Author: Ricardo Gibert
Date: 17:52:55 04/18/02
Go up one level in this thread
On April 18, 2002 at 14:25:36, Dann Corbit wrote:
>On April 17, 2002 at 23:59:39, Ricardo Gibert wrote:
>
>>On April 17, 2002 at 15:43:23, Jesus de la Villa wrote:
>>
>>>
>>>Have someone defined the general rule(s) where null move
>>>is unable to find simple combinations?, and if so, which
>>>are those rules ?
>>>
>>>"Obviusly" is more expensive to check it than to not
>>>use Null Move.
>>>
>>>Thanks for asking
>>>
>>>
>>>
>>>PS. I hope you undertand my poor English :)
>>
>>Nullmove assumes that having the move is better than not. This assumption is not
>>correct for zugzwangs, so nullmove will have a problem with those positions
>>where zugzwang is relavant. It fails quite badly, so some care is needed in
>>avoiding this. Not using it in the endgame where zugzwang occurs with
>>significant frequency is one way. Another idea is "double nullmove".
>>
>>Pseudo-zugzwang, where having the move results in a delay in attaining a
>>favorable result, is fine for nullmove. In fact, it makes out like a bandit by
>>allowing a program to find a favorable result more easily than it would
>>otherwise. Unfortunately, these also only occur with significant frequency in
>>the ending where nullmove is typically turned off :-(
>
>Could you explain pseudo-zugzwang or show an algorithm snippet that demonstrates
>it? I have never heard of it before.
There is no "algorithm snippet" for it. It is a chess term. It is not a true
zugzwang. Some just call it a zugzwang even though it isn't really. Here is a
position to illustrate:
[D]3k4/8/p1PK4/P7/8/8/8/8 w - - 0 1
White is in a kind of zugzwang, since if it were Blacks move instead, he would
win immediately by
1...Kc8 2.c7 Kb7 3.Kd7 Ka7 4.Kc6 Ka8 5.c8=Q+ Kh7 6.Qb7#
Having the move delays the win by 9 ply, i.e.
1.Kd5
if 1.c7? Kc8 2.Kc6 is stalemate. If 1.Kc5 Kc7 2.Kd5 Kc8 3.Kd4 (3.Kd6?! Kd8
repeats the original position) 3...Kd8 4.Kc4 Kc8 5.Kd5 Kd8 6.Kd6 so this is 2
ply slower.
1...Kc8
If 1...Kc7?! 2.Kc5
2.Kc4
Not 2.Kc5?! Kc7 3.Kd5 Kc8 only repeats
2...Kd8 3.Kd4 Kc8 4.Kd5 Kd8 5.Kd6
After a 9 ply maneuver (triangulation), White has succeeded in obtaining the
original position , but with Black to move instead.
The position is not a true zugzwang, since the result