J. Math. 19(1974), 284-291 (1975; Zbl 0303.46012)] proved that each separable metric space can be Lipschitz embedded into \(c_0(\omega_0)\), \textit{P. Assouad} [ibid. 31, 97-100 (1978; Zbl 0387.54003)] simplified his proof, improved constants of Lipschitz embeddings and suggested a generalization of his result. We present the final improvements of constants for Lipschitz embeddings into \(c^+_0(\omega_0)\) and another generalization of Aharoni's result which comes from the theory of uniform spaces. Namely we prove that each metric space \((X,\rho)\) whose metric uniformity has a base consisting of point-finite uniform covers (such a base of uniform covers will be called a point-finite base; a base of uniform covers is a collection of uniform covers such that each uniform cover is refined by some cover that belongs to a base) can be uniformly embedded into \(c^+_0(\kappa)\) where \(\kappa=\text{dens }X\). We also prove that a uniformity of each \(c_0(\kappa)\) has a point-finite base of uniform covers, hence the \(c_0(\kappa)\) are universal spaces for metrizable uniformities with a point-finite base. It implies a well-known fact [\textit{G. Vidossich}, Proc. Am. Math. Soc. 25, 551-553 (1970; Zbl 0181.50903)] that each separable uniformity has a point-finite base. Investigating the Lebesgue number of point-finite uniform refinements we point out the reason why a uniform embedding cannot be generally replaced by a Lipschitz one in the nonseparable case. As a byproduct of this analysis we obtain full information on the Lebesgue number of point-finite uniform refinements in \(c_0(\kappa)\) and an internal characterization of metric spaces which are Lipschitz embeddable into \(c_0(\kappa)\). The difference between \(c_0\) and \(c^+_0\) is mentioned. \((c_0(\kappa)\) denotes a Banach space the underlying set of which is formed by all real-valued mappings \(f:\kappa\to\mathbb{R}\) such that \(\text{card(supp } f)\leq\omega_0\) and \(\lim f=0\), and which is equipped with the sup-norm. \(c^+_0(\kappa)\) denotes its metric subspace consisting of all mappings the values of which are nonnegative.) For a real number \(t\), \((t)^+=\max(t,0)\), \((t)^-= \max(-t,0)\). For a cardinal number \(\alpha\), \((\alpha)^+\) denotes the successor of \(\alpha\).

Related Organizations
Keywords

point-character of uniformities, Metric spaces, uniformly continuous embeddings, Lipschitz embeddings, Metric spaces, metrizability, Uniform structures and generalizations, Geometry and Topology, Uniformly continuous embeddings, Point-character of uniformities, Embedding

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    citations
    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).
    13
    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.
    Top 10%
    influence
    This indicator reflects the overall/total impact of an article in the research community at large, based on the underlying citation network (diachronically).
    Top 10%
    impulse
    This indicator reflects the initial momentum of an article directly after its publication, based on the underlying citation network.
    Average
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citations
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!
13
Top 10%
Top 10%
Average
hybrid