Source:
Problem diagram.
Black plays.
Win a prize if you can find a killing sequence!
Analysis:
Myron Souris
Multiple pro solutions to this problem try to show that Black can kill,
and give the marked exchange as the starting moves. (Although one pro's
analysis doesn't even mention White's marked play, which is the best
defense.)
However, the analysis below seems to show instead that White is almost alive with a favorable 2-step ko! Can the pros be wrong?!
If you can find a convincing killing sequence for Black, please
send your analysis to the Problem of the Week Editor at
potw@usgo.org. The earliest correct
response wins a back issue of Go World magazine. Be
sure to include any non-obvious variations as appropriate.
Analysis follows (click here to download the SGF with more variations)
"Official" solution?
After Black's natural hane at 1, the "official" solutions give
only the immediate White throw-in at 2.
Black 5 is a brilliant move that kills White after White's slack play at 2.
After Black 7, White doesn't even have a direct ko.
But White seems to have a much better defense.
See the next diagrams.
White refutation?
No "official" solution gives the defense at White 2,
which seems to lead to a favorable 2-step ko for White!
Black 3 is necessary to prevent a White play there to form 2 eyes.
Black can try to cause trouble with 5, but White can simply connect at 6.
See the next diagram for the continuation.
Continuation diagram.
Black 7 captures the corner stone.
Black 9 is forced, otherwise, White lives outright.
White 10 represents a ko threat.
With Black 11 the ko becomes direct, but White has already made the extra move elsewhere at 10.
So from the starting position, White has a favorable 2-step ko.
This result is a lot better for White than being unconditionally killed!
Black variation.
Varying from the 2nd diagram above with the immediate capture
of the White corner stone at Black 5 is no better.
Through 8, White transposes to the previous diagram,
where White has a favorable 2-step ko.
Email the Problem Of The Week editor at potw@usgo.org
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