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China Mathematics Olympiads (National Round) 2008 Problem 5

Source:

November 28, 2010
geometrytrapezoidpigeonhole principlecombinatorics

Problem Statement

Find the smallest integer nn satisfying the following condition: regardless of how one colour the vertices of a regular nn-gon with either red, yellow or blue, one can always find an isosceles trapezoid whose vertices are of the same colour.