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Group
    Keywords: Go term

One or more isolated stones or chains (strings) of stones of one colour, hanging together as if effectively connected. This is a major term used in discussion, but is informally defined only.


See weak group, strong group, virtual groups. Many particular names are given to the groups which are interesting examples for life-and-death.


KarlKnechtel contributes his understanding and formalism: A group of stones is a set of stones on the goban, such that

  • all stones in the set are the same colour;
  • each pair of stones in the set is joined by some path of orthogonally adjacent points (I am not about to attempt to define "path" formally; it should be obvious anyway ;) ), such that every point on that path is either empty or covered by a stone of the same colour;
  • the set is not a proper subset of another group. (For those unhappy with that recursion: There does not exist another stone of the same colour on the goban which is connected by any path as described above, to any stone in the set.)

Unfortunately, the term is sometimes also used to mean something similar to the above, except that empty points are not allowed in the paths. I.e, a set of stones of the same colour, all connected to each other by orthogonal adjacency. However, it seems that the term string has become preferred for such an entity.

These sorts of things may be intuitive even to beginners, but those with an interest in ComputerGo programming really need mind-boggling precision in these definitions, and it's often a lot harder to give that precision than you'd think (given that you don't have any difficulty with the concepts when you're actually playing Go ;) ). See also EyeDefinitionDiscussion.

Surely this definition isn't very useful? In many games, there will only be two groups on the board (one black, one white) until late in the middle game.



Thanks for the definition Karl. To paraphrase then, a maximal subset of the stones of one color connected by stones of the same color or empty points. For the definition of string or unit, just remove the 'or empty points' clause. I agree that it seems like this definition may not be useful in the early or mid-game. We may have to invoke some sort of localness condition. Again, thanks. --DaveFinlay


BobMcGuigan: I think the second point in the definition needs some refinement. According to the definition given the black stones in the diagram below are one group. Is that really what you want?

[Diagram]
19x19 diagram

The points marked with squares show the required path. I think of stones in a group as being connected somehow, even if not strictly connected.

The problem is how to capture this sense of "wholeness" of the group when the stones could be quite far apart.



This is a copy of the living page "Group" at Sensei's Library.
(OC) 2004 the Authors, published under the OpenContent License V1.0.