Gokyo Shumyo, Section 1, Problem 37 / Solution

One continuation  

at a would also live.

Variation at  

Black can block at , due to the presence of the marked stone. He wins in territory, but leaves the aji of the cut at a.

Another variation at  

Black could try here, but it doesn't work. In this variation White also lives, and Black makes less points than in the main line.

Failure 1  

Simply descending at is wrong. Black ataris and connects ...

Failure 1 - variation  

... or connects underneath. It is this combination which the preparatory move by White at a makes impossible.

Failure 2  

Simply capturing at is insufficient to make two eyes. The jump at kills.