Gokyo Shumyo, Section 1, Problem 20b / Solution

Main line  

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.

Variation at 2  

This dies easily.

Variation (ii) at 2  

Death in bulky five. If Black continues with atari at a, White simply connects at .

Variation (iii) at 2  

After , a and b are miai.

Variation (iv) at 2  

Resistance at is useless here.

Variation (v) at 2  

The black group dies as well: a and b are miai.

Mistake at 3  

If White plays at here, Black lives in seki: either he makes seki at a, or if White plays there, Black can capture her bent four in timely fashion.

Mistake at 1  

With the clamp at , Black lives.

continuation  

If White plays at a to prevent a seki, then Black plays at b to form a bent four live group.

 

After (oshitsubushi), White is short of liberties and cannot play at to form a bulky five, so Black lives.

Variation after the mistake  

White 4 here doesn't work... after , White is caught in a connect-and-die.

Another variation  

Same fate here if White chooses to take the ko at instead.

MMM  

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.

Escape?  

No escape. --unkx80