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 Post subject: James Davies - life and death - question
Post #1 Posted: Sat Nov 25, 2017 2:00 am 
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Greetings,

I have a question about one of the diagrams from the book of James Davies - Life and Death.

Dia4 on the pic below. Why is why alive? Doesnt white need to capture the 3 black stones inside in order to try to make 2 eyes - meaning capturing 3 stones is a dead shape?

Or black must first try to capture white?

Image

This really got me confused for some basic principles which I thought i knew :scratch:

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 Post subject: Re: James Davies - life and death - question
Post #2 Posted: Sat Nov 25, 2017 2:35 am 
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White must not actually capture (take off the board) the three Black stones, otherwise Black will respond by playing in the middle of where his three stones were, thereby killing White by reducing him to one eye.

As it stands, both White and Black share two liberties. White cannot capture Black for the reason stated above, but equally Black cannot capture White because when Black fills one of the two inside shared liberties, White will capture four stones and Black will have no way to reduce this shape to one eye.

So the best course of action for both players is not to play inside that shape. That means both are alive and both have 0 points in that area. This is called "seki". At the end of the game, the area will be left as is.

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 Post subject: Re: James Davies - life and death - question
Post #3 Posted: Sat Nov 25, 2017 2:53 am 
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Thanks for the answer. Then it is just an error in the book? Cause at the end of the explaination of Dia 4 the authos says "white is alive", and not "white is alive in seki".

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 Post subject: Re: James Davies - life and death - question
Post #4 Posted: Sat Nov 25, 2017 3:00 am 
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Korulez wrote:
Thanks for the answer. Then it is just an error in the book? Cause at the end of the explaination of Dia 4 the authos says "white is alive", and not "white is alive in seki".


"White is alive in seki" would make it clearer, but generally, "is alive" covers "is alive with points" as well as "is alive without points" (i.e., seki).

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 Post subject: Re: James Davies - life and death - question
Post #5 Posted: Sat Nov 25, 2017 3:05 am 
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Thanks! It just a bit confusing cause later in the book at other examples the author uses "white/black is alive in seki".

I think the topic can be closed now. Thanks again for the replies.

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Post #6 Posted: Sat Nov 25, 2017 4:56 am 
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Quote:
"is alive without points" (i.e., seki).
Seki can have points in area scoring.

I don't know a 'formal' definition of alive;
I wonder about: "impossible for the opponent to remove from the board, given perfect play by both sides"... ? But maybe that's cyclical if a definition of 'perfect play' involves including seki.

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Post #7 Posted: Thu Apr 26, 2018 8:30 am 
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EdLee wrote:
I don't know a 'formal' definition of alive;


The usual definiton is: Stones are 'alive' if they cannot be captured.

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Post #8 Posted: Thu Apr 26, 2018 8:44 am 
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Akura wrote:
EdLee wrote:
I don't know a 'formal' definition of alive;

The usual definiton is: Stones are 'alive' if they cannot be captured.

These Black stones are not alive, then?
Click Here To Show Diagram Code
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$$ - X X X X X X X X X O .
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$$ - . . . . . . . . . . .[/go]

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 Post subject: Re: James Davies - life and death - question
Post #9 Posted: Thu Apr 26, 2018 8:54 am 
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They are not unconditionaly alive, because it's possible (but very very unlikely) that they can be captured in a game.

But if you consider "alive" as "if black answer to white moves, they cannot be captured", then it is alive.

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 Post subject: Re: James Davies - life and death - question
Post #10 Posted: Thu Apr 26, 2018 9:01 am 
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For formal life definitions and correction of the wrong "Stones are 'alive' if they cannot be captured.", see viewtopic.php?p=230294#p230294

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 Post subject: Re: James Davies - life and death - question
Post #11 Posted: Thu Apr 26, 2018 9:46 am 
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Life and death in go do not have single, unambiguous meanings. Stones may be capturable, even killable, but capturing them or killing them may be costly.

Under area scoring, dead stones at the end of the game may be captured without cost. So we may define live stones under area scoring as stones remaining on the board at the end of play. If that is our definition, then the players will capture dead stones at the end of play. But why force them to do so? Why not let the players agree which stones are alive or dead, and only force them to capture dead stones if they disagree? In that case our definition is that stones are alive or dead if the players agree that they are. ;) :)

Anyway, the White stones in Diagram 4 are alive because if Black attempts to capture them she must add one stone to the three inside stones, and then White can capture the four stones and make 8 pts. of territory instead of 0. It is costly for Black to try to capture the White stones. Note the assumptions of alternating local play. Those assumptions are practical during a game, but do not guarantee that the White stones will remain on the board at the end of play. So that's another definition of life. It is possible to construct a board where correct play is for Black to put the White stones in atari and for White to ignore that atari and play elsewhere, and then Black captures the White stones. OC, that would involve a humungous ko. Formally, we might say that the White stones are alive with probability 1 - ε, where ε is small. To put it informally, the White stones are alive, except when the aren't. ;) :)

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 Post subject: Re: James Davies - life and death - question
Post #12 Posted: Fri Apr 27, 2018 7:30 am 
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Most books and clubs players aren't using precise language. There is a kind of red pill / blue pill choice in go rules. You can take the blue pill, play games, go around telling people that go has simple rules and have fun. Or if you can take the red pill, read Berlekamp, Spight, Jasiek, etc. and this may also be fun. No one writes a computer program based entirely on a few sentences in Davies or some informal explanation they heard on a bus and if you want to run a bigger tournament you may want to at least lick the red pill. But it's your choice. :D


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