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liberty discussion
  Difficulty: Advanced   Keywords: Tactics, Theory, Go term

Page edited down 2003/06/09 by Charles Matthews. To see other contribution see older versions (up to version 32).


mAsterdam ...I do think that a Beginner page on liberty should only offer one meaning.

Charles I think this is what I have said from the start. Just a point though: the 'capture metric' sense isn't loose at all, but it's a book-length discussion.


BobMcGuigan: I see the problem of "secondary liberties" as being, yet again, a matter of context. I like best the idea of "moves to capture" because it encompasses everything with one term and factors in the context. Given the bizarre shortage-of-liberty shapes that can occur near the edge it might be necessary to make a move 10 or more spaces away from a group in order to capture it. In other words this is a property of a position rather than a group. Some of the same issues come up in approach move kos. There, too, the key idea is number of moves needed to reach a certain state on the board.

Charles I have to say that in introductory writing on go, using 'group' sometimes to mean chain and sometimes not, is disastrous. Not so for most players. Isn't this the same kind of distinction?

[Diagram]
A group



If you make the liberties of the WC stones depend on those on the WS stones (which are in the same group, naturally), you have also to answer such questions as : in a capturing race, does White need to save the WS chain?

Bob's suggestion that we need position-context, which is quite true, makes a third level chain/group/position.


Charles The obvious definition is like liberties sense 2 - liberties sense 1. We have one problem: that this is like pears - apples. That is, sense 2 is a metric: it measures in time units how long it may take to capture a group. While sense 1 is a cardinality, attached to a set of adjacent intersections of a chain. In a number of cases we can get away with saying sense 2 comes from sense 1 by counting in some multiplicities; and in that case I think it makes sense to speak of secondary liberties lying at those points that need to be counted twice or more. But you might say that things are generally much worse than that. For example the saying 'a false eye liberty is -1 liberty' - the case where the opponent can force you to fill in a false eye in sente - refers to an effect like a negative secondary liberty,

Bill: I agree that it is like pears and apples, or at least like different varieties of apples.


HolIgor: Perhaps, because I come from the country where chess is very popular, I always understood and called what as named here as "secondary liberty" as tempi. White's group has two liberties but three tempi to capture.

[Diagram]
Two liberties, three tempi

On White's turn, White wins the capturing race. On Black's turn, Black can form a group with three liberties but four tempi to capture, because White cannot attack from the corner. Her group has only two liberties. Shortage of liberties.

Charles Yes, tempo might be a step in the right direction ...



Charles I would never mention 'secondary' liberties to complete beginners; then you certainly need the shortage of liberties concept to study tactics at all seriously, and the approach move is a good example at this point. For players around 10 kyu, perhaps, thinking of 'extra' liberties caused in certain ways should be clarifying. So here, with the pedagogic purposes we have on certain pages, we can talk about secondary liberties as a kind of working concept - not very computable or at least not trivially computable.

I have to say that it is groups as a whole that have these 'secondary' liberties, not chains (as far as I can see). Or at least for a given chain you have to label adjacent chains that are or might be connected in some way ...

Richard Hunter I use physical liberties for the actual points adjacent to stones and then add approach moves to get the liberty count, which is how many moves it takes to capture the stones.

Bill Are we justified in calling such moves liberties? Yes, indeed. Consider a dame liberty. As long as a chain of stones has at least 1 of them it stays on the board during play. Why do we count more than 1? Very simply, to capture the stones unless they are added to, the opponent must fill each of them. The dame count tells us the minimum number of moves he must make to capture them. The reason for counting liberties has to do with that fact. So the underlying idea in talking about liberties (plural) rather than simply a condition of being captured vs. not has to do with how many plays it will take for capture. Such plays are rightly called liberties.[1]

On the question of what to tell a beginner:

You have to inform the beginner about capture, and so must tell her about dame. In English, you call them liberties. "A stone or chain may be captured by filling its last liberty."

But when you start talking about a chain or stone having, say, 3 or 4 liberties, you are really talking about moves to capture. Otherwise you would not bother. We do not go around talking about how a group has 4 or 5 eyes. Who cares? 2 is enough. The only go reason to count liberties has to do with considering the possible future capture of a stone or chain of stones. (And that includes recognizing seki and classifying capturing races.)


[1] I have to record my disagreement with Bill here, and all along. Charles



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