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Seki
Path: LifeAndDeath   · Prev: BasicLiveShapes   · Next: LGroup
  Difficulty: Beginner   Keywords: Life & Death, Go term

Seki means 'dual life'. It can be seen as a sort of symbiosis where two groups share liberties which neither of them can fill without dying.

This definition does not take into account rare positions where the last dame can be filled without dying, but it is in neither player's interest to do so in terms of score. See Seki with eyes question. --jvt Nor double ko seki.

[Diagram]
Diag.: seki

Although in general a group needs two eyes to live, there are exceptions where groups of different colours, each with fewer than two eyes, keep a 'balance'. The simplest form of a seki is as shown in this diagram: the marked white and black groups have no eyes, and share two liberties at a and b. If either player plays on one of these points, the opponent will play the other and capture.


[Diagram]
Diag.: seki with eyes

A slightly more complex variant is formed by the marked black and white groups. Both groups have one eye, and they share a liberty. Again if either player plays at a, the other one will capture.


[Diagram]
Diag.: one eye-no eye-one eye

The two white groups both have one eye. The black group has none, but shares a liberty with both of the white groups. Again no player can capture any stones of the other player, and the position is seki.

{TakeNGive 11k: If I understand correctly, in Japanese Counting (territory scoring), neither Black nor White gets any points from the seki; but with Chinese Counting (area scoring), Black gets 2 points from the stones living in seki, while White gets 12 points from this seki (10 points from the stones living in seki plus 2 points from the eyes a and b). Right?

unkx80: You are right.}

Hmm. I think that saying that he gets '10 points from the stones in the seki' can be misunderstood. With area scoring all alive stones on the board (+ territory) count as points. Stones involved in a seki are alive and thus count as points. That is not because they're in a seki, but because they are alive. In practice, under area scoring, there are extra no points from a 'balanced' (same number of eyes/liberties) seki either - if Black has 10 stones in the group and White only 2 then White has 8 stones (=points) somewhere else on the goban.
-- MortenPahle


[Diagram]
Diag.: 3-in-5

A final example of a seki. Black can capture the three white stones, but if he does, White will then come back at the point where the marked stone is. Black will then be left with a dead shape, so Black will not do so. Instead, this position will remain until the end, with the three white stones alive through seki.


[Diagram]
Diag.: No points in a seki

If you play using Japanese counting, as is usual in Europe and Japan, this is a position to watch out for. The Japanese rules say that there are no points counted in a seki, so Black cannot count 3 points for a and the marked white stone. Instead, after the game is over, during or after the filling of the dame, Black should capture at a, and that way get at least one point out of the situation. This is also of importance in the position of the third diagram: The points a and b are not territory points in Japanese counting, but they are in Chinese counting (for example in a game using Ing rules).

A final note: although neither side can 'win' a seki, a seki can be a source of ko threats - as long as the ko is larger than the seki.



[Diagram]
Diag.: One more interesting example

Allow me to provide just one more interesting example on seki, but involving kos. Go figure. :-)

--unkx80

A related shape can be found on the Double Ko page. See also Seki with eyes question. --MortenPahle



Here's an interesting seki that showed up in a game last night. I've simplified the shapes of the territories somewhat, but black's false eye position and the corner are exact.

This is a zero score under Japanese rules. Note that under area scoring rules, however, Black has at least one more eye than White. (If White can win a ko, I think, then Black's false eye in the corner can be filled.)

I would welcome scoring analysis from some of the experts. --Thomas Bushnell, BSG

(TakeNGive copied it to SekiWithEyesQuestion2 -- I still don't have a firm enough handle on this to answer authoritatively; so I eagerly await enlightenment...)

[Diagram]
Diag.: an interesting seki


Proposal for seki

Definition:

  1. A mutual liberty is an empty point that reaches two chains of different color.
  2. Two chains of different color live in simple seki if they reach no empty points but the same two mutual liberties.

Addition:

  1. A group of stones is alive in simple seki, if for any sequence of moves by the opponent, there is a sequence of answers that turns the group into a chain which is part of a pair of chains, complying with definition 2.

[Diagram]
Diag.: Seki

(TapaniRaiko: The proposal is not enough, as these two examples show. I have seen an example where a 35x35 board was filled with small groups all living in a single seki.)


[Diagram]
Diag.: Not a seki


[Diagram]
Diag.: Is this a seki?

It seems to me that neither Black nor White wishes to play further in this space. So is it a seki? -- nuance

KarlKnechtel: Yes, it is, if I recall correctly. Though, both sides can make a couple more moves, but it won't change anything :) Refer to nakade where this situation is considered.




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