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Two disjoint infinite sets A and B of points in the plane

Source: IMO Shortlist 1996, C4

August 9, 2008
combinatorial geometrycombinatoricspoint setTriangleIMO Shortlist

Problem Statement

Determine whether or nor there exist two disjoint infinite sets A A and B B of points in the plane satisfying the following conditions: a.) No three points in AB A \cup B are collinear, and the distance between any two points in AB A \cup B is at least 1. b.) There is a point of A A in any triangle whose vertices are in B, B, and there is a point of B B in any triangle whose vertices are in A. A.