Beginner Exercise 140 / Solution

Two stage ko.  

The solution to this problem is a two stage ko. If White cannot win this ko, then he can back out with a subsequently.

White dumpling.  

A connection at is highly undesirable for White. After , the ko becomes a direct ko, and the White group is extremely heavy.

Direct ko.  

The descent at is not good too, as it leads to a direct ko too.

SilverHawk: Why not at a? White cannot approach on either side due to shortage of liberties, and if White ever captures the marked stone, Black can play b to put White in atari.

fractic; at a captures the two white corner stones but it lets the other 4 white stones off the hook.

Failure

Failure.  

The atari at is a failure. Instead of capturing at a, White can descend at and there is nothing else Black can do.

Failure.  

The atari at here does not work either. The key move for White is , which prevents the ko.

Discussion

The following discussion is pitched at a slightly higher level than the problem, but it is still revelant to the problem.

Discussion.  

Without the black stone at a, can extend and say arigato to Black for the free gift. Obviously White should not play at b otherwise black can play at and thank White instead.

Discussion.  

So without Black a, Black should atari at . The correct response is , regardless of whether there is a Black stone at a. This is probably a IGS 1k* level indicator.