Preventing first player from getting 11 Xs in a row
Source: IMO Shortlist 1994, C6
October 22, 2005
combinatorics unsolvedcombinatoricsIMO ShortlistCombinatorial gamesgame strategygame
Problem Statement
Two players play alternatively on an infinite square grid. The first player puts an in an empty cell and the second player puts an in an empty cell. The first player wins if he gets adjacent 's in a line - horizontally, vertically or diagonally. Show that the second player can always prevent the first player from winning.