impu1se: I think this is right. White must play to keep black from making an eye. Black will now live due to the double ko. However, white has a nice source of ko threats.
IlyaM: Yep, this is correct solution. Playing before
also works. Fortunaly in my game where this position appeared it appeared very late in the game so there was no ko-fights.
EricBackus: I'm missing something. At any given move, black will hold either the 'a' ko or the 'b' ko, and white will hold the other.
If white is allowed to play 'c' and 'd', then this reduces to Double Ko Seki (right?) which we're told is a failure. To prevent this, black must take 'd'. But to do that requires winning a ko fight. It seems to me that black gets to choose between double-ko seki and a single-ko fight, and we're told that seki isn't good enough so this must be a single-ko fight. Is this really the best solution available to black?
if white play at , black
live.
EricBackus: Wait, if white plays and black plays
, then if white plays at 'c', isn't that seki, and therefore a failure for black?
can also play at c, but no difference made.
EricBackus: I must be missing something obvious in the above two diagrams. Can't white play 'c', putting black in atari, and this reduces to a ko for life in the corner? So it's not life, it's ko, and thus a failure if unconditional life was possible?