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

Seki means mutual life. In its simple form, it is a sort of symbiosis where two live groups share liberties which neither of them can fill without dying.

Below you will find several types of seki. Be aware that although neither side can 'win' a seki, a seki can be a possible source of ko threats.[1] There are also scoring issues in seki.


Example of a simple seki (no eyes)

[Diagram]
Simple seki

The simplest form of a seki is as shown in this diagram: the BC and WC 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. So neither player will.[2]

Example of a seki with eyes

[Diagram]
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.



Example of a seki with and without eyes

[Diagram]
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.



Example of a seki with 1 eye in total

[Diagram]
1-eyed Seki

b is an eye, which would count in Chinese (and de facto Internet) rules, but not in Japanese scoring.

The only play for either side which doesn't immediately lose the corner, is White at a. However that costs 1 point in Gote, as Black captures at b and White must recreate the original position by recapturing at WC. This is an example of Sending Two Returning One.



Example of a seki with a partly filled eyespace

[Diagram]
3-in-5

Another type 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.



Example of a seki with double ko

[Diagram]
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


Another example of seki with partly filled eyespace

[Diagram]
Another seki

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

See butterfly seven.



Seki with strings in atari

[Diagram]
Hane seki

Both sides have a string in atari, but for either side, capturing starts a capturing race that the other side can win. Best play is to leave this, therefore it's seki.



A pathological seki

[Diagram]
Twisted, huh?

If black plays on 'a', white answers on 'b' and black loses the capturing race.

I don't know whether this is a classic strange position or not. It doesn't seem likely that such things could appear in real games. I stumbled across it years ago while trying to construct an algorithmic definition of seki. (It convinced me to work on something easier.:-) I think it's interesting as a suggestion of how to create sekiish positions with arbitrarily large numbers of shared liberties. -- WilliamNewman


[Diagram]
Is this the continuation?

I think the correct way to continue is this, resulting in an "ordinary" seki. Weird position, though. --Dansc
I don't see why that move 4 is the best move for white. Why not play to reduce the liberties of the weaker black group, instead of the stronger black group? After all, if white can capture the weak black group, the stronger black group has no eyes and will die. Thus, why not play 4 at 5? After that, the upper right black group has only three liberties, while the white groups have four liberties each, so it seems as though black should lose the capturing race. -- WilliamNewman

[Diagram]
Ist das nicht ein Snitzelbank?

Bill: Nope, but he does lose sente.

WilliamNewman: I have since found somewhat similar shapes [ext] elsewhere



Benjamin Geiger:

The Japanese Rules say that "eyes in seki" are not counted as territory.

This may sound like a stupid question, but what does that refer to? Are all eyes of the groups living in seki ignored, or just the shared eyes?

[Diagram]
Eyes in seki?

Are the circled points counted as territory for White?

No, they are not.

A group living in Seki should have no more than one eye. --Confused

Benjamin Geiger: If I understand correctly, each group living in seki can have one eye.

Charles Matthews This diagram can become a permanent seki if the central white group can get two eyes. If not, it is a temporary seki.



Sending Two Returning One with an eye for each side

[Diagram]
Yet one more interesting example

In the 4-move cycle Black plays 2 stones, White only one. With territory rules, this costs Black one point. After some (finite) number of cycles have been played, White could simply admit Black's claim that the white group is dead, and still have enough points to win. So Black does not start the cycle.



Other examples of seki:


[1]

Ko threats for both sides, that is. The interaction with the remove double threats idea is one of the more bizarre corners of go theory, though of little practical importance. Charles Matthews

[2]

A recapture under the stones, although something conceivable here, is not possible in this case; it isn't an idea that has much interaction with seki.



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This is a copy of the living page "Seki" at Sensei's Library.
(OC) 2003 the Authors, published under the OpenContent License V1.0.