Bent four in corner and Japanese rules.

Toge · Post by **Toge** » Tue Jun 21, 2011 6:55 am

Why is bent four in corner dead under Japanese rules?

daal · Post by **daal** » Tue Jun 21, 2011 7:21 am

Isn't it only considered dead if left so until the end of the game when there are presumably no ko threats?

Toge · Post by **Toge** » Tue Jun 21, 2011 7:27 am

daal wrote:Isn't it only considered dead if left so until the end of the game when there are presumably no ko threats?

- That's exactly the problem. Is there any case when there are absolutely no threats under Japanese rules? Why shouldn't black have to play out that ko?

If we declare it dead without playing ko white has 0 points in lower right, black has 16. If black plays ko and wins it (no threats) white has 4 points and black has 18. White gains 2 points.

yoyoma · Post by **yoyoma** » Tue Jun 21, 2011 7:35 am

Toge wrote:
daal wrote:Isn't it only considered dead if left so until the end of the game when there are presumably no ko threats?
- That's exactly the problem. Is there any case when there are absolutely no threats under Japanese rules? Why shouldn't black have to play out that ko?

There are no threats because that's the way they made the rules (pass only for ko threat in the resolution of life&death at the end of game).

Cassandra · Post by **Cassandra** » Tue Jun 21, 2011 7:36 am

Under "usual" Japanese rules, status of life and death is evaluated per single group.

As far as Bent-Four is concerned (and so a bit shortened) they say that a group is alive if you cannot take it off the board. A group that is not alive, is dead. Dead groups inside opponent's alive ones can be taken off the board as prisoners without further play.

In your Bent-Four in the lower left, White cannot take Black's three stones off the board, so these are alive.

You have found a sequence, with which Black can take White's group in the lower right off the board, so is is dead. If you assume Black's outside group as alive, White's group will become prisoners.

It does not matter, what happens on the rest of the board in the meantime, because the position at game end will remain unchanged as basis for scoring.

Bill Spight · Post by **Bill Spight** » Tue Jun 21, 2011 7:45 am

The question can be answered in more than one way. One is to appeal to the current Japanese rules, under which bent four is dead because the defender has no effective ko threat. The defender has no effective ko threat because the only threat is a pass for that specific ko, after which the attacker will win the ko. Another is to address the reasons that the current rules are as they are. That is more speculative.

The current Japanese rules make explicit what has been implicit in the rules, namely virtual play after the end of regular play. Dead stones are removed without actual play, but may be removed by virtual play. The play is virtual because, if the defender is allowed to pass without penalty, the attacker loses points by capturing dead stones. Under virtual play Black, the attacker in this example, could remove the White ko threats before playing the bent four. Then Black could win the ko and capture the White stones. Since Black could do that, the White stones are dead, and may be removed without actual play.

Now, on rare occasions White may have an irremovable ko threat. In that case Black cannot remove the threat through virtual play, and, under current Chinese rules this bent four would be seki. However, back in the mists of time the Japanese decided that bent four was simply dead. Perhaps they were unaware of the rare occasions that there would be a bent four and an unremovable ko threat, or perhaps they felt that accommodating such rare positions made the rules unworkable or unappealing.

I have added a couple of comments and illustrative variations to the SGF file.

Edit: Even though under area scoring, which the Chinese rules have used for centuries, Bent Four is not necessarily dead, there is some evidence that it was considered dead under earlier Chinese rules, because there are ancient Chinese life and death problems where Bent Four is treated as dead.

Magicwand · Post by **Magicwand** » Tue Jun 21, 2011 8:03 am

i will state it simply with very few words.

white group can not go after black group because it will kill him for sure.
black can go after the white group and make ko out of it.
so black has more "right" than white group.

if you want more reasons... black can eleminate all ko threats and go after white group.
so it is considered dead.

yes yes..i know that there are ko threats that you can not eleminate.
but...it is like 0! = 1
with that convention it is easier to eleminate the bent four issues.

Redundant · Post by **Redundant** » Tue Jun 21, 2011 9:34 am

Magicwand wrote: but...it is like 0! = 1

This is horribly off topic, but 0!=1 can be justified by using a good definition for the factoria. If you define n! as the number of bijections from a set with n elements to itself, then 0! is the number of bijections from the empty set to itself. There is only one, the empty function.

