Sunday Problem #18
Published 4 Oct 2020 by antti
(last edited 11 Oct 2020)
This shape appeared in one of Oscar’s games.
Black’s task, of course, is to live, but there are several ways to do so. The real problem is in finding the best way to live.
Black to play.
Solution
Even when the task is to live, you want to live with as much territory as possible. Therefore, in this kind of a problem, you first want to consider the eye-shape expanding move of black 1.
Unfortunately, white 2 is a good response, and Black is forced to play 3–5, giving up two stones in the process. Black can do better than this.
If instead of 3 Black plays at 5, then white a, black b, white c, and black d start a kō for the whole black group.
Another way for Black to try to optimise her territory is to not to try to expand the eye shape, but to play a move on its border that does not fill up black territory, such as 1 here.
This time, white responds with 2. Black 3 is then good for building eye shape but, up to 10, White manages to create a gote seki. Still, this is slightly better for Black than the previous outcome.
What if Black goes directly to try to secure two eyes with 1?
This time, white 2 turns out to be a good response. The best Black can do is then set up a seki with 3–9, but this is considerably better for White than the previous diagram because White also gets sente.
Black’s best move is the non-obvious empty triangle of 1, which makes a miai of 2 and 3. This way, Black avoids having to sacrifice two stones and also secures five points of territory.
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