Under the Stones Problem 4 / Solution

Ko  

This is a ko. Assuming is a ko threat and black answers with then black cannot kill by playing under the stones as shown in the next two diagrams.

Continuation 2  

And white is alive because of his second eye at a.

White's resistance is not enough  

is not the best move. under .

Ko threat  

How about this ko threat at ? ~ Gregory

Variations of reply #1  

If Black replys to ko threat at then White takes ko and Black has to find the big ko threat to force White's response and then retake ko.

Variations of reply #2  

If Black ignores the ko threat and takes with , then .

If @ b then @ c, and if attacks on top at e or f, then at a captures the four Black stones with the double cut; and if connects on the bottom then at f gets the second eye.

If on top at d, e, or f, then still at c, Black connects on the bottom at D-1 or F-1 to protect against the double cut, and at b makes White's second eye. (In the case of at e white may fill to protect and threaten the cut at f making f and b miai for the second eye.)