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In the Cartesian coordinate plane define the strips S_n = \{(x,y)|n\le x < n + 1\}, n\in\mathbb{Z} and color each strip black or white. Prove that any rectangle which is not a square can be placed in the plane so that its vertices have the same color.

IMO Shortlist 2007 Problem C5 as it appears in the official booklet:In the Cartesian coordinate plane define the strips S_n = \{(x,y)|n\le x < n + 1\} for every integer n. Assume each strip S_n is colored either red or blue, and let a and b be two distinct positive integers. Prove that there exists a rectangle with side length a and b such that its vertices have the same color.

Edited by Orlando Döhring

Author: Radu Gologan and Dan Schwarz, Romania

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