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HMIC 2013 Problem 4

    A subset U⊂RU \subset \mathbb{R} is open if for any x∈Ux \in U, there exist real numbers a,ba, b such that x∈(a,b)⊂Ux \in(a, b) \subset U. Suppose S⊂RS \subset \mathbb{R} has the property that any open set intersecting (0,1)(0,1) also intersects SS. Let TT be a countable collection of open sets containing SS. Prove that the intersection of all of the sets of TT is not a countable subset of R\mathbb{R}. (A set Γ\Gamma is countable if there exists a bijective function f:Γ→Zf: \Gamma \rightarrow \mathbb{Z}.)

    Dataset text NuminaMath olympiads. Quoted with source.

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