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 Post subject: Size of ko for surrounded small groups
Post #1 Posted: Fri Aug 30, 2019 1:36 am 
Lives in gote

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I have a question about defending small groups that are completely surrounded by living opponent stones and where the opponent can get a ko.

On the top, White can get a ko. Black can play 'a' to live or he can play another big point such as 'b'.

If White wants to win the ko, he needs two moves. The swing value is about 22 points I guess. In return, Black gets two moves elsewhere. So each White move is only worth about 11 points.

Therefore, at this stage of the game it might be bigger for Black to leave the group and play elsewhere.

Is this reasoning correct?

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 Post subject: Re: Size of ko for surrounded small groups
Post #2 Posted: Fri Aug 30, 2019 1:41 am 
Judan

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Seem reasonable. It's also important that the surrounding white group is not alive. So there are a few ways black could use that:
- If black lives, does white need to add a move, say around h12 or e12. If so is black happy or sad?
- Or would black rather play on the outside, say g13, so that if white starts the ko it's heavier for him, as black winning ko, white following up on a non-local threat, and then black another move might kill or severely attack white?
- If black could somehow get f15 peep in sente then it's smaller for the group to die, and maybe can still get semedori.


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 Post subject: Re: Size of ko for surrounded small groups
Post #3 Posted: Fri Aug 30, 2019 4:14 am 
Tengen

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I ignore aji, attacks on the white group and influence because the evaluation question is more fundamental and must be understood first.

In an ordinary ko, it is wrong to divide the difference value (swing) by 2. It is correct to divide it by 3 because this is the tally, which is the excess plays 2 of the ko winner (White) plus the excess play 1 of the ko preventer (Black). The difference value divided by the tally is the move value (modern endgame theory).

You compare it to the move value of a move elsewhere (ko threat). If it is a play in a local gote (such as a moyo contructor), it has a gote move value with the tally 2.

In your example and simply believing you, the ko difference value is 22 so the ko move value is 22/3 = 7 1/3. We still need to calculate the move value of the ko threat, which is not so easy because the local gote has lots of follow-ups.

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 Post subject: Re: Size of ko for surrounded small groups
Post #4 Posted: Fri Aug 30, 2019 4:46 am 
Honinbo

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It's nice to consider the point value that these stones are actually worth, but more than a precise calculation like that, I think it's very important to note that the surrounding white group can be attacked in the future - I disagree that this concept is less fundamental.

In general, when you have cutting stones, you can consider whether it's important to keep those stones, or whether they can be sacrificed. One aspect of this is whether those cutting stones are cutting off a group that can be weak, or if you are cutting to alive groups. If you are cutting two groups that are already alive, the cutting stones aren't that important.

Here, though, the surrounding white group is not yet alive, so I think the cutting stones (i.e. the black group on top) is important to keep alive. But I'm not sure I'd play 'a' to live - I think F19 might be better - and reduces white's eye space a little bit more on the top. You could also consider E19 to live locally, but I don't like that very much, because it's easier for white to get eye space on top.

In general, at this stage in the game, I'm more concerned about the attackability of the G16 group than the precise point value stuff is worth in that area. If white were totally live on both sides, then it becomes just a point thing.

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 Post subject: Re: Size of ko for surrounded small groups
Post #5 Posted: Fri Aug 30, 2019 5:31 am 
Tengen

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With "more fundamental" I refer to the topic of the thread title. From a general POV, you are right, of course, Kirby.

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 Post subject: Re: Size of ko for surrounded small groups
Post #6 Posted: Fri Aug 30, 2019 6:21 am 
Honinbo

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RobertJasiek wrote:
With "more fundamental" I refer to the topic of the thread title. From a general POV, you are right, of course, Kirby.


Thanks, Robert - I see what you mean, now.

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