HMIC 2013 Problem 4
A subset is open if for any , there exist real numbers such that . Suppose has the property that any open set intersecting also intersects . Let be a countable collection of open sets containing . Prove that the intersection of all of the sets of is not a countable subset of . (A set is countable if there exists a bijective function .)
Dataset text NuminaMath olympiads. Quoted with source.
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