BrianMarshall: Well, I guess I'll make an attempt.
After , doesn't this depend on the ko?
Another attempt resulting in ko:
But after White 13, Black can again live by winning the ko. White 11 at 12, while tempting (to me, at least), is worse. at gives Black unconditional life.
Now an example of one failed attempt by White.
After , I think a and b are miai for life.
Brian: Ah, I see. Thank you for the correction.
Now Black can easily get two eyes and live.
unkx80: If is played at , then Black dies. Also, what happens when is played at ?
Brian: Hmm. If is at , then Black cannot split at , or he'll lose . But blocking reduces the eye to three in a row, which White can kill. OK, I see that now.
But when is at ,
White can just extend with , preventing an eye in the corner. Black needs an eye on the left, so . Ah! kills, then, by threatening a white move at . defends, but prevents the eye.
Is this the correct answer this time?
unkx80: What if is played at ?
Brian: If is played at , then played at prevents an eye on the left.
The situation after .
In my mind, for Black to form an eye at z he needs to occupy one set of: a-b-c-d, a-b-c-e, a-b-d-e, or b-c-d-e.
Alternately, Black could attempt to form an eye at d. This would require points c-z-e.
Since, at best, Black requires three of six points, White can prevent this.
Dieter: I have a feeling this is coming pretty close. Just another minute ...