TwT: @Elim: It is no seki. Attacker playing elsewhere. There is still small live in gote.
caranthir: I can't see @Elim's comment any more. Anyway, to clarify to @JPortman: in a real game, "finalizing the seki" will be too small for a long time in many cases, not the least because after P19 it still takes two moves to kill the entire thing. If you put this position into Katago with an otherwise empty board, then opponent playing O19 is ~9 points loss (provided that we don't answer it locally but instead play something elsewhere that makes sense), and continuing to kill the corner with Q19 is another ~9 points loss for them. It's very, very small value for your moves. If the outside is strong like it is in this position, it's just endgame.
JPortman: The final move at D19 should be added to the sequence to finalize the seki. So the full sequence will be C19 - E19 - D19 . Because in the real game the attacking player will play at F19, and it will be a sente move forcing the defender to connect at D19 ( or else the group will die).
rosagold: Currently only the first move C19 is the marked solution. I would like to make C19-E19-D19 the correct sequence. Because C19-E19-A18 actually is dead. i guess the whole problem aims more for the A18 vs D19 part, instead of finding the 1st move.
Farkas: I disagree with your comment.
A18 is obviously not good, as it is self-atari, but D19 isn't good, either, as it is unnecessary.
I can't see @Elim's comment any more. Anyway, to clarify to @JPortman: in a real game, "finalizing the seki" will be too small for a long time in many cases, not the least because after P19 it still takes two moves to kill the entire thing. If you put this position into Katago with an otherwise empty board, then opponent playing O19 is ~9 points loss (provided that we don't answer it locally but instead play something elsewhere that makes sense), and continuing to kill the corner with Q19 is another ~9 points loss for them. It's very, very small value for your moves. If the outside is strong like it is in this position, it's just endgame.