Rectangular Eight in the Corner

    Keywords: Life & Death

Table of contents

1. No outside liberties

[Diagram]
Black lives with gote  


If Black plays first, Black has several moves which grant life, A, B, C, D, E, F and G.

[Diagram]
No outside liberties  


This position is hyperactive. Its value and play depends upon the ko threat situation. If White is komaster, she can often make seki with sente.


W1 is the strongest attack. B2 is the point to avoid ko. B6 is necessary or White plays there (or to the right of it) to capture the corner. B2 at W5 or B6 will also make a seki, but White would get an extra ko threat.

Nikok34 (KGS) : Can't in this position White can make a bent four in the corner with 1-1?

willemien : No not all bent four's are bent four in the corner. If White plays 1-1 and Black captures, you get the "Not corner bent four 3" on the bent four in the corner page and Black is alive.

Nikok34 : Now I see why ... thanks.

[Diagram]
Variation at 4  


If B4 here, W5 makes a ten-thousand-year ko. Despite taking sente, this result is generally worse for Black than a seki.

[Diagram]
Variation at 2  


B2 at the 2-2 point invites an approach ko. Black takes first and White owes an approach (square), but nevertheless ko.

If after W5, White gets to play square (the approach move), the ko becomes direct.

[Diagram]
Variation at 2 - White's mistake  


W5 in this variation invites a seki. Without B6 either player can make a throw-in ko.

The Count: This isn't necessarily a mistake, is it? If White has one big ko threat and no others then White can't win the approach ko above, but can win the throw-in ko, and so Black will play B6 and White will make seki which is better than nothing.

IronChefSakai: It seems to me that White will rarely want to play the throw-in in the previous diagram (creating the approach ko), because assuming both players know this shape well, both White and Black would have to prefer a ko over seki and White would not only have to prefer a ko over seki, but would have to be willing to settle for a disadvantaged approach ko. Can anyone verify this thinking?

Bill: OC, if White is komaster White will throw in. Sometimes that will also be correct when neither player is komaster.

[Diagram]
Wrong start: gote seki  

2. Outside liberty

[Diagram]
Outside liberty  


If there is at least one outside liberty, Black is alive with seven points.

[Diagram]
Defenses  


Now the marked points all work.


Conclusion for the endgame

[Diagram]
Outside liberty  


W1 and W3 each threaten at least mannen-ko in sente, and W5 threatens to make seki in sente. B6 makes seven points in reverse sente.

[Diagram]
the mannen-ko  


For the mannen-ko, the stone in the ko will be black and Black will still have the outside liberty. (black+square can be either B2 or B4.)




See also


Rectangular Eight in the Corner last edited by 107.210.159.110 on June 19, 2020 - 18:50
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