Alternative solution to problem #155 in GGPFB Vol. 1?

Teaching
1

Hello!

Working through Graded Go Problems for Beginners Volume 1 by Kano Yoshinori 9p published by Kiseido, I found what looks to me like a solution to problem #155, but it is not the given solution.

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Problem 155. Black to play. (1 move)
How can Black kill the white group?

The given solution is A. I thought it was B, which isn’t discussed in the answer section. C is the wrong move, which is discussed.

Is B also correct, and if not, why not? Playing it out it looked okay to me, but perhaps I missed something or A is better?

2

No.

3

If black B, white can answer at A and it’s a ko. So black A is considered better as it kills unconditionally.

2 Likes
4

Sometimes problems do have multiple solutions, in which case the correct answer will be the best possible move(s), either in terms of profit or economy. And without looking at anything else, A is the best here: it kills with a single move, and that’s that.

If nothing else, though, B isn’t economical: if you played there, White could capture above B, then you’d have to play at A anyway, making B an unnecessary move.

But, as @SanDiego pointed out, it goes beyond that. If White answered Black B at A instead, Black would have to capture the stone below B (otherwise White would capture B and live). So, Black captures… and that starts a ko.

And if White can find a good ko threat somewhere on the board, she can capture Black’s capturing stone and, unless Black can find a good ko threat of his own, capture B next and live.

So, you see, you have a certain solution and one that could either work or lead to some real trouble.

2 Likes
5

Aah, got it. I hadn’t spotted the ko. So if black B and white A, black plays in the lower eye and takes a stone to start the ko. The outcome then depends on how many ko threats each player can find. If white doesn’t have any good ko threats, black can connect to end the ko, but if white does have a good ko threat, she will retake the ko and so on.

Thank you!

1 Like
6

Haha @smurph! I’ll read that again :wink: