Two players alternately write O's (first player) and X's (second player) in the unoccupied cells of an n x n grid.
The first player to occupy the vertices of a square with horizontal and vertical sides is the winner. What is the smallest n such that the first player has a winning strategy?
Note: Roland Bacher and Shalom Eliahou proved that every 15 x 15 binary matrix contains four equal entries (all 0's or all 1's) at the vertices of a square with horizontal and vertical sides. So the game must result in a winner (the first player) when n=15.