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J Groups
Difficulty: Beginner
Keywords: Life & Death
The black shape in the diagram is the basic J-group. It has the same status as an L+1 group (see The L Group is Dead): it lives or dies according to sente.
The correct way for White to kill this shape looks like a complete display of the principal killing techniques: reducing the eye shape with a hane, followed by a placement on the vital point, followed by a descent and a throw-in. The best way for Black to live is by playing at 3, giving him 6 points of territory. The order of 1 and 3 is reversible.
Black can improve on the status of his corner by descending to the edge instead of making a hanging connection. The best White can do now is turn the corner to ko.
Here is her best sequence to produce the ko. White gets to make the first capture.
Here is another way to set up a ko, but this one only works when the black group has no external liberties. Black gets to make the first capture, but if White wins the ko, she removes a part of the black group from the board.
However, I would like to add to the discussion by adding this diagram. Black can play at 4, aiming to escape at 'a'.
White has to respond at 1 (if white responds at 3 instead, black can start a ko at 'a'), and the result is a ko.
However, this version of the J-group is definitely dead after 1, 3 and 5. There is no chance for a ko.
Life and death evaluation of shapes is normally done without the ability to escape. Look at the funny white 'wings' on the outside of the L group in the first diagram of The L Group is Dead. Even the L Group is alive in sente if descending to either edge is a threat to escape. The present shape would normally be studied/evaluated with a white stone at 'b' to prevent such an escape. In such a case (no escape) W 5 finishes off B. In studying life and death then we need to learn the basic techniques. Next, memorization of the fundamental cases can be a powerful tool to improve play in real games. However, as much as the vital points and main continuations, we need to thoroughly understand the various conditions such as no escape, no/limited outside liberties, relationship to the corner, etc. that may apply to each case. Small differences in game situations can turn around any of the proverbs/rules of thumb that we have studied so hard! That's why I try to teach fellow beginners the merit of a net above a ladder. The ladder is a NonLocalMoveVersusALocalMove: you need to look at more parts of the goban. Ladders may be easier to pattern-match ; sure - but a ladder's status changes more rapidly than a net's. --JanDeWit I totally agree with DaveSigaty and JanDeWit on their views. Actually, I have wanted to add the part provided by DaveSigaty after I made the changes but I was unable to get onto this site then.
--unkx80
Practical application
Black should under almost no circumstances ignore the double kakari and let White play 1. But how does Black continue if he decides that he has to play this line after all?
Black retreats to san-san and White tries to keep his eyespace as small as possible.
Black plays the kikashi of 1 and 3 before living in the corner, in accordance with the proverb.
--Stefan This is a copy of the living page "J Groups" at Sensei's Library. (C) the Authors, published under the OpenContent License V1.0. |