The black group in this diagram is called the tripod group. It is one of the smallest shapes in the corner that is alive as it stands. Black needs to remember the correct reply to three white attacks. See probe for an example of how it may form.
We will explore the following lines of attack for white:
- a : First attack.
- b : Second attack.
- c : Third attack.
In response to the attack at , black should defend at . We will investigate some follow-ups from this position.
The descent at is answered by as well.
If captures, then and lives by oshitsubushi as well.
Note that when White plays at , is necessary for unconditional life.
If plays elsewhere, then and can create a ko.
here lives too, but it appears not to be the recommended move. Probably the reason is that a White descent at a becomes a definite sente, in the sense that White will get a unconditional kill if Black plays elsewhere. See Xuanxuan Qijing Unnumbered Problem 2 for the reason.
The crawl at here may or may not be a good move, and depends heavily on whether the stone exists, and the White stones in the vicinity of . In this case, the clamp at takes away all the eye space and kills. Therefore, in life-and-death problems, the main line always have at .
Subsequently, if , then and can lives by oshitsubushi.
Similarly, if captures, then Black lives by oshitsubushi as well.
RhysD: Suppose white eventually has a second-line stone in support; does black have an answer to the first-line crawl?
fractic: The second line stone needs to be a lot closer. Black can capture white with the sequence to . If now white plays a then black plays b and if white plays c then black plays a.
Again, for the case when is present, can kill. falsifies the eye on the side.
So he invaded at . White can play one point higher, but then Black has a cut at , which he can use to isolate the two white stones at the top. Either way Black creates a nice position.
From an endgame point of view, it is better for White to play like this diagram (1 replaces 8 above): Black will end up with two points less in the corner. However in the game White followed the previous diagram, probably because he didn't like the remaining weaknesses on the left side following this sequence at a.