Notions of compactness for special subsets of ℝ I and some weak forms of the axiom of choice

Journal of Symbolic Logic 75 (1):255-268 (2010)

We work in set-theory without choice ZF. A set is Countable if it is finite or equipotent with ${\Bbb N}$ . Given a closed subset F of [0, 1] I which is a bounded subset of $\ell ^{1}(I)$ (resp. such that $F\subseteq c_{0}(I)$ ), we show that the countable axiom of choice for finite sets, (resp. the countable axiom of choice AC N ) implies that F is compact. This enhances previous results where AC N (resp. the axiom of Dependent Choices) was required. If I is linearly orderable (for example $I={\Bbb R}$ ), then, in ZF, the closed unit ball of the Hilbert space $\ell ^{2}(I)$ is (Loeb-)compact in the weak topology. However, the weak compactness of the closed unit ball of $\ell ^{2}(\scr{P}({\Bbb R}))$ is not provable in ZF
Keywords Axiom of Choice   product topology   compactness   sequential compactness
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DOI 10.2178/jsl/1264433919
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References found in this work BETA

Products of Compact Spaces in the Least Permutation Model.Norbert Brunner - 1985 - Zeitschrift fur mathematische Logik und Grundlagen der Mathematik 31 (25-28):441-448.
The Compactness of 2^R and the Axiom of Choice.Kyriakos Keremedis - 2000 - Mathematical Logic Quarterly 46 (4):569-571.
Products of Compact Spaces in the Least Permutation Model.Norbert Brunner - 1985 - Mathematical Logic Quarterly 31 (25‐28):441-448.
The Axiom of Choice.Thomas J. Jech - 1976 - Journal of Symbolic Logic 41 (4):784-785.

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