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On countable choice and sequential spaces
Author(s) -
Gutierres Gonçalo
Publication year - 2008
Publication title -
mathematical logic quarterly
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.473
H-Index - 28
eISSN - 1521-3870
pISSN - 0942-5616
DOI - 10.1002/malq.200710018
Subject(s) - mathematics , countable set , cosmic space , metric space , axiom of choice , axiom , space (punctuation) , closure (psychology) , discrete mathematics , linear subspace , urysohn and completely hausdorff spaces , context (archaeology) , topological space , combinatorics , pure mathematics , set (abstract data type) , set theory , computer science , hausdorff dimension , paleontology , geometry , economics , hausdorff measure , market economy , biology , programming language , operating system
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 $ \hat \sigma $ ‐completion. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)