Points and circles in the plane...
Source: JBMO 2009 Problem 4
June 27, 2009
ceiling functionanalytic geometrygeometrycircumcirclecombinatorics proposedcombinatorics
Problem Statement
Each one of 2009 distinct points in the plane is coloured in blue or red, so that on every blue-centered unit circle there are exactly two red points. Find the gratest possible number of blue points.