Tesuji - Capturing Races

[jauscpa17]

The following is end-of-chapter 5, problem 4 from Tesuji (James Davies), regarding capturing races. It’s black to play and the given answer is black at “b” which makes sense, but I initially thought that “a” would be the answer and despite having spent much time going through many, many variations, I can’t find a way which white can unconditionally win if black plays at “a” So, it seems like “a” would also be a correct answer, but I’m sure the author would have mentioned that if that were the case. I would love it if someone can either confirm that “a” is also correct or show a sequence which white can use to successfully counter that.

Problem 4.sgf (1.4 KB)

[Myrsilos]

If black b, then black captures all 5 white stones, right? Black a doesn’t remove any of white’s liberties, so white can play at N3 and connect by capturing the one stone

[Sadaharu]

If black plays A, white can try to eat the corner black stones through ko. Try finding the move.

[jauscpa17]

If this is what you mean (white at 1), it seems black can still kill with this sequence.

[jauscpa17]

I assume this is what you mean by white making a ko at 3 and then playing next move (white 5) at D. Not a favorable ko for white, but black would obviously be better by unconditionally killing by playing the correct first move one space to left of C.