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At some places (see the references) Martin Erickson describes a certain game:  "Two players alternately write O's (first player) and X's (second player) inthe unoccupied cells of an n x n grid. The first player (if any) to occupy fourcells at the vertices of a square with horizontal and vertical sides is thewinner."  Then he asks "What is the outcome of the game given optimal play?" or  "What is the smallest n such that the first player has a winning strategy?"  For n lower than 3 a win is obviously impossible.  The aim of this article and the additionally (in the source package) providedcomputer program SQRGAME2 is to give and prove sure strategies for the secondplayer not to lose if n is 3 or 4, and for the first player to win if n is 5.

Guaranteed successful strategies for a square achievement game on an n by n grid