Tripod Group
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The tripod group
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.
and
is probably the strongest attack, threatening a ko. The throw-in at
is neccessary for life. The result is oshitsubushi.
The descent at is a mistake, because
is the eye-stealing tesuji, making a placement and aiming at setting up a snapback at a. Black is dead.
Second attack
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 1-2 point problem 1 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
.
Third attack
Of course, White would aim for a ko with . To avoid a ko,
and
would be neccessary. The rest is pretty much a one lane road, with
living unconditionally by a double ko.
is the wrong way to answer
. After the exchange of
and
,
is possible. The result is bent four in the corner, and Black is dead.
Surrounding stones
Surrounding stones do matter for the status of the tripod group. For example, if there is a White stone at a or b, then the status becomes unsettled.
Again, for the case when is present,
can kill.
falsifies the eye on the side.
The tripod group with the extra leg
While the tripod group is unconditionally alive, Black should not make the exchange of and
.
The reason is that after the exchange, and
turns the whole group into a ko. See tripod group with extra leg for more detailed discussion for this.
Practical application
This is a position from the final game of the 2000 German Championship, between Franz-Josef Dickhut (Black) and Christoph Gerlach (White). Black wanted to do something about the upper left corner.
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: 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.