Bent four sharing thread

[richyfourtytwo]

I knew about bent four since decades. I believe I have really understood it only recently. (But who knows, maybe that moment is still in the future.) It didn’t happen in my games very often yet though.

We have a thread for sharing sekis already, so why not one for bent fours?

I’ll make a start: here is one from a recent game of mine involving around 45 points in the lower right corner.

Freundschaftsspiel

[stone.defender]

Did anyone have 2 bent-four in 1 game?
So only one of 2 groups can be actually killed with it if both are same color.
And I can’t imagine what happens if they are opposite color.

(There is bureaucracy instead of mechanics in Japanese rules, so I’m talking about Chinese)
https://senseis.xmp.net/?BentFourInTheCornerIsDead

[yebellz]

I have a thread where I work through how bent-four works under Chinese rules. Basically, the bent-four is dead in ko, which means that if there are big enough, unremovable ko threats, it could actually live.

Life and Death under Chinese Rules

Consider the following board position under Chinese rules with a 5.5 point komi [game1] Which stones are alive or dead? Who will win this game and by how much? What is the best line of play? The purpose of this example is to explore how life and death is handled under Chinese rules. Pause here to consider the above questions if you wish to treat this example as a problem, before reading on into the rest of the discussion. Discussion Note that white’s group in the top-left forms a “bent fo…

Another example is in the last post of that thread:

@yebellz

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@stone.defender

Did anyone have 2 bent-four in 1 game?
So only one of 2 groups can be actually killed with it if both are same color.

Under Japanese rules, if one player has two bent-four formations in separate areas, then they would (most likely, unless some other weird connected things are happening) both be dead groups. The life and death resolution phase of Japanese rules applies special ko rules that would essentially isolate each position and nullify any ko threats. See:

Odd Cases in the Japanese Rules

Special Ko Rules when Determining Life and Death After Passing During the normal course of play, the basic ko rule (which is universal across all go rules) is in effect: Article 6. Ko A shape in which the players can alternately capture and recapture one opposing stone is called a “ko.” A player whose stone has been captured in a ko cannot recapture in that ko on the next move. When a ko arises that both players want to win (e.g., the life/death of a large group may depend on ko), they must …

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@stone.defender

And I can’t imagine what happens if they are opposite color.

Two bent-fours in opposite colors could actually result in a seki (under both Japanese and Chinese rules):

@yebellz

It should also be noted that under the modern Japanese rules, it is not automatic that bent four in the corner is always dead either. Here is an example where two interacting bent fours in the corner produces a seki: http://warp.povusers.org/go/UndeadBentFours/

Unfortunately, the website that I linked there seems to now be down, but basically, it showed how the position is a hanezeki-like oddity, where it behaves like a strange capturing race, with the person who starts it would lose.

[yebellz]

@stone.defender

Did anyone have 2 bent-four in 1 game?
So only one of 2 groups can be actually killed with it if both are same color.
And I can’t imagine what happens if they are opposite color.

Found the example and posted about it here:

@yebellz

I found that the original position (although without the detailed analysis) can be found via Sensei’s library here: Strange Sekis at Sensei’s Library

Here is the pattern, which is seki under all rulesets:

This is seki in a manner analogous to hanezeki, in the sense that whoever plays another move in the local position will surprisingly start a capturing race that they lose. In other words, you can only win the capturing race if your opponent plays first (and hence no one should want to start it).

[Paige]

I was actually just looking at this yesterday after encountering bent four in one of my games. I was curious how the different rules handled it and encountered the bent four seki under all rulesets.

[antonTobi]

Here’s a fun position, not quite bent four, but pretty closely related:

What is best play for black/white playing first under chinese rules?
(I have no idea about japanese rules, maybe @yebellz can analyze that for us!)

Although the position is presented on a compact small board here, it does not in fact rely on the two groups being adjacent (at least in chinese rules). This sort of situation could happen in a normal 19x19 game, but I don’t know of any examples from real games.

[Paige]

Ah its because if white plays first he dies. If black plays first it gives white a ko threat. White is inevitably alive in this position after all ko threats are removed.

(i.e A1 A4 A2 E4 F4 B2 making the KO and A3 miai)

If white play’s first he will just lose the ko and die if there are no ko threats.

Edit: but, I don’t know how this would play out under the rules.

[yebellz]

That’s a nice position. If I’ve understood it correctly, I believe that this is one of the cases that behaves very differently under the Japanese rules.

I’ll have to think about it a bit to make sure, but I’ll eventually post an analysis in my Odd Cases thread.

[yebellz]

Under Chinese rules, both players should pass, and all stones are alive in seki.
Under Japanese rules, both players should pass, but all White stones are dead (and removed), and all Black stones are alive*.

*Even though it seems redundant, I’m pedantically saying that all of the Black stones are alive and that the dead White stones are removed, just to clarify that there are not any other weird situations going on, like dead stones not being removed due to anti-seki or residing in the eye of a seki.

Odd Cases in the Japanese Rules

Never Mind the Ko, Here’s the Dead Group In another thread, @le_4TC threw down the proverbial, pedantic gauntlet… Chinese Rules Analysis In order to appreciate and understand how peculiar this position is under the Japanese rules, we should first analyze and make sense of it under the Chinese (or similar area-scoring) rules, where complicated life-and-death is settled by the principal of “playing it out” (which is not what is done under Japanese rules, of course). [0root] Let’s first exam…