Beginner Exercise 5 / Solution

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 , Black can turn the status of this group into a ko. This is similar to bent four in the corner.

Comment  

White cannot escape by , which can be blocked by .

Solution

Solution  

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

Comment  

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

When Black wins the ko  

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

When White wins the ko  

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

Variation  

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

Variation  

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

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 any effect.

Tas: But for black to play rather than tenuki is surely a picnic ko, he has nothing to lose by doing it, but might kill the group.

Failure

Failure  

occupies the other middle point, but lives.

Failure  

This lives too.

Additional information

Two external liberties  

When White has two outside liberties, he can live without ko.

Comment  

Even if black attacks with and ...

Comment  

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

Comment  

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

Matthias Ernst

Internal ko threat  

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

Not out of the woods  

at
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.

If black tries to pin at b then white lives  

Previous comment was not true. If black tries to pin white, she can play oshitsubushi at circle point and live. Black cannot disregard white's move on previous diagram.

Srdjan Mitrovic

Corrected, oshitsubushi not possible  

The above comment looks incorrect to me. In the "Not out of the woods" diagram above, is the threat to escape, and then occupies the corner spot allowing the escape. Since the corner is already a bent three, it's just a single eye.

Eric Backus

My mistake, I thought that B plays tenuki with in previous comment.

Srdjan Mitrovic

First few squences in the solution.  

Actually, white can escape.

white can escape  

If black plays b, white can play at a, and white would have two eyes. If black plays a, white can play at c, and white can escape depending on what else goes on on the board.--Daniel Wang

Sorry Daniel, but if black plays b and white plays a then the ko may continue, provided of course that black has sufficient ko threats.--Rick Lush

Actually, this also shows that white cannot escape simply because black plays at "b." Escaping refers to moving out of danger areas. This white isn't going anywhere.--ProtoDeuteric