IMO Shortlist 1977 problem 16
Dodao/la:
arhiva2. travnja 2012. Let

be a set of

points in the plane

whose coordinates are integers such that any three points from

are vertices of a nondegenerate triangle whose centroid doesnt have both coordinates integers. Determine the maximal
%V0
Let $E$ be a set of $n$ points in the plane $(n \geq 3)$ whose coordinates are integers such that any three points from $E$ are vertices of a nondegenerate triangle whose centroid doesnt have both coordinates integers. Determine the maximal $n.$
Izvor: Međunarodna matematička olimpijada, shortlist 1977