It's ambiguously a double sente. It is a sente for Black. White to play can choose whether to play it as a double sente or as a reverse sente.

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At some point, doesn't thinking have to go on?
— Winona Adkins

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What is W's sequence when chose to play it as reverse sente?

Click Here To Show Diagram Code

[go]$$Wc White 1 pt. reverse sente
$$ ----------------------
$$ | . . . . . . 8 7 9 . .
$$ | . . . . . . X X O . .
$$ | . X . X . . X O . O .
$$ | O X O X 2 X X O . , .
$$ | X X X X 1 O X O . . .
$$ | O X O 6 X O X O . . .
$$ | O X 4 5 X O X O . . .
$$ | O O O O O 3 O . . . .
$$ | . . . . . . . . . . .
$$ | . . . , . . . . . , .[/go]

White keeps going after to gain one point.

_________________
The Adkins Principle:
At some point, doesn't thinking have to go on?
— Winona Adkins

Visualize whirled peas.

Everything with love. Stay safe.

Now I'm confused. This is the same sequence that is W's sente (for E15 area), isn't it? Or is there a different way for W if chose to play E15 area as double sente, as you mentioned?

And why is this reverse sente? B plays last there (E15 area), hence that part seemed simple double sente (with B's sente leaving a double gote there, W's sente nothing, and top being an independent double gote in any case)...

The E-15 area by itself is more complicated. Adding the hane-and-connect actually simplified matters. The combination is a Black sente.

Click Here To Show Diagram Code

[go]$$Bc Black 1 pt. sente
$$ ----------------------
$$ | . . . . . . . . . . .
$$ | . . . . . . X X O . .
$$ | . X . X . . X O . O .
$$ | O X O X . X X O . , .
$$ | X X X X 1 O X O . . .
$$ | O X W . X O X O . . .
$$ | O X . . X O X O . . .
$$ | O O O O O 2 O . . . .
$$ | . . . . . . . . . . .
$$ | . . . , . . . . . , .[/go]

After the hane-and-connect and the area around are miai. End of story, locally.

For this to be a 1 pt. sente, White has to gain 1 pt. with a reverse sente. White does that with the above sequence.

White also has the option of stopping locally after , for a double sente that gains no points.



Yes, each of these two regions is independent. But their combination alters correct play. For instance, in combination with the hane-and-connect, saving the stone is incorrect, as Kano points out. White's only correct play is the one given.

All of the above assumes no ko fight, OC.

_________________
The Adkins Principle:
At some point, doesn't thinking have to go on?
— Winona Adkins

Visualize whirled peas.

Everything with love. Stay safe.

I think what confused me is that you were talking about the whole position complex, while I'm only about the E15 area... But now I think that even if that area is double sente, that still not necessarily means that the best line always starts there (in other, different positions).

Well, if we are only talking about the E-15 region, things are even more complex. Each player has both a sente option and a gote option. What do we call it? An ambiguous double sente/gote?

Click Here To Show Diagram Code

[go]$$Wc White 1 pt. sente
$$ ----------------------
$$ | . . . . . . . X O . .
$$ | . . . . . . X X O . .
$$ | . X . X . . X O . O .
$$ | O X O X 2 X X O . , .
$$ | X X X X 1 O X O . . .
$$ | O X O 6 X O X O . . .
$$ | O X 4 5 X O X O . . .
$$ | O O O O O 3 O . . . .
$$ | . . . . . . . . . . .
$$ | . . . , . . . . . , .[/go]

Click Here To Show Diagram Code

[go]$$Wc White 1 pt. gote
$$ ----------------------
$$ | . . . . . . . X O . .
$$ | . . . . . . X X O . .
$$ | . X . X . . X O . O .
$$ | O X O X . X X O . , .
$$ | X X X X 2 O X O . . .
$$ | O X O . X O X O . . .
$$ | O X 1 . X O X O . . .
$$ | O O O O O 3 O . . . .
$$ | . . . . . . . . . . .
$$ | . . . , . . . . . , .[/go]

