MathDB
Sad Combinatorics

Source: 2020 USOMO 5, by Carl Schildkraut

June 21, 2020
combinatoricsHi

Problem Statement

A finite set SS of points in the coordinate plane is called overdetermined if S2|S|\ge 2 and there exists a nonzero polynomial P(t)P(t), with real coefficients and of degree at most S2|S|-2, satisfying P(x)=yP(x)=y for every point (x,y)S(x,y)\in S. For each integer n2n\ge 2, find the largest integer kk (in terms of nn) such that there exists a set of nn distinct points that is not overdetermined, but has kk overdetermined subsets.
Proposed by Carl Schildkraut