A Bend in the Corner - or: Death by Tenuki

Today I re-discovered a concept I have read about a couple of months ago: The 'bent four in the corner'-shape. I could never actually put it to use, so it was kind of dead knowledge to me.

I came across it while solving problems (again). It took me quite a while to get through the problem. I will try to not bore you by discussing every detour I took. But: One of the detours towards the solution led me to the bent four. And that is the one I want to talk about.

Here is the tsumego. I will also present the solution. So do not expand carelessly, if you want to solve it at your own.

Diagram 1

Black to start a ko.

Diagram 2 - Black 5 on D19

First, let's quickly outrule a wrong variation. It is rather tempting to start with an atari. In this case this leads to white being alive, though. So that couldn't be it. I figured that one out on my own and I'm rather proud on it ;)

Diagram 2

The solution starts with black 1 as shown here. The atari at A is the weakness that begs to be exploited, but it needs to be done indirectly.

Diagram 3

Diagram 3 shows the solution. It is a ko for life. I was unsatisfied at first, because I could not figure out why that should be the only way for white to play.







I'm glad I followed up at my dissatisfaction. It was what led me to what is coming now. However, anything from here on wasn't in my book - so take it in with care. I also concentrate on the variation that leads to the bent 4 in the corner.

Diagram 4

What happens when white plays 2 like here in diagram 5 and then connects solidly with 4 after the atari? I believed there might be something to it. Maybe another ko, a seki or even a second eye for white.

Diagram 5

This even looks as if I was right and white could at least force a ko (w-c18, b-a19, w-a17, b-b19, w-a19, b starts ko with a18) . I still come up a lot with moves like that and it did take me very long to realize that there is a better possibility for black to play.

EDIT: If I see it right, white only has the opportunity to force a ko, as long as black plays tenuki and does not close the last outside liberty. Otherwise it is seki.

Diagram 6

The continuation of diagram 4 should look like this. In the beginning, I made another mistake and mistook that for another ko, because of the shape in the corner. I thought black would play on A and white take on B. Diagram 7 shows the resulting ko.

Diagram 7

Black 12 at A would start a ko for life.








So the question is: If this variation also leads to a ko, why does my book not talk about it? It took me some time, but preparing the diagrams for this post I finally got it: It is a bend four in the corner shape. So it is dead. But: Why is it dead? Lets take another look at diagram 6!

Diagram 6

Black has absolutly no reason to play A as long as there are any ko-threats left on the board. He could easily wait until the very end of yose and then kill the stones by starting a ko white can not win. If white tries to approach, she dies. For that reason the 'bent four in the corner''-shape is considered to be dead.

It was just difficult for me to stop. In essence, black kills by playing tenuki.

I have read about it quite some time ago, now I got it and will not forget again. Nice!