/off topic

Kirby · Post by **Kirby** » Tue Jun 21, 2011 10:19 am

So I read online that the following position is a seki by Japanese rules　（http://www.nihonkiin.or.jp/joho/kiyaku/shikatsu-13.htm）:

Click Here To Show Diagram Code: [go]$$W Seki $$ ---------------- $$ | X X . X O X . . $$ | . O X . O X . . $$ | O O O O O X . . $$ | X X X X X X . . $$ | . . . . . . . . $$ | . . . . . . . .[/go]

If we compare this to the rationale we have for bent 4 being dead, though, it seems to me that, under Japanese rules, this shape should also be dead.

That's because, ignoring irremovable threats (as is done for bent 4), black can remove all of white's threats, and then play as follows:

Click Here To Show Diagram Code: [go]$$Bm1 $$ ---------------- $$ | X X . X O X . . $$ | 1 O X . O X . . $$ | O O O O O X . . $$ | X X X X X X . . $$ | . . . . . . . . $$ | . . . . . . . .[/go]

Click Here To Show Diagram Code: [go]$$Bm1 $$ ---------------- $$ | 4 3 2 X O X . . $$ | 5 O X . O X . . $$ | O O O O O X . . $$ | X X X X X X . . $$ | . . . . . . . . $$ | . . . . . . . .[/go]

This seems like a similar situation as bent 4 - I mean, hey, it's almost the same shape - but it's apparently seki under Japanese rules.

What's up with that?

emeraldemon · Post by **emeraldemon** » Tue Jun 21, 2011 10:41 am

but if W4 is pass, doesn't white get to take the ko first?

Kirby · Post by **Kirby** » Tue Jun 21, 2011 10:44 am

emeraldemon wrote:but if W4 is pass, doesn't white get to take the ko first?

Good point, but isn't this the same with bent 4?

Click Here To Show Diagram Code: [go]$$W $$ ---------------- $$ | X X X O X . . $$ | . O . O X . . $$ | O O O O X . . $$ | X X X X X . . $$ | . . . . . . . $$ | . . . . . . .[/go]

Click Here To Show Diagram Code: [go]$$Bm1 $$ ---------------- $$ | X X X O X . . $$ | 1 O 2 O X . . $$ | O O O O X . . $$ | X X X X X . . $$ | . . . . . . . $$ | . . . . . . .[/go]

Click Here To Show Diagram Code: [go]$$Bm3 White 4 pass $$ ---------------- $$ | . 1 . O X . . $$ | 3 O O O X . . $$ | O O O O X . . $$ | X X X X X . . $$ | . . . . . . . $$ | . . . . . . .[/go]

Although, I suppose black will not do this, but terminate the position:

Click Here To Show Diagram Code: [go]$$Bm3 White 4 pass $$ ---------------- $$ | 3 1 . O X . . $$ | . O O O X . . $$ | O O O O X . . $$ | X X X X X . . $$ | . . . . . . . $$ | . . . . . . .[/go]

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

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]

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.

yoyoma · Post by **yoyoma** » Tue Jun 21, 2011 10:49 am

Kirby wrote:
emeraldemon wrote:but if W4 is pass, doesn't white get to take the ko first?
Good point, but isn't this the same with bent 4?

In bent 4, if white passes instead of starting the ko, black will just extend to the 1-1 point and kill.

There is an explanation for many shapes here:
http://www.cs.cmu.edu/~wjh/go/rules/Japanese.html

Kirby · Post by **Kirby** » Tue Jun 21, 2011 10:51 am

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?

Kirby · Post by **Kirby** » Tue Jun 21, 2011 10:52 am

yoyoma wrote:
Kirby wrote:
emeraldemon wrote:but if W4 is pass, doesn't white get to take the ko first?
Good point, but isn't this the same with bent 4?
In bent 4, if white passes instead of starting the ko, black will just extend to the 1-1 point and kill.

There is an explanation for many shapes here:
http://www.cs.cmu.edu/~wjh/go/rules/Japanese.html

Good point. I thought of this and edited my post, but you were too quick!

oren · Post by **oren** » Tue Jun 21, 2011 10:53 am

Kirby wrote: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?

Yeah, and I think this is done generally to avoid having to do area scoring. Bent 4 will die in all but very unusual circumstances, but you have to fill in your own territory to do it. If you use AGA rules for example, there's no penalty in removing all the possible ko threats (in all but very unusual circumstances).

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