Bent four sharing thread

[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.