(n+1)^2 points inside a n x n square. Looks classical.
Source: 1st Romanian Master in Mathematics (RMIM) 2008, Bucharest, Problem 4
February 9, 2008
geometrypigeonhole principleperimetercombinatorial geometrycombinatorics proposedcombinatorics
Problem Statement
Consider a square of sidelength and (n\plus{}1)^2 interior points. Prove that we can choose of these points so that they determine a triangle (eventually degenerated) of area at most .