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Bent four in the corner
Path: LifeAndDeath   · Prev: CarpenterSSquare   · Next: TwistedFourInTheCorner
  Difficulty: Advanced   Keywords: Life & Death, Rules

Bent four in the corner is an expression referring to two things.

  1. The eye shape of a group in the corner being bent four, which we will discuss here.
  2. The fact that under Japanese rules, a group which is surrounded by a live group and whose eye shape can be reduced to this shape, is dead. Discussion at Bent four in the corner is dead.

1. Fewer than two outside liberties

[Diagram]
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.


[Diagram]
Main line

White responds with an atari at W2. Black starts the ko at B3.

White can't play oshitsubushi at a, because of shortage of liberties; and the fate of the group is decided by a ko fight. Finishing the ko



2. At least two outside liberties

[Diagram]
Bent four in the corner

If White has an extra liberty on the outside, however, she can play the atari at W4. Black can't fill at W2 because suicide is forbidden (an example of oshitsubushi).


[Diagram]
Bent four in corner

So, if B5 takes away a liberty, White plays W6 (capturing two stones) and makes life with two eyes.



3. Making bent four in the corner

The position becomes more complicated when the eye shape is almost filled in the following ways.

[Diagram]
Situation 1 with a potential bent four

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.


[Diagram]
Situation 2 with a potential bent four

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.



[1] Finishing the ko

[Diagram]
Finishing the ko - a

With a single outside liberty, Black finishes the ko by almost filling the eyespace with a bent three. Playing 1 at a reignites the ko and gives White a second chance.


[Diagram]
Finishing the ko - b

With no outside liberties, Black can finish the ko by capturing all the stones. Playing B1 at a leaves White a strong ko-threat.



Avoiding The Bent Four

[Diagram]
Mistake

B3 at W4 would be the bent four in the corner. B3 avoids this but at a cost. Although he lives, B1 is a mistake.


[Diagram]
Correct

B1 here instead lives with four points, avoiding the bend altogether.



Some exercises involving bent four in the corner:


Authors



Path: LifeAndDeath   · Prev: CarpenterSSquare   · Next: TwistedFourInTheCorner
This is a copy of the living page "Bent four in the corner" at Sensei's Library.
(OC) 2003 the Authors, published under the OpenContent License V1.0.