Bent four in corner and Japanese rules.

[Kirby]

Thanks for the responses, folks. I think that I've got the nuance. I'm trying to think of another shape with the same properties as bent-4, but that is not bent-4, but I'm having difficulty doing so.

[yoyoma]

So maybe this can be generalized to say that, when we are analyzing shapes under Japanese rules, we must assume that neither side has any ko threats?

http://www.cs.cmu.edu/~wjh/go/rules/Japanese.html

Article 7. Life and death

1. Stones are said to be "alive" if they cannot be captured by the opponent, or if capturing them would enable a new stone to be played that the opponent could not capture. Stones which are not alive are said to be "dead."
2. In the confirmation of life and death after the game stops in Article 9, recapturing in the same ko is prohibited. A player whose stone has been captured in a ko may, however, capture in that ko again after passing once for that particular ko capture.

[Kirby]

So maybe this can be generalized to say that, when we are analyzing shapes under Japanese rules, we must assume that neither side has any ko threats?

http://www.cs.cmu.edu/~wjh/go/rules/Japanese.html

Article 7. Life and death

1. Stones are said to be "alive" if they cannot be captured by the opponent, or if capturing them would enable a new stone to be played that the opponent could not capture. Stones which are not alive are said to be "dead."
2. In the confirmation of life and death after the game stops in Article 9, recapturing in the same ko is prohibited. A player whose stone has been captured in a ko may, however, capture in that ko again after passing once for that particular ko capture.

Cool. Thank you.

[SpongeBob]

Wow - one can become KGS 1d without understanding bent 4 - I am impressed.

(No offence intended, Toge, I wish I will reach 1d myself one day ...)

[Toge]

Wow - one can become KGS 1d without understanding bent 4 - I am impressed.

(No offence intended, Toge, I wish I will reach 1d myself one day ...)

- Well this virtual playing thing was completely new to me. I've never played Go over the board. I've also had just intuitive understanding what constitutes living group. I think Kirby's example is excellent.

[hyperpape]

Don't feel too left out. I didn't hear about virtual play from any time I played face to face games. It came from sensei's library and rules discussions on these boards.

[jts]

Wow - one can become KGS 1d without understanding bent 4 - I am impressed.

(No offence intended, Toge, I wish I will reach 1d myself one day ...)

- Well this virtual playing thing was completely new to me. I've never played Go over the board. I've also had just intuitive understanding what constitutes living group. I think Kirby's example is excellent.

Out of curiousity: do you think you've ever marked a dead group as living because you didn't understand this wrinkle in the rules?

[Toge]

Out of curiousity: do you think you've ever marked a dead group as living because you didn't understand this wrinkle in the rules?

- Possibly when I was a beginner. I can't remember. I've known that bent four is dead shape, but not the reasons why. Usually it would turn into ko.

[RobertJasiek]

To answer the OP question, bent-4 is dead under Japanese rules because,

1) if the question is for historical reasons, the standard answer is that the rules authors among the Japanese professionals overlooked both unremovable threats and bent-4-seki shapes,

2) if Japanese rules are pre-1989 style rules and the question is for the rules application, the rules say so in a precedent,

3) if Japanese rules are the 1989 rules and the question is for the rules application before a game stop, application of rules is irrelevant,

4) if Japanese rules are the 1989 rules and the question is for the rules application after a game stop, the pass for ko rule (regardless of whether applied in all its ambiguity or in the unambiguous context of J2003 interpretation) creates the effect that bent-4 is dead (i.e. the stones around the corner nakade are dead),

5) if certain modern written forms of verbal Japanese rules are used and the question is for the rules application before a game stop, application of rules is irrelevant,

6) if certain modern written forms of verbal Japanese rules are used and the question is for the rules application after a game stop, the basic ko rule creates the effect that bent-4 is dead (i.e. the stones around the corner nakade are dead).

[Mef]

In the case of the shape I brought up, black cannot do that because he will repeat the board position, maybe:

$$Bm1
$$ ----------------
$$ | X X 2 X O X .
$$ | 1 O X . O X .
$$ | O O O O O X .
$$ | X X X X X . .
$$ | . . . . . . .
$$ | . . . . . . .

Click Here To Show Diagram Code

[go]$$Bm1
$$ ----------------
$$ | X X 2 X O X .
$$ | 1 O X . O X .
$$ | O O O O O X .
$$ | X X X X X . .
$$ | . . . . . . .
$$ | . . . . . . .[/go]

$$Bm1
$$ ----------------
$$ | 5 3 . X O X .
$$ | . O X . O X .
$$ | O O O O O X .
$$ | X X X X X . .
$$ | . . . . . . .
$$ | . . . . . . .

Click Here To Show Diagram Code

[go]$$Bm1
$$ ----------------
$$ | 5 3 . X O X .
$$ | . O X . O X .
$$ | O O O O O X .
$$ | X X X X X . .
$$ | . . . . . . .
$$ | . . . . . . .[/go]

Is that why it's different? Black can still start the ko, though, so there must be some nuance in the rationale here.

Japanese rules don't have superko, so B is free to repeat this position as much as he wants (though he loses 2 points every time he does from the capture difference). Eventually if B repeats it enough times, white just gives up the group (because he has enough points to win). As mentioned B can't win the ko (because white captures first, and no threats other than pass are allowed).

Thanks for the responses, folks. I think that I've got the nuance. I'm trying to think of another shape with the same properties as bent-4, but that is not bent-4, but I'm having difficulty doing so.

Not quite the same thing, but another position you may find interesting is three points without capture

[Kirby]

Not quite the same thing, but another position you may find interesting is three points without capture

Thanks, that is interesting.