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What are the symmetries of the tic tac toe board game? Ie, what are the ways you can rotate, reflect, and/or flip the tic tac toe board, such that the next best move to a board(after it was rotated, reflected, etc) is still the next best move after the board was rotated/reflected/fipped? How would i also construct a group multiplication table for these symmetries?

Thank You!

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Please show some work. –  Henning Makholm Oct 17 '11 at 0:17

3 Answers 3

This may be a more subtle question than it seems at first sight.

The easy answer might be that the board is a $3\times 3$ square and so you are looking at the symmetry group of a square.

However, the number of possible different games is known to be 255,168 ignoring symmetry and 26,830 taking symmetry into account. Surprisingly, the latter number is less than one-eighth of the former. The way I once tried to explain this was

  • the first diagram below is equivalent to the second using a reflection in the line between the top right and bottom left, so they can both be considered as being the third;
  • therefore the fourth must be equivalent to the fifth, since they are both essentially the sixth, which is simply the third with two extra moves.

enter image description here

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On the other hand, if one simply numbers the moves consecutively, the first and second are equivalent under a reflection, but the fourth and fifth aren’t: in the fourth move $4$ is adjacent to move $1$, while in the fifth it’s adjacent to move $3$. Under this stricter (and in my view more reasonable $-$ the order of the moves is part of the game) notion of equivalence I believe that you do get the expected number of different games. –  Brian M. Scott Oct 17 '11 at 0:57
    
@Brian. Indeed. You get 31,896 possible games (one eighth of 255,168) with your rather less exciting definition of symmetry. –  Henry Oct 17 '11 at 0:59
    
May you live in interesting ti- definitions! :-) –  Brian M. Scott Oct 17 '11 at 1:01

Why isn't the total number of possible games 362880? That would be 9*8*7*6*5*4*3*2.

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Because it is possible for the game to end before all 9 squares are filled in. –  fixedp Sep 14 at 14:03
    
Oh yeah. Thanks! –  Tech Curmudgeon Sep 14 at 14:14

I once made a ticktacktoe AI, and in order to keep my potential outcomes small I rotated the board. When the user would make a choice I rotated the placements on the board so that the mouse would be over one of two options that the AI was concerned with. If AI takes mid he now thinks the only choices for the user are bottom-right and bottom-middle. If you chose top I would pass the second choice bot-mid and do a 180 flip to the board so you would choose what appears to be top-mid. I hope this helps.

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