olymmath

SCHWEITZER 2015 Problem 1

    Let KK be a closed subset of the closed unit ball in R3\mathbb{R}^3 . Suppose there exists a family of chords Ω\Omega of the unit sphere S2S^2 , with the following property: for every X,Y∈S2X,Y\in S^2 , there exist X′,Y′∈S2X',Y'\in S^2 , as close to XX and YY correspondingly, as we want, such that X′Y′∈ΩX'Y'\in \Omega and X′Y′X'Y' is disjoint from KK . Verify that there exists a set H⊂S2H\subset S^2 , such that HH is dense in the unit sphere S2S^2 , and the chords connecting any two points of HH are disjoint from KK .

    EDIT: The statement fixed. See post #4

    Dataset text AI-MO AoPS. Quoted with source.

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