IMO Shortlist 1996 problem C4


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Determine whether or nor there exist two disjoint infinite sets A and B of points in the plane satisfying the following conditions:

a.) No three points in A \cup B are collinear, and the distance between any two points in A \cup B is at least 1.

b.) There is a point of A in any triangle whose vertices are in B, and there is a point of B in any triangle whose vertices are in A.
Izvor: Međunarodna matematička olimpijada, shortlist 1996