Parallelogram Hater
Source: Rioplatense L2 2023 #4
December 6, 2023
geometrycombinatorics
Problem Statement
A set of points on the plane is antiparallelogram if any four points of the set are not vertices of a parallelogram. Prove that for any set of points on the plane, no three of them are collinears, there exists a subset of points, such that this subset is antiparallelogram.