Synthetic Two Stage Ko

    Keywords: Ko, Tactics, Go term

This page is in need of attention.


Karl Knechtel: I have recently seen positions of this sort crop up fairly often in mid-dan level KGS games.

[Diagram]

Synthetic two-stage ko, "middle" position

[Diagram]

Synthetic two-stage ko, "white leading" position

[Diagram]

Synthetic two-stage ko, "black leading" position

The groups share a liberty and each has an external simple ko; they do not share the kos. As a result, this plays not like a double ko, but as a two-stage ko determining the status of all inside stones. Having won one ko, neither player can fill, but must instead start the other ko, which then behaves as the next "stage".


[Diagram]

Synthetic two-stage ko, "middle" position

We can also form such a position by giving each group an eye, and no shared liberties.


[Diagram]

With both an eye and a shared liberty

With both an eye each and a shared liberty, the position can be seki or mannenko. This is the only construction of two-stage mannenko I am aware of.

It is, presumably, somewhat less likely to be played than an ordinary mannenko, because a two-stage ko starting in the "unfavourable" position is harder to win than a direct ko from the "unfavourable" position. However, one player has the option of capturing an outside ko, and then starting the mannenko in "neutral" position, while opponent gets a gote move in response to the initial capture (as opposed to replying by capturing on the other side to preserve seki). And of course, there is the double-ko potential too (a 4-move cycle exists) - very complicated!


unkx80: The part "one player has the option of capturing an outside ko, and then starting the mannenko in "neutral" position" is wrong --

[Diagram]

With both an eye and a shared liberty

Suppose B1 takes first, then W2 also takes. Then Black a would be self-atari.

[Diagram]

With both an eye and a shared liberty

Later, B5 captures, then W6 captures too. This reverts back to the starting position.

[Diagram]

With both an eye and a shared liberty

Therefore, the only way for Black to win this capturing race is to play at B1, putting himself into an unfavourable two-stage ko.


I meant this way, hence "and then":

[Diagram]

W2 tenuki, W4 captures B1

hhw: I think if White does not immediately take the other ko with W2, then he should just keep on playing tenuki. Black will use B1, B3, and B5 to capture a few points, but White will get 3 moves in a row elsewhere.


This is a copy of the living page "Synthetic Two Stage Ko" at Sensei's Library.
(OC) 2011 the Authors, published under the OpenContent License V1.0.
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