Sunday Problem #72

Published 17 Oct 2021 by antti (last edited 24 Oct 2021)

This is a common corner shape that all dan players should know by heart.

Black to play.

If there is something in the corner, it has to start from Black’s placement of 1. After this, White has two eye-forming bumping moves to choose from; generally, forming the farmer’s hat with white 2 here is best. After 3–6, it might look like Black cannot achieve much, but she can in fact set up a one-move approach kō in the corner with 7–13. After 13, White can already start fighting the kō, or he can play elsewhere once, wait for Black to fill his last liberty, and then fight it.
A one-move approach kō is considerably worse than a direct kō, and so Black should not be in a hurry to start this during a game. I like to think that, in the problem setting, the white corner group is basically alive but forms no territory; if White adds a move to defend his group, the move is roughly worth the territory he forms.
Solution (alternate)
White can also bump with 2 here. 3–11 follow, and again the result is a one-move approach kō in White’s favour.

Comments (3)

Oscar wrote 1 year, 5 months ago:

I learned this shape as a 4d while playing a real game. I was on the white side of it and got unpleasantly surprised!

antti wrote 1 year, 5 months ago:

It makes you wonder if you should descend to q18 at all as white, or if the hanging connection of r18 would be better. Apparently the difference between these is only about half a point!

antti wrote 1 year, 5 months ago:

Solution added.