Author: Paul
Date: 11:06:15 02/04/01
Go up one level in this thread
On February 04, 2001 at 11:48:44, Hermano Ecuadoriano wrote: >Since I'm not an expert, my verbiage might not be right. That's a nice word: 'verbiage' ... gotta remember that! >In the old days of full width programs and alpha-beta only we had provably >shortest mates. (This interesting question was proven.) These must be very old days ... coz most old programs I know gave an upper bound that could be exact, but you never knew that (unless you were a strong chessplayer). >Now, with forward pruning, these can't be provably shortest (unless the >program goes back and proves it, full-width A-B). >Furthermore, they can't be upper bounds: in the above position different >programs announced mates of different depths, using the tablebases. They >can't all be upper bounds. (At last I get to use my education.) This position is not an example of the upper bounds rule, coz it's a 'the *opponent* wins' position ... not a 'mate in X' one for the side to move! That's what I was talking about. In this position the side to move is losing, and tries to avoid that by maximizing the score for itself, as is shown by the 'evidence' below: >Some evidence: > >snip Crafty .log >White(1): setboard 8/6kn/3B3p/5K1B/8/8/8/8 b -- >(snip text board display) >(Next is my first attempt to make a diagram in this forum!) >[D]8/6kn/3B3p/5K1B/8/8/8/8 b - - > >Black(1): go >end-game phase > time surplus 0.00 time limit 1000:00 (1000:00) > nss depth time score variation (1) > 9 0.70 Mat68 Kg8 Kg6 Ng5 Be2 Ne6 Kxh6 <HT> > 10 1.34 0.01 Ng5 Be5+ Kh7 Bf4 Nh3 Bxh6 <HT> > 10-> 1.35 0.01 Ng5 Be5+ Kh7 Bf4 Nh3 Bxh6 <HT> > 11 2.67 0.01 Ng5 Be5+ Kh7 Bf4 Nh3 Bxh6 <HT> > 11-> 2.69 0.01 Ng5 Be5+ Kh7 Bf4 Nh3 Bxh6 <HT> > 12 5.61 0.01 Ng5 Be5+ Kh7 Bf4 Nh3 Bxh6 <HT> > 12-> 5.63 0.01 Ng5 Be5+ Kh7 Bf4 Nh3 Bxh6 <HT> > 13 13.85 0.01 Ng5 Be5+ Kh7 Bf4 Nh3 Bxh6 <HT> > 13-> 13.87 0.01 Ng5 Be5+ Kh7 Bf4 Nh3 Bxh6 <HT> > 14 40.59 0.01 Ng5 Be5+ Kh7 Bf4 Nh3 Bxh6 <HT> > 14-> 40.62 0.01 Ng5 Be5+ Kh7 Bf4 Nh3 Bxh6 <HT> > 15 51.75 -- Ng5 > 15 1:59 Mat72 Ng5 Be5+ Kf8 Kg6 Ke7 Bg7 Ke6 Bxh6 <HT> > 15-> 2:01 Mat72 Ng5 Be5+ Kf8 Kg6 Ke7 Bg7 Ke6 Bxh6 <HT> > >1. That Mat68 was not an upper bound! >2. It was not proof that there was ANY mate: if it can change it's mind at > ply 10, it could change it at ply 16. Here it's finding a mate in 68 at ply 9 and later sees that it can improve on that by avoiding the mate, and later it again sees a mate etc... not strange at all. I also see that you've left a line out of this 'evidence' ... there's no '9->'? >The tablebases are being reached by different paths, and are being probed >with different positions. > >I think this is an important misunderstanding (which should be explained to >the user). In the past, and in popular usage, a mate announcement was an >exact solution. A 'mate announcement' from the winning side still is an upper bound which can be exact, you never know, but one from the losing side doesn't have to be right. At least, that's my take on this ... Groetjes, Paul
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