IMO Shortlist 2004 problem C2


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April 2, 2012
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Let {n} and k be positive integers. There are given {n} circles in the plane. Every two of them intersect at two distinct points, and all points of intersection they determine are pairwisely distinct (i. e. no three circles have a common point). No three circles have a point in common. Each intersection point must be colored with one of n distinct colors so that each color is used at least once and exactly k distinct colors occur on each circle. Find all values of n\geq 2 and k for which such a coloring is possible.
Source: Međunarodna matematička olimpijada, shortlist 2004