Life In 19x19

Hi everybody,

I'm currently reading a book about Yose ,

where that "double sente" situation is exposed :

Click Here To Show Diagram Code

[go]$$B
$$ | . . . . . . . . . . . . . .
$$ | . . . . . . . . . . . . . .
$$ | . . X . . . . . . . . . . .
$$ | . . . . . O . . . . . . . .
$$ | . . . X . . . . O , . . . .
$$ | . . . . . O . . . . O . . .
$$ | . . . X . , . . . . . . . .
$$ | . . . . . . . . . . . . . .
$$ +----------------------------[/go]

The sente for black is this one :

Click Here To Show Diagram Code

[go]$$B
$$ | . . . . . . . . . . . . . .
$$ | . . . . . . . . . . . . . .
$$ | . . X . . . . . . . . . . .
$$ | . . . . . O . . . . . . . .
$$ | . . . X . . . . O , . . . .
$$ | . . . . . O . . . . O . . .
$$ | . . . X . 1 . . . . . . . .
$$ | . . . . . . . . . . . . . .
$$ +----------------------------[/go]

I'm ok with that, but the sequence for white is this one :

Click Here To Show Diagram Code

[go]$$W
$$ | . . . . . . . . . . . . . .
$$ | . . . . . . . . . . . . . .
$$ | . . X . . . . . . . . . . .
$$ | . . . . . O . . . . . . . .
$$ | . . . X . . . . O , . . . .
$$ | . . 2 a . O . . . . O . . .
$$ | . . . X 1 , . . . . . . . .
$$ | . . . . . . . . . . . . . .
$$ +----------------------------[/go]

And here I don't understand why black didn't play at "a", is there anything white can do if black reply at "a" ?

Click Here To Show Diagram Code

[go]$$W
$$ | . . . . . . . . . . . . . .
$$ | . . . . . . . . . . . . . .
$$ | . . X . . . . . . . . . . .
$$ | . . . . . O . . . . . . . .
$$ | . . 6 X . . . . O , . . . .
$$ | . . 5 2 . O . . . . O . . .
$$ | . . 3 X 1 , . . . . . . . .
$$ | . . . 4 . . . . . . . . . .
$$ +----------------------------[/go]

maybe something like that but then, I cannot find a sequence for white to live in the corner ?

So I don't understand why is not the "best" reply here...

PS : There are a few free yose problems given by the authors of the book here :
https://docs.google.com/file/d/0B3rabuV ... JxME0/edit (but it is in french...)

oca wrote:And here I don't understand why black didn't play at "a", is there anything white can do if black reply at "a" ?

Hi oca,

One thing to consider: study the end-game sequences if W gets the first line hane D1 ( in your last diagram ).
What's the difference in end-game between the tiger's mouth and the solid connect ?

EdLee wrote:

oca wrote:And here I don't understand why black didn't play at "a", is there anything white can do if black reply at "a" ?

Hi oca,

One thing to consider: study the end-game sequences if W gets the first line hane D1 ( in your last diagram ).
What's the difference in end-game between the tiger's mouth and the solid connect ?

Hi EdLee,

Thanks for the hint, that's to keep sente is it ?

Click Here To Show Diagram Code

[go]$$W black ends in sente
$$ | . . . . . . . . . . . . .
$$ | . . . . . . . . . . . . .
$$ | . . X . . . . . . . . . .
$$ | . . . . . O . . . . . . .
$$ | . . . X . . . . O , . . .
$$ | . . 2 . . O . . . . O . .
$$ | . . . X 1 . . . . . . . .
$$ | . . 4 3 5 . . . . . . . .
$$ +--------------------------[/go]

Click Here To Show Diagram Code

[go]$$W black ends in gote
$$ | . . . . . . . . . . . . .
$$ | . . . . . . . . . . . . .
$$ | . . X . . . . . . . . . .
$$ | . . . . . O . . . . . . .
$$ | . . . X . . . . O , . . .
$$ | . . . 2 . O . . . . O . .
$$ | . . 6 X 1 . . . . . . . .
$$ | . . 4 3 5 . . . . . . . .
$$ +--------------------------[/go]

Hi oca, yes, very good. Sente v. gote.

