The best way to face off against this computer program is to mark a corner. If the program does not select the middle square, you can always win. If it does (1/8), you select one of the 2 spots adjacent to your first mark. If the computer does not block you, you win. If it does block you (1/6), you block them, and have a chance to win. If the computer blocks you again (1/4), it has forced a tie, but otherwise you win. So, the best strategy yields a 1-(1/8)(1/6)(1/4) = 191/192 chance of winning.

Congratulations to those of you who submitted correct answers this month, especially Didrik Jonassen, this month’s winner of a Jane Street t-shirt!