is the only move. The only correct answer for Black is at . White's correct answer is at , and note that White must capture the black stone at , resulting in ten thousand year ko.
Death in bulky five. If Black continues with atari at a, White simply connects at .
After (oshitsubushi), White is short of liberties and cannot play at to form a bulky five, so Black lives.
White 4 here doesn't work... after , White is caught in a connect-and-die.
Mistake upon mistake upon mistake
In Gokyo Shumyo, Section 1, Problem 20 was already a mistake. White's hane - as we now know - is another one and then is yet another one.
makes miai of a and b: b would enable White to almost fill Black's eye space with a bulky five and White a would make an eye inside Black's group. If Black rushes to capture White's stones, he's left with a farmer's hat space inside his group.
Can't Black also live by escaping with a monkey jump in this case?
No escape. --unkx80