Gokyo Shumyo, Section 1, Problem 18 / Solution

Black lives  

(See Oki Wants Ikken Tobi.)

Main line  

Black 1 comes in handy when 7 gives atari and threatens to make two eyes. If White connects at 3, Black captures at a and has an eye in the corner and one where he captured four stones.

(variations later - please add some yourself)

Variation 1  

White 2 takes a vital point, and 4 and 6 reduce the eyespace but still a and b are miai for life.

Wrong 1 (seki)  

Black 1 creates a long L group, which White can turn either into seki or (worse) into ten thousand year ko, depending on Black 5. Life with points as in the correct diagram is better than seki (or ten thousand year ko).

Wrong 1 (White mistake)  

If White resists at 6, Black 7 is a tesuji. Black 9 recaptures at 7 and if White connects underneath at a, Black can give atari at b.

Wrong 1 (ten thousand year ko)  

Here Black misses the tesuji and ends up with a ten thousand year ko.

Wrong 2  

Black 1 is badly placed for catching the placement stone 2.

Addendum  

If the marked stone is present Black 1 doesn't work anymore. White 8 is double atari.

Addendum  

In this case it is better to create a long L group with 1. The marked stone now alters the status from seki to ten thousand year ko. See Long L Group for details.