MathDB
2012-2013 Winter OMO #33

Source:

January 16, 2013
Online Math Opengeometrygeometric transformationreflectioncombinatorial geometry

Problem Statement

Let nn be a positive integer. E. Chen and E. Chen play a game on the n2n^2 points of an n×nn \times n lattice grid. They alternately mark points on the grid such that no player marks a point that is on or inside a non-degenerate triangle formed by three marked points. Each point can be marked only once. The game ends when no player can make a move, and the last player to make a move wins. Determine the number of values of nn between 11 and 20132013 (inclusive) for which the first player can guarantee a win, regardless of the moves that the second player makes.
Ray Li