A common pitfall is to play the descent at , assuming that capturing at a and the connection at
are miai.
This problem is deliberately set up in such a way that capturing the four Black stones directly does not live - it is a farmer's hat big eye.
However, one should notice that if the stone (which was
) is at a instead, then White at b can make the second eye and live. This observation leads to the improved sequence below.
An improved sequence is the diagonal at , threatening a connection at
as well as a ko at
.
If does not play the ko and destroys the eye at the side instead, then
is sente, so White has time to live at the vital point of
. However, the improved sequence is not yet the solution, since Black takes the ko first.
White makes a throw-in at followed by an atari at
. Observe that
cannot connect at
.
If insists on killing the White group, then
takes the ko first. From the usual life-and-death perspective, this is the best play by both players.
If Black avoids this ko, White can live, as the variations below show.
Black may try this way after takes the ko.
Must White fight this ko to live?
at
.
White can play this way.
goes all out to kill.
takes four stones, but cannot make an eye there because of under the stones play.
But White takes sente and lives.
Backing up, Black can do no better than , allowing
to live.
Since this ko is so large, it is reasonable to assume that neither player has a ko threat, and also that the temperature of the board is not too high, so that there will be no ko exchange. Then the best way for Black to play is to simply capture at , allowing
to connect to the group outside.