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2007-06-04

Problem diagram.
Black plays.

Source:
Igo Hatsuyo-ron #11 : 300 year old problem set for student professionals.

This classic problem's status changes depending on whether the Japanese or Chinese rule sets are in effect. The position arises near the end of main line "solution". (You can thank me later for skipping over the nightmare of variations leading up to this diagram.)

Analysis:
Myron Souris

Solution follows (or click here to download SGF)

One way street.
Black 1 must prevent White from connecting out down the side. White 2 must prevent a Black play at 2, which would prevent White from forming a rabbity-six shape in the corner to kill Black. And again, Black 3 must prevent the rabbity-six.

But a conundrum...
Black 5 repeats the original position from 4 moves back. Under Japanese rules, Black has an Eternal Life ko, i.e., if neither player stops repeating the cycle, the game has no outcome.

BUT NOT SO FAST! Under Chinese (Situational Superko) rules, Black cannot play at 5 to repeat the position from 4 moves back. So Black must make a big threat elsewhere, to which White responds. With the overall board position changed, Black can then go back to play at 5 and the cycle repeats.

Note that White does not have to find any ko threats, so Black eventually runs out of big "elsewhere" moves, and dies by not being able to restart the 4 move cycle along the top right edge.

In the original Igo Hatsuyo-ron problem (that started well before the top of page diagram), Black's only resource for life is this variation that ends in an Eternal Life ko. So with the Japanese rules, Black avoids death; with the Chinese rules, Black dies.

Fascinating! Kind of makes you want to study Go the rest of your life, doesn't it?

Small board Go problems often require the logical rigor of the Chinese rule set. Here are two other examples from the archive:
Problem 95
Problem 96