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Atari Go Counting at Sensei's Library

Atari Go Counting

  Difficulty: Intermediate   Keywords: Variant, Theory

Atari Go Counting

Atari-go is a go variant, where the first player to capture any stones instantly wins the game.

Usually a game of atari-go is played to the bitter end, so that if no stones are captured before the territories are settled, players play the dame, and then add stones to their own territories until one of the players will be forced to put some of his own stones into atari.

Counting territories

[Diagram]
5x5 Atari go basic counting  

Counting points in atari-go is done very much the same way as in normal go. [1]

At this point of the game, all the territory is settled. Black has 7 points of surrounded territory, white has 6. As the players play inside their territories, white will be the first to put himself in a self-atari, so black will win, if he plays with a correct strategy.


Seki

[Diagram]
5x5 Atari go counting: seki  

On this board, there are two independent sekis. The sekis count as zero points for both, so the next player to move will lose, because only self-atari moves are available.


Group tax

Because every group needs two liberties to avoid capture (in atari-go they don't even have to be in separate eyes), having more groups costs some of the points as group tax.

[Diagram]
5x5 Atari go group tax  

Here white has two groups against black's one. As you can see, black can play one more move without putting himself in atari, after which white is forced into playing self-atari.


"Square tax"

[Diagram]
5x5 Atari go square tax  

A surprising feature of atari go is that the square shape in this diagram counts as zero points for white. This happens when the white group does not have another eye elsewhere.


[Diagram]
5x5 Atari go square tax - seki!  

Even though black originally had no move inside the white square, after W2 black can play B3 to turn white's group into a seki. White is forced to playing a self-atari on his next move.


[Diagram]
5x5 Atari go endgame with square tax  

The square tax seems to be two points, just like the group tax. Because a group with a two point eye is worth zero (2 points minus group tax), and a group with a single square eye is worth zero (4 points minus group tax and the square tax), here it does not matter whether white plays a or b.

If white chooses to play b, he gets rid of the square (or, "pays the square tax"), but black a will force white to play inside his own territory once more, so the price was two points anyway.


Other interesting bits

Bass could not come up with any more strange atari-go scoring quirks, so add yours here!

[1] RobertJasiek: Why? Pass-fights are possible and can let "scores" be fake predictions! Oops, now I get it: One needs a proof that pass-fights would require captures, and in atari go, they can't occur prematurely.


Atari Go Counting last edited by RobertJasiek on March 25, 2013 - 14:57
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