In mathematics, a Menger space is a topological space that satisfies a certain a basic selection principle that generalizes σ-compactness. A Menger space is a space in which for every sequence of open covers
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History
In 1924, Karl Menger introduced the following basis property for metric spaces: Every basis of the topology contains a countable family of sets with vanishing diameters that covers the space. Soon thereafter, Witold Hurewicz observed that Menger's basis property can be reformulated to the above form using sequences of open covers.
Menger's conjecture
Menger conjectured that in ZFC every Menger metric space is σ-compact. Fremlin and Miller proved that Menger's conjecture is false, by showing that there is, in ZFC, a set of real numbers that is Menger but not σ-compact. The Fremlin-Miller proof was dichotomic, and the set witnessing the failure of the conjecture heavily depends on whether a certain (undecidable) axiom holds or not.
Bartoszyński and Tsaban gave a uniform ZFC example of a Menger subset of the real line that is not σ-compact.
Combinatorial characterization
For subsets of the real line, the Menger property can be characterized using continuous functions into the Baire space
The cardinality of Bartoszyński and Tsaban's counter-example to Menger's conjecture is