Author: Robert Hyatt
Date: 04:58:36 09/22/00
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On September 22, 2000 at 00:14:34, Mike Adams wrote: > Here is the position Pulsar is white in this fen string and to move. >1k4r1/ppp4R/3p4/4p3/q1n1P3/1NP5/P1P2rPP/1K2Q1R1 w - - > It is a forced mate in 6 but to make it even easier for chess engines to >see this position requires a queen sacrafice on move 3 to delay it to move 6 >otherwise its mate in 4. Pulsar should need depth 5 to see this. It doesnt >process that it is in check in qsearch so after 4 moves its checkmated but it >can pass on moving in qsearch and be blind to the mate. My check extensions dont >help eathier because they look at if position is in check before move and it is >not in check or actually checkmated tell its move 5 for pulsar. Of course the >game is over then. Pulsar reported depth 6 in this position so it should have >seen it but instead it grabbed the free rook and reported a winning position >even though it had fallen into a forced mate in 6. So i did some tests. > >With Null move at R2 (depth-3) Pulsar plays Qxr untill depth 7 then it plays Kb1 >to Ka1. >With Null move at R1 (depth-2) Pulsar plays Qxr untill depth 6 then it also >plays Kb1 to Ka1. >With Null move off Pulsar plays Qxr untill depth 5 as expected then on depth 5 >it plays Knight B3 to d4. > > Whats the deal? Why is Pulsar missing a mate in 6 when it should see it in >depth 5 and it gets depth 6. Am i doing something wrong with Null Move? Is >there some trick to prevent this. I would be interested in a explanation of how >this can happen and if it happens often. > thanks Mike Adams A queen sac is the worst thing that can happen. The opponent is now way ahead in material. A null-move search will always fail high because it will shorten the depth to the point that the mate is not seen, and then say "hey, I am a queen ahead, this is great." Make sure you are not trying null-moves while in check. And be prepared to need a couple of extra plies to see a mate in some cases.
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