Algebra, Logic and Topology

 

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Author(s)
Gonçalo Gutierres;

Title
On countable choice and sequential spaces

Abstract
Under the axiom of choice, every first countable space is a Fréchet-Urysohn space. Although, in its absence
even R may fail to be a sequential space.
Our goal in this paper is to discuss under which set-theoretic conditions some topological classes, such as
the first countable spaces, the metric spaces, or the subspaces of R, are classes of Fréchet-Urysohn or sequential
spaces.
In this context, it is seen that there are metric spaces which are not sequential spaces. This fact raises the
question of knowing if the completion of a metric space exists and it is unique. The answer depends on the definition
of completion.
Among other results it is shown that: every first countable space is a sequential space if and only if the axiom
of countable choice holds, the sequential closure is idempotent in R if and only if the axiom of countable choice
holds for families of subsets of R, and every metric space has a unique σ-completion.

Journal
Mathematical Logic Quarterly

Volume
54

Year
2008

Issue
2

Page(s)
145-152

 
     
 

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