The key move for White is to descend with . White lives unconditionally with 9 points, capturing Black stones.
If blocks here, then
wins the race. Note that Black cannot approach at a.
If at
, then
at
captures the Black stones just as well.
The hane at might be an instinctive move, but in this problem it is a mistake. Now
can make a throw-in, and Black gets an opportunity to play
White cannot approach at either a or b. The result is seki. (
is not immediately necessary.
is an endgame play worth 8 points in gote.)
Starting a ko with is a risky preposition - it puts the life of the entire White group at stake. In life-and-death problems, by convention ko for life is always treated as inferior to unconditional life - and rightly so.
Black can kill unconditionally with the descend, .
Unlike most other problems, the proverb the opponent's vital point is my vital point does not apply in this problem.
Although White at is the key for making unconditional life, Black playing at
here only results in a seki after
and
. (Black's attempts at under the stones play all fail. see /BlackAttempts)