Interesting problem - Choose no fewer than n/4 squares
Source: IMO LongList 1979 - P13
May 29, 2011
combinatorics unsolvedcombinatorics
Problem Statement
The plane is divided into equal squares by parallel lines; i.e., a square net is given. Let be an arbitrary set of squares of this net. Prove that it is possible to choose no fewer than squares of in such a way that no two of them have a common point.