Seki

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  Difficulty: Beginner   Keywords: Life & Death, Go term

Chinese: 共活 (gong4 huo2)
Japanese: セキ (seki)
Korean: 빅(bik), 비김 (bigim)

Seki, a Japanese go term adopted into English, 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.

Table of contents Table of diagrams
Simple seki
Seki with eyes
One eye-no eye-one eye
1-eyed Seki
3-in-5
One more interesting example
Hane seki
Eyes in seki?
Yet one more interesting example
Yet another seki

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 (false) 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. If b were not a false eye, there would be no seki; Black would be dead.

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.

Note that a White play at c will only put his group into self-atari, because Black will then play at b and capture the whole group.



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.

Jef: So, when scored, the black group only gets two points?

Bill: The Black group gets zero points.



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 1. --MortenPahle



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.


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.[3]


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.

Chris L After one cycle the board position will have been recreated. Is this not prohibited by some form of ko rule?

chrisg I'd be interested if someone could add a reference to a page explaining if this isn't a seki, and under what rules.

El Draco With the positional superko rule, it would be prohibited to complete a cycle, but with situational superko, after one cycle it's white's move instead of black's. If he plays somewhere else, black can try playing there again, with the same result.

If, at some point, white passes and black tries playing again, he may not move again at a1 if white passes again, since that would be prohibited by either superko rule.

There is one devious way black could play however, which is playing a ko threat right after white captures the two stones. After this, white can't pass, lest black plays a1 again and white may not capture the two stones without repeating a previous position with same player to move. This way, black could win if he has more ko threats than white has moves to play. If white plays in black's territory just to not-pass, black must pass and it wouldn't have cost either player a point. If white has no legal (or plain dumb) moves left however at some point, and black has at least one ko threat, black can win.

Correct me if I'm wrong, since I just thought this up. ;)

Bill: Good thinking, El Draco! :-)

But you are not alone. I think Frank Janssen was the first to have that idea in the '90s. See Rules Beast 1.


[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.

[3]

Bill: While Charles has a point, the usual assumption that outer stones in a diagram are alive applies. Also, there is a slight inaccuracy. White's outer group does not need two eyes to preserve the seki.

[Diagram]

Yet another seki


Japanese Seki Definitions

In strict Japanese rules, a seki is defined as any group that is touching dame (where dame are points that aren't inside eyes). It is normal for players not to fill dame points before passing. However, the rude player is pefectly within their rights to demand that any group of his opponent's (even if it has 15 eyes) is in seki if it is touching dame, and therefore its territory isn't to be counted.

However, this isn't really important as there is another Japanese rule that says if one player asks that the game be resumed - the other player MUST oblige but the other player plays first (of course they can pass). So if your opponent tells you (rightly) that one of your groups is in seki, you can just resume play.

See also


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This is a copy of the living page "Seki" at Sensei's Library.
(OC) 2005 the Authors, published under the OpenContent License V1.0.
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