I have not seen this problem before, so here's my take.
Continued at [1].
Hmm... I failed to notice this one. I will try again some other time.
DaveSigaty: Based on the above...
Does waiting until this point to play make miai of the left and the right? White at
(the key point in the previous try) no longer seems to work.
Oops! recaptures and ends the threat on the right while keeping
in shortage of liberties.
So White has to play here and Black makes ko with
. Note that a can be used as a local ko threat.
dnerra: It seems to me your last diagram does not work, Dave:
One eye for White.
Q: Why not start with the key point here?
A: Because Black plays .
[1] unkx80: From the third diagram by Dieter and Dave's suggestion, how about...
Dave: This looks about right for the play at . As dnerra shows above, this move by itself is not enough to live. The timing must be right, when Black is fully committed on the left. Let's see what the official solution is. Does White have something better? Even if she does, this has been an interesting analysis.
Dieter: This is the main line as given by Hashimoto Utaro. I'm a bit puzzled why all of a sudden the solution is a ko. There is definitely a ko section later in the book. Well it is a good thing, for now we can't even be sure any more of the kind of solution we're looking for in the next problems.
dnerra: I was sure I'd found the solution before I found the above diagram:
at
,
at
.
But it's indisputable that the ko in unkx80's solution is better. What an intriguing problem!
unkx80: I am not sure this is yet another line. What if plays at
instead?