TUYMAADA 1996 Problem 3
Nine points of the plane, located at the vertices of a regular nonagon, are pairwise connected by segments, each of which is colored either red or blue. It is known that in any triangle with vertices at the vertices of the nonagon at least one side is red. Prove that there are four points, any two of which are connected by red lines.
Dataset text AI-MO AoPS. Quoted with source.
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