hoshi911: I disagree with the solution
using w-S2!
After white S2, black answers Q1
and white should be dead!
(w-S1 b-r2 or b-S1 w-r2 give
no escape for white)
Ben Chaney: I don't see any way for black to make a double ko. White appears to be just alive.
caranthir: I think I see what you mean. In my opinion it still is technically a kind of double ko, although once in every cycle, the defender has a local ko "threat" to end the double ko by capturing at C2. I write "threat" in quotation marks, because a thing living in a double ko is quite alive anyway.
flemenga: People are saying it is Alive in double ko... the only way I can see this is if you mean it is alive in *sente* with double ko. Otherwise, it seems to be totally alive in gote to me. I’m only 13kyu so still not sure if that is the way people normally refer to the life or death of a group.
caranthir: Totally accurate would be to say that the life and death is unsettled, and the defender can live (in double ko). Being alive in sente or gote, for that matter, would refer to the situation at the end of the move or sequence that has been played to settle the life and death. If the defender lived "in sente", the opponent would even be forced to defend themselves locally in some way after the sequence to live - unless globally somewhere else on the board there was even more value.
No, doesn't work. S2-Q1-S1-R2-T5