Click Here To Show Diagram Code

[go]$$Bc Black 1 pt. sente
$$ ----------------------
$$ | . . . . . . . X O . .
$$ | . . . . . . X X O . .
$$ | . X . X . . X O . O .
$$ | O X O X . X X O . , .
$$ | X X X X 1 O X O . . .
$$ | O X O . X O X O . . .
$$ | O X . . X O X O . . .
$$ | O O O O O 2 O . . . .
$$ | . . . . . . . . . . .
$$ | . . . , . . . . . , .[/go]

Click Here To Show Diagram Code

[go]$$Bc Black 1 pt. gote
$$ ----------------------
$$ | . . . . . . . X O . .
$$ | . . . . . . X X O . .
$$ | . X . X . . X O . O .
$$ | O X O X . X X O . , .
$$ | X X X X 1 O X O . . .
$$ | O X O 5 X O X O . . .
$$ | O X 3 4 X O X O . . .
$$ | O O O O O 2 O . . . .
$$ | . . . . . . . . . . .
$$ | . . . , . . . . . , .[/go]

_________________
The Adkins Principle:
At some point, doesn't thinking have to go on?
— Winona Adkins

Visualize whirled peas.

Everything with love. Stay safe.

Actually this may also be shown right here. Suppose B to play, and he takes top gote first. Then W's best line seems to also ignore the double sente at E15 area, and take the gote there instead.

So taking double sente CAN be a mistake (even at roughly correct temperatures, when the opponent actually responds, though this seems a special case here since W's sente forces B to take a valuable gote as well - BTW is white's sente in E15 area really a +1? Not a 0?)

You mean this sequence, right?

Click Here To Show Diagram Code

[go]$$Bc Black takes gote first
$$ ----------------------
$$ | . . . . . . . 3 1 2 .
$$ | . . . . . . X X O . .
$$ | . X . X . . X O . O .
$$ | O X O X . X X O . , .
$$ | X X X X . O X O . . .
$$ | O X O . X O X O . . .
$$ | O X 4 . X O X O . . .
$$ | O O O O O . O . . . .
$$ | . . . . . . . . . . .
$$ | . . . , . . . . . , .[/go]

OC, now both locally and globally Black needs to take her sente, nothing ambiguous about it.

Click Here To Show Diagram Code

[go]$$Bc Black 5 takes double sente
$$ ----------------------
$$ | . . . . . . . 3 1 2 .
$$ | . . . . . . X X O . .
$$ | . X . X . . X O . O .
$$ | O X O X . X X O . , .
$$ | X X X X 5 O X O . . .
$$ | O X O . X O X O . . .
$$ | O X 4 . X O X O . . .
$$ | O O O O O 6 O . . . .
$$ | . . . . . . . . . . .
$$ | . . . , . . . . . , .[/go]

Absolutely.



Well, once Black has taken the hane-and-connect, what is left is doubly ambiguous between sente and gote, so it's a very special case.



The size of the sente is determined by how much the reverse sente gains, and the reverse sente gains 1 pt. If Black took her sente option instead, it would gain 0 pts. but that's normal. Sente gains nothing.

_________________
The Adkins Principle:
At some point, doesn't thinking have to go on?
— Winona Adkins

Visualize whirled peas.

Everything with love. Stay safe.

Hm. I guess you mean B's reverse sente sequence that gains 1pt in E15 area is C13 D13 D14 (since starting with atari would not really seem reverse sente, but a sente followed by an optional gote)?

These complicated positions strain the standard classifications. Indeed, sente followed by an optional gote is a kind of reverse sente in this position. Each player has the choice of playing sente or gote, and that is how Black makes the choice: sente and stop, or sente and continue.

I have more to say about this position, but let me make this observation. I have to use a game tree, because I don't know of any such actual position on the go board, but one probably exists. In the tree / represents a move by Black and \ represents a move by White. Numbers are scores from Black's point of view. BIG stands for some big number or other.



This may look like a kind of ambiguous double sente, but, unless there is a ko fight, the White move from A to C is not good. How come? Because the result for White is a local score of 0. A is better for White than that because White has the option of playing from A to a local score of -1, while the best that Black can do is to play to B with sente, for a local score of 0. A can be classified as a Black sente.



A is now like the position around E-15. The count of D is 0, but now the White move from A to C is a possibility. Why? Because Black has the move from D to a score of 1, to act as a reverse sente for C. Without that Black move, the White move from A to C has no value, outside of a ko fight.

