Beginner Exercise 5 / Solution

Sub-page of BeginnerExercise5

This problem assumes that the reader knows how a ko fight works.

Introduction

[Diagram]

Introduction

This shape looks like the twisted four, but it is at the corner. We note that when the group has only one outside liberty at [circled point], Black can turn the status of this group into a ko. This is similar to bent four in the corner.

[Diagram]

Comment

White cannot escape by W1, which can be blocked by B2.



Solution

[Diagram]

Solution

B1 occupies one of the middle points in the twisted four. When W2 occupies the other middle point, B3 takes the ko first.

[Diagram]

Comment

W4 attempts to set up a oshitsubushi, but this leaves the White group with only one liberty, so B5 simply captures all the White stones.

[Diagram]

When Black wins the ko

If Black wins the ko, he will connect at B1. The White group becomes a bent three big eye and dies.

[Diagram]

When White wins the ko

If White wins the ko, he will capture at W1, making two eyes and live.

[Diagram]

Variation

If W2 connects the 1-1 stone, which is under atari, White dies in gote as a bent three big eye.

[Diagram]

Variation

If W2 here, B3 leaves the White group in a bent three.



Failure

[Diagram]

Failure

B1 occupies the other middle point, but W2 lives.

[Diagram]

Failure

This lives too.



Additional information

[Diagram]

Comment

When White has two outside liberties, he lives without ko.

[Diagram]

Comment

Even if B1 and B3 attacks...

[Diagram]

Comment

W4 sets up a oshitsubushi. The atari at B5 is too late, W6 captures two Black stones and makes two eyes.



Discussion

This discussion is at a slightly higher level than the intended level of the problem.

Whether this is a picnic ko is not clear. If White wins, he lives; if Black wins he dies. Hence, we cannot say that this ko is lopsided. Perhaps White invaded Black's corner and managed to make ko. That kind of thing is not unusual. It is a mistake to think of something as your territory unless your opponent cannot invade to no effect.


Authors


[Diagram]

Comment

B3 at WS
What about White trying to escape? If White answers with W2, Black still needs to play B3, so White can connect at W4.

Matthias Ernst



[Diagram]

Internal ko threat

W4 at WS
Brent: No, W2 is an internal ko threat and Black would respond by blocking the connection at B3. White would then retake the ko with W4, and it would be Black's turn to find a ko threat.



[Diagram]

Not out of the woods

B3 at WS
Coconuts: Even if black disregards the ko "threat" here and lets white connect, white is not away yet. B at a or b pretty effectively pins the white group down, so unless white has a strong position on the top side of the board, she is still in a lot of trouble.



This is a copy of the living page "Beginner Exercise 5 / Solution" at Sensei's Library.
(OC) 2004 the Authors, published under the OpenContent License V1.0.
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