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IMO 2013 Problem 2

  • Day 1

A configuration of 40274027 points in the plane is called Colombian if it consists of 20132013 red points and 20142014 blue points, and no three of the points of the configuration are collinear. By drawing some lines, the plane is divided into several regions. An arrangement of lines is good for a Colombian configuration if the following two conditions are satisfied:

i) No line passes through any point of the configuration.

ii) No region contains points of both colors.

Find the least value of kk such that for any Colombian configuration of 40274027 points, there is a good arrangement of kk lines.

Proposed by Ivan Guo from Australia.

Official text Transcription from AI-MO AoPS, checked word for word against the official paper. Quoted with source.

Official text: IMO 2013 official English paper, page 1.

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