Author: Heiner Marxen
Date: 12:16:30 10/24/99
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On October 24, 1999 at 12:55:22, James Robertson wrote:
>r4q1k/p2bR1rp/2p2Q1N/5p2/5p2/2P5/PP3PPP/R5K1 w - - bm Rf7; id WAC008;
>
>My program has always solved WAC 8 easily. But last night I noticed the latest
>version chose Nf7+ instead of Rf7. I looked at the position with Crafty 16.18,
>and it gives mate in 7 after Rf7. After Nf7+, Crafty gives mate in 10 on ply 12,
>so this value may not be trustworthy.
>
>Can anyone look at the position and find out how far Nf7+ really is from mate?
First, I can confirm that WAC008 is a mate in 7, with Re7 as only key move
(treated as a mate problem).
Then, after Nf7+ there follows either
(a) ...Qxf7 and a mate in 5 (starting Rxe7), or
(b) ...Kg8 and a mate in 8, starting with Nh6+, followed by the only legal
move Kh8. This is the initial position, again.
So, technically Nf7+ leads to a forced mate, but the shortest way to do so
reproduces the initial position and uses the mate in 7 starting with Re7.
That does not look very clever to me.
OTOH, you may have another PV, which may be longer, but also a forced mate.
You should check your PV, it might be different (and probably ok).
Heiner
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