There are probably other reasons, to do with the shape and corner aji. Maybe others can help you out.

The position can, but need not, be a double sente. It depends on whether the black group is weak or strong on the left side and whether the white group is weak or strong on the lower side and in the center. Depending on global context, it could even be a gote. IOW, the book author seems to have imagined a strong white group on the left side and a strong black group in the center and on the far right side.

Double sente depends upon the rest of the board. Typically each player's threat must be greater than anything else on the board. One flaw with older yose books is that they present certain positions as inherently double sente. It has been known for well over 40 years that there is something wrong with the idea of unconditional double sente. Still, writers often follow tradition and classify some positions as double sente. One who does not is O Meien, 9 dan, who does not even mention double sente in his recent yose book.

Bill Spight wrote:Double sente depends upon the rest of the board. Typically each player's threat must be greater than anything else on the board. One flaw with older yose books is that they present certain positions as inherently double sente. It has been known for well over 40 years that there is something wrong with the idea of unconditional double sente. Still, writers often follow tradition and classify some positions as double sente. One who does not is O Meien, 9 dan, who does not even mention double sente in his recent yose book.

I think that is fair comment, but I would also say that I have always assumed that when doing yose problems that they are considered in isolation and so the remainder of the board do not affect the outcome. With this understanding double sente does make sense.

DrStraw wrote:

Bill Spight wrote:Double sente depends upon the rest of the board. Typically each player's threat must be greater than anything else on the board. One flaw with older yose books is that they present certain positions as inherently double sente. It has been known for well over 40 years that there is something wrong with the idea of unconditional double sente. Still, writers often follow tradition and classify some positions as double sente. One who does not is O Meien, 9 dan, who does not even mention double sente in his recent yose book.

I think that is fair comment, but I would also say that I have always assumed that when doing yose problems that they are considered in isolation and so the remainder of the board do not affect the outcome. With this understanding double sente does make sense.

Quite the opposite. In isolation the only double sente is seki. Otherwise the value of a double sente is undefined, because of division by zero.

Bill Spight wrote:

DrStraw wrote:I think that is fair comment, but I would also say that I have always assumed that when doing yose problems that they are considered in isolation and so the remainder of the board do not affect the outcome. With this understanding double sente does make sense.

Quite the opposite. In isolation the only double sente is seki. Otherwise the value of a double sente is undefined, because of division by zero.

If the follow up move is large enough then the value is undefined, precisely because of the divide by zero.

If the only consideration is preventing the first line hane, why not here?

Click Here To Show Diagram Code

[go]$$W
$$ | . . . . . . . . . . . . . .
$$ | . . . . . . . . . . . . . .
$$ | . . X . . . . . . . . . . .
$$ | . . . . . O . . . . . . . .
$$ | . . . X . . . . O , . . . .
$$ | . . . . . O . . . . O . . .
$$ | . . 2 X 1 , . . . . . . . .
$$ | . . . . . . . . . . . . . .
$$ +----------------------------[/go]

or here?

Click Here To Show Diagram Code

[go]$$W
$$ | . . . . . . . . . . . . . .
$$ | . . . . . . . . . . . . . .
$$ | . . X . . . . . . . . . . .
$$ | . . . . . O . . . . . . . .
$$ | . . . X . . . . O , . . . .
$$ | . . . . . O . . . . O . . .
$$ | . . . X 1 , . . . . . . . .
$$ | . . . 2 . . . . . . . . . .
$$ +----------------------------[/go]

Marcel, DrStraw, It is the responsibility of book authors to present correct examples instead of making dubious assumptions.

