A008907 - OEIS

%I #16 Mar 30 2012 18:35:06

%S 1,3,12,38,108,174,204,153,57,15

%N Number of legal tic-tac-toe (or noughts and crosses) positions after n plays, up to rotation and reflection.

%C As in chess, a "play" is an action by a single player, a "move" is two contiguous plays by each opponent.

%H <a href="/index/Th#TTT">Index entries for sequences related to tic-tac-toe</a>

%e Example: a(2) = 12 since after 2 plays there are 12 legal positions:

%e -x- -x- -x- -x- -o- x-- x-- x-- x-o xo- o-- ox-

%e --- --- -o- o-- -x- --- --o -o- --- --- -x- ---

%e -o- o-- --- --- --- --o --- --- --- --- --- ---

%K nonn,fini,full

%O 0,2

%A _Russ Cox_

%E Additional comments from _David W. Wilson_, Jun 03 2001