ASU 249 All Soviet Union MO 1977 1000 squares with parallel sides
Source:
July 6, 2019
combinatorial geometrySquares
Problem Statement
Given squares on the plane with their sides parallel to the coordinate axes. Let be the set of those squares centres. Prove that you can mark some squares in such a way, that every point of will be contained not less than in one and not more than in four marked squares