Bill, I disagree that the value of double sente would be non-existent. Only a specific kind of value is non-existent: the miai value because the number of local excess moves is zero and, in classic maths, we do not divide by zero. However, a double sente does have a different value: the local swing. At a moment, when it is correct to play a double sente because its threat (a second value!) is so big that the opponent must answer, the locally starting player takes the swing value. If he fails to do so, then the opponent takes it and "gains" it for the according expected score change. Therefore, books should not take pride in hiding double sente.

emeraldemon wrote:If the only consideration is preventing the first line hane, why not here?

$$W
$$ | . . . . . . . . . . . . . .
$$ | . . . . . . . . . . . . . .
$$ | . . X . . . . . . . . . . .
$$ | . . . . . O . . . . . . . .
$$ | . . . X . . . . O , . . . .
$$ | . . . . . O . . . . O . . .
$$ | . . 2 X 1 , . . . . . . . .
$$ | . . . . . . . . . . . . . .
$$ +----------------------------

Click Here To Show Diagram Code

[go]$$W
$$ | . . . . . . . . . . . . . .
$$ | . . . . . . . . . . . . . .
$$ | . . X . . . . . . . . . . .
$$ | . . . . . O . . . . . . . .
$$ | . . . X . . . . O , . . . .
$$ | . . . . . O . . . . O . . .
$$ | . . 2 X 1 , . . . . . . . .
$$ | . . . . . . . . . . . . . .
$$ +----------------------------[/go]

or here?

$$W
$$ | . . . . . . . . . . . . . .
$$ | . . . . . . . . . . . . . .
$$ | . . X . . . . . . . . . . .
$$ | . . . . . O . . . . . . . .
$$ | . . . X . . . . O , . . . .
$$ | . . . . . O . . . . O . . .
$$ | . . . X 1 , . . . . . . . .
$$ | . . . 2 . . . . . . . . . .
$$ +----------------------------

Click Here To Show Diagram Code

[go]$$W
$$ | . . . . . . . . . . . . . .
$$ | . . . . . . . . . . . . . .
$$ | . . X . . . . . . . . . . .
$$ | . . . . . O . . . . . . . .
$$ | . . . X . . . . O , . . . .
$$ | . . . . . O . . . . O . . .
$$ | . . . X 1 , . . . . . . . .
$$ | . . . 2 . . . . . . . . . .
$$ +----------------------------[/go]

It's not the only consideration. Making the 4-4 stone stronger is another, preventing this sente sequence for example.

Click Here To Show Diagram Code

[go]$$W
$$ | . . . . . . . . . . . . . .
$$ | . . . . . . . . . . . . . .
$$ | . . X . . . . . . . . . . .
$$ | . . 4 3 . O . . . . . . . .
$$ | . . . X 5 . . . O , . . . .
$$ | . . . 6 . O . . . . O . . .
$$ | . . 2 X 1 , . . . . . . . .
$$ | . . . . . . . . . . . . . .
$$ +----------------------------[/go]

RobertJasiek wrote:Marcel, DrStraw, It is the responsibility of book authors to present correct examples instead of making dubious assumptions.

It is the responsibility of an author to make himself understood. Excessively verbose explanations are a hindrance to learning. If reasonable assumptions are made in order to simplify the learning process than the overall quality of the book is improved. Fortunately, the majority of authors follow this principle. I can only think of a very few who do not.

Bill Spight wrote:It has been known for well over 40 years that there is something wrong with the idea of unconditional double sente.

Hi Bill, could you elaborate on this a little bit. Thanks.

Since double sente depends on the (global) positional context, examples ignoring it must be accompanied by an explanation of the made simplifying assumptions. One does not learn what double sente is by saying that it was double sente, period. And it really is not that hard for a book author to present a proper version of the example:

Click Here To Show Diagram Code

[go]$$B
$$ | . . . . . . . . . . . . . .
$$ | . . O . . . . . . . . . . .
$$ | . . . . . . . X . . X . . .
$$ | . . X . . . . . . . . . . .
$$ | . . . . . O . . . . . . . .
$$ | . . . X . . . . O , . . . .
$$ | . . . . . O . . . . O . X .
$$ | . . . X . , . . . . . . . .
$$ | . . . . . . . . . . . . . .
$$ +----------------------------[/go]

EDIT: oca,

Click Here To Show Diagram Code

[go]$$W
$$ | . . . . . . . . . . . . . .
$$ | . . O . . . . . . . . . . .
$$ | . . . . . . . X . . X . . .
$$ | . . X . . . . . . . . . . .
$$ | . . . . . O . . . . . . . .
$$ | . . . X . . . . O , . . . .
$$ | . . 2 . . O . . . . O . X .
$$ | . . . X 1 , . . . . . . . .
$$ | . . . . . . . . . . . . . .
$$ +----------------------------[/go]