_________________
The Adkins Principle:
At some point, doesn't thinking have to go on?
— Winona Adkins

Visualize whirled peas.

Everything with love. Stay safe.

Easy tsumego by yours truly

All my problems are easy, eh?

Click Here To Show Diagram Code

[go]$$Wc White to play
$$ --------------
$$ | . . O . X O .
$$ | . X . . X O .
$$ | X . X X X O .
$$ | X X O X O O .
$$ | X O O O . . .
$$ | O . . . O . .
$$ | . O . . . . .
$$ | . . . . . . .[/go]

_________________
The Adkins Principle:
At some point, doesn't thinking have to go on?
— Winona Adkins

Visualize whirled peas.

Everything with love. Stay safe.

Not sure if I'm right about this,
just thought I'd give it a go.

Close.

_________________
The Adkins Principle:
At some point, doesn't thinking have to go on?
— Winona Adkins

Visualize whirled peas.

Everything with love. Stay safe.

Close but no cigar. It was a fun problem

This seems to imply one follows the other. But even if C is not better than A in points (as far as EV is comparable to a collapsed result), W may still be interested (even outside ko fights), if it leaves better parity on board (parity of last big gote moves etc.). Or are these dynamics out of scope here?

Endgame problem

Click Here To Show Diagram Code

[go]$$Wc White to play
$$ --------------
$$ | . . O . X O .
$$ | . X . . X O .
$$ | . . X X X O .
$$ | X X O X O O .
$$ | X O O O . . .
$$ | O . . . O . .
$$ | . O . . . . .
$$ | . . . . . . .[/go]

As I said, endgame and tsumego go together well.

_________________
The Adkins Principle:
At some point, doesn't thinking have to go on?
— Winona Adkins

Visualize whirled peas.

Everything with love. Stay safe.

_________________
Help make L19 more organized. Make an index: https://lifein19x19.com/viewtopic.php?f=14&t=5207

Back to the game tree for E-15



Let's simplify it by removing White's sente option.



What is this? Another ambiguous sente/gote?

Not really, even though Black could play A'-> B -> D as a ko threat. Kos aside, A' = D. Maybe that is obvious to you, but let me show that.

Now, with no ko fight, games add and subtract. So A' = D means A' - D = 0. How do you subtract D? You form the negative of D and add it. So how do you form the negative of D? In go it's easy, you just reverse the color of the stones. For a game you flip the tree and reverse the signs of the scores.

Let's do that for D.

First, flip the tree:

Second, reverse the signs:

So the negative of D is D! How about that?

Now to find out if A' - D = 0. That is so if the result with best play, after each player has made the same number of moves, is 0, no matter who plays first. It is also the case with a seki, or standoff, where neither player wants to play or is able to play. For instance, D' above = 0. Black prefers a score of 0 to -1, and White prefers 0 to 1 (for Black), so neither play wants to play.

Let's look at A' - D = A' + D.


First, suppose that Black plays first.
1) Let Black play from A' to B; then White will reply from B to D. As we now know, D + D = 0.
2) Let Black play from D to 1. Then White will play from A' to -1. 1 - 1 = 0.

Next, suppose that White plays first.
1) Let White play from A' to -1. Then Black will play from D to 1. 1 - 1 = 0.
2) Let White play from D to -1. Then Black will play A' -> B -> D -> 1. 1 - 1 = 0.

So A' = D. It's just a gote, except in ko fights.

In summary, in A, D is necessary to retain the option, C, by providing a possible reverse sente for it; and C is necessary to keep A from reducing to D.

_________________
The Adkins Principle:
At some point, doesn't thinking have to go on?
— Winona Adkins

Visualize whirled peas.

Everything with love. Stay safe.

Why would White want to remove his option to play reverse sente? Maybe Black will play the sente and it will come to the same thing, but for White to play sente (except possibly in case of a ko fight) simply does Black's job for her. It is never better than doing nothing.

_________________
The Adkins Principle:
At some point, doesn't thinking have to go on?
— Winona Adkins

Visualize whirled peas.

Everything with love. Stay safe.