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Butterfly Seven
Keywords: Life & Death
(Term coined by Dieter) This shape is alive. The best Black can get in one move is a seki in gote by playing at a (see next diagram). White playing there makes six points in gote.
After 1, if Black occupies two of the four marked points, White should occupy the other two. Further elaboration: a question of ko threats1. Black's threat
If Black plays 1, he threatens to kill the group. If White does nothing ...
Black 1 (or any of the other two marked points) sets up a killing shape. The other two marked points are then miai for the kill. If White does nothing at all, Black will eventually be able to almost fill the eyespace with a rabbitty six. If White plays one of the marked points, Black takes the other. The kill then branhces into a farmers hat almost fill or a bulky five almost fill.
So, White answers at 2. Black can either continue at 3 (because otherwise his ko threat loses points: White 3 makes seven points) or leave it as another ko threat (depending on the value of the ko). In either case, White must respond at 4 for the same reason as explained above. The final seki position is reached. Black has yet another ko threat but that is one definitely losing points: White will capture and live with a bent four and 8 points.
2. White's threat
White's threat is certainly less valuable. If Black ignores it, White 3 makes 6 points. If Black answers ...
... meaning this threat is even too large to bear, White 3 and Black 4 follow in the same logic.
Conclusion: It is very unlikely that the group gets killed as a result of a ko elsewhere. Black's first move inside the eye space indicates that the endgame stage is reached where the most valuable moves are about six points. It is unlikely that later a ko will develop whose value is large enough to sacrifice the white group. Of course you never know with amateurs ... Note that Black will have to play 17 unanswered moves to capture the 16 stone group, making the value of each move worth only about 2 points each ( depending on whether it becomes a 6 point live shape or seki ). White may benefit from either forcing the capture or using the liberty excess to kill all or part of the wall.
Bill: Assuming that the surrounding Black stones are alive, Black will have to make only 3 unanswered moves to kill White. The other moves, if they are made at all, would occur after gainful play is over, to demonstrate that the White stones are dead. So they cost nothing. In the original position White has on average 3 points of territory. Black's first move is worth 3 points (miai), making a seki. Each of the other moves gains 18 points, for a Black territory worth 36 points. (That's a large ko threat. :-)) I was alluding to something like this:
Defend the butterfly or take an outside liberty? Bill, in what I said above, I was assuming the outside stones were not absolutely alive. SiouxDenim Dieter: OK, but then the issue is how to win the capturing race, not the value of each move. Still, making the inside seki, captures the 12 Black stones on the right. That is, assuming the White stones ... oh, no... Bill: None of the above.
Even if Black plays first he cannot capture the White stones, as Dieter indicated.
If White ignores B 1 or B 3, Black will kill for a swing of 83 points.
To show how wrong I was, here's the details:
OK, not quite enough liberties. I should have added a stone at the circled spot.
SiouxDenim
Bill: I have removed 1 Black stone on the right and added one in the middle. Black is still dead, but now has 18 humungous ko threats! E. g., B 1 - W 2 and B 3 - W 4. (White can remove 15 of them at the cost of one point by playing at 1 or 3 before Black plays here.) This is a copy of the living page "Butterfly Seven" at Sensei's Library. ![]() |