Much later on the left lower side, this will be followed up by one of the following gote sequences:

Click Here To Show Diagram Code

[go]$$B
$$ | . . . . . . . . . . . . . .
$$ | . . O . . . . . . . . . . .
$$ | . . . . . . . X . . X . . .
$$ | . . X . . . . . . . . . . .
$$ | . . . . . O . . . . . . . .
$$ | . . . X . . . . O , . . . .
$$ | . . X . . O . . . . O . X .
$$ | . . . X O , . . . . . . . .
$$ | . . . 3 1 2 . . . . . . . .
$$ +----------------------------[/go]

Click Here To Show Diagram Code

[go]$$W
$$ | . . . . . . . . . . . . . .
$$ | . . O . . . . . . . . . . .
$$ | . . . . . . . X . . X . . .
$$ | . . X . . . . . . . . . . .
$$ | . . . . . O . . . . . . . .
$$ | . . . X . . . . O , . . . .
$$ | . . X . . O . . . . O . X .
$$ | . . . X O , . . . . . . . .
$$ | . . 2 1 3 . . . . . . . . .
$$ +----------------------------[/go]

For both together, one can imagine the simplifying average:

Click Here To Show Diagram Code

[go]$$B
$$ | . . . . . . . . . . . . . .
$$ | . . O . . . . . . . . . . .
$$ | . . . . . . . X . . X . . .
$$ | . . X . . . . . . . . . . .
$$ | . . . . . O . . . . . . . .
$$ | . . . X . . . . O , . . . .
$$ | . . X . . O . . . . O . X .
$$ | . . . X O , . . . . . . . .
$$ | . . . X O . . . . . . . . .
$$ +----------------------------[/go]

Now compare this with

Click Here To Show Diagram Code

[go]$$W
$$ | . . . . . . . . . . . . . .
$$ | . . O . . . . . . . . . . .
$$ | . . . . . . . X . . X . . .
$$ | . . X . . . . . . . . . . .
$$ | . . . . . O . . . . . . . .
$$ | . . . X . . . . O , . . . .
$$ | . . . X . O . . . . O . X .
$$ | . . . X 1 , . . . . . . . .
$$ | . . . . . . . . . . . . . .
$$ +----------------------------[/go]

after which White has the sente / privilege follow-up

Click Here To Show Diagram Code

[go]$$W 4 might also be at a, depending on global context
$$ | . . . . . . . . . . . . . .
$$ | . . O . . . . . . . . . . .
$$ | . . . . . . . X . . X . . .
$$ | . . X . . . . . . . . . . .
$$ | . . . . . O . . . . . . . .
$$ | . . . X . . . . O , . . . .
$$ | . . b X . O . . . . O . X .
$$ | . 4 a X O , . . . . . . . .
$$ | . . 2 1 3 . . . . . . . . .
$$ +----------------------------[/go]

Result: Black has lost 1 point in comparison to initially responding at b.

This is the pure local value explanation. However, b also makes better eyeshape and leaves behind fewer weaknesses for a white invasion if White should become even stronger around the corner. E.g., if Black can later invade the bottom, the white group there will become stronger and then b is in a better position. IOW, b is also the proper move and the better preparation for a possible black invasion.

Another consideration is this aji in case of Black's tenuki:

Click Here To Show Diagram Code

[go]$$W
$$ | . . . . . . . . . . . . . .
$$ | . . O . . . . . . . . . . .
$$ | . . . . . . . X . . X . . .
$$ | . . X . . . . . . . . . . .
$$ | . . . . . O . . . . . . . .
$$ | . . . X 3 . . . O , . . . .
$$ | . . 2 1 . O . . . . O . X .
$$ | . . a X O , . . . . . . . .
$$ | . . . . . . . . . . . . . .
$$ +----------------------------[/go]

Usually a is the attacking move, but White can also create this ko.