Superko

    Keywords: Ko, Rules

Superko rules prohibit recurrences of an earlier board position under certain circumstances. The recurrence of such positions could be caused by long cycles, such as triple ko, eternal life or round-robin ko.

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Seki

Forms of superko rules

Superko rules come in various forms, depending on when they forbid recreating a board position; factors include:

  • on whose turn the position was previously created;
  • whether players have passed, immediately before or at some intervening time;[1]
  • the type of move recreating the position (see Chinese Superko below).

The main variants of the superko rule in use are:

The following variants have been used in theoretical discussions:

Example

The most commonly used example why a superko rule is necessary is triple ko. See below for a more simple case that also illustrates the rule:

[Diagram]
Seki  

This situation is a seki in the corner. Assume Black just passed, so that this position has occurred with either side to play. If White plays at a, Black plays b. With a superko rule in place, White cannot retake at white+circle, so Black can take the corner. Of course, this comes from a White initial mistake, but see Rules Beast 1 for a case where White breaks the seki and wins the fight...


See also

Test cases

Related rules

  • Fixed ko rule: repeated positions may occur but you are not allowed to make the same move in them.
  • Long cycle rule: Rather than forbidding the repetition a board position, this rule assigns a result to the game if a repetition occurs. The game result depends on the number of stones captured by either since the first occurrence of the position.
  • Short cycle rules?: The result depends on the number of moves between the repeats. If there are 3 or fewer moves between the repeats the game is a loss for the player who makes the repeat. If there are more moves between the repeats the game is a draw.
  • Long cycle seki rule: The groups involved in a long cycle are treated as alive in seki
  • No result: The Japanese rules and Korean rules say that if a board position is repeated, the game has no result; this usually means the game must be replayed.
  • Colour-blind superko: Ignore the colours, suggested as a ko-rule for multi-colour go.

Further information

Discussions


Notes

[1] Bill Spight: There are also versions of the superko rule that take passes into account, such as the earliest superko rule proposed, by Yasunaga Hajime in an article in Kido in 1929:

Article 7: The repetition of the same pattern shall be prohibited unless the right to an alternate move is disregarded.
([ext] http://harryfearnley.com/go/shimada/intro.html)

In modern terms, "disregarding the right to an alternate play" means passing. When a pass lifts a ko or superko ban, this is sometimes called "pass for ko threat". There are different versions of the superko rule in which an intervening pass allows a repetition. Yasunaga's seems to say that any intervening pass lifts the restriction on repetition, but it may mean a pass by the player who makes the repetition. The interaction with passes can pose problems with ending the game, but not taking passes into account can lead to strange results. My home page, when I get it up, will have a discussion, and my solution to the problem, based on Combinatorial Game Theory. Meanwhile, for my solution, see Spight Rules.

Yasunaga proposed a three-consecutive-pass rule, which is how my solution will usually work. Ing uses a four-pass rule.


Superko last edited by PJTraill on February 11, 2019 - 19:43
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