Sadaharu: Also I just realized but what if white answer C16 with B17? Still goes back to the same variation it seems
posetcay: B17 is a brilliant tesuji, I completely missed that. It looks like it is impossible for black to avoid ko then if white really wants to make a ko. I changed the solution tree completely.
Sadaharu: Honestly I think the Ko is better than sacrificing the group just to live in the corner. I’m not sure whether that was the intended answer.
posetcay: Yes, I also think ko result is fine since it is a heavy ko for white, but there are so many different ways to make ko here yet only one way to live without ko. Thats why I thought it was the most interesting sequence and the strongest resistance by the opponent. I put (no ko) on the problem to clarify that the challenge is to avoid ko, because sometimes your opponent has a lot more threats than you and even though the ko is locally favorable you might still end up losing points.
Sadaharu: B16 as the first move should work as well? Should be better than the actual answer.
After the variation up to white eating the stone at B19, black can extend at A18 followed by D19 to make another eye.
My bad. B17 doesn’t work since white can just cut.at C18.