The attempts on this page are based on a wrong problem statement.
Well, after White 3, I don't see how Black can kill anything. Every move inside I seem to be able to find an answer for and no refutation. Is this the solution? For example:
--Dieter: Well, I have to check the problem book again. Even simply descending to 3 immediately saves the group. It feels as if there is a superfluous stone, probably the marked one, as unkx80 indicated. My apologies if it turns out to be so.
unkx80: I make the claim because I think I have seen this before somewhere. So I modify the marked stone and present what I think is the solution.
The tesuji is White 1.
The next tesuji is White 3, sacrificing yet another stone.