Bent four in the corner
Bent four in the corner is an expression referring to two things.
- The eye shape of a group in the corner being bent four, which we will discuss here.
- The fact that under some Japanese rules, a group which is surrounded by a live group and whose eye shape can be reduced to this shape, is --as a rule-- dead. Discussion at Bent four in the corner is dead.
1. Fewer than two outside liberties
If the defending shape has fewer than two liberties on the outside, Black can prevent unconditional life for White's group and make a ko by playing at a.
White responds with an atari at . Black starts the ko at
.
White can't play oshitsubushi at a because it would be self-atari; and the fate of the group is decided by a ko fight.
1.1 Finishing the ko
With a single outside liberty, Black finishes the ko by almost filling the eyespace with a bent three. Playing at a reignites the ko and gives White a second chance.
With no outside liberties, Black can finish the ko by capturing all the stones. Playing at a leaves White a strong ko-threat.
2. At least two outside liberties
If White has an extra liberty on the outside, however, she can play the atari at , an oshitsubushi: Black cannot form a killing shape at
because this would be suicide, which either is forbidden or includes self-capture.
3. Making bent four in the corner
The position becomes more complicated when the eye shape is almost filled in the following ways.
White can play at neither a nor b because having her eyespace almost filled with straight three means death. Therefore Black can fill the marked outside liberties at any time, before playing a. As we now know this starts a ko. We now refer to the page bent four in the corner is dead to explain why this is dead as a rule.
Black cannot play either at a or b, because White will play c to almost fill the eye space with bent three, which kills. Therefore White can fill the marked outside liberties at any time, before playing c then a or b. Again, as we now know this starts a ko and we refer to the page bent four in the corner is dead to explain why this is dead as a rule.
4. Avoiding The Bent Four
at a would be the bent four in the corner.
avoids this but at a cost. (Later Black a, White b, and Black has three points.) Although he lives,
is a mistake.
5. Corner Non-Bent Four
The following are Bent Four and are in the corner but are not examples of what is meant by Bent Four in the Corner. In each, 'a' and 'b' are miai for life:
6. Links
Some exercises involving bent four in the corner
See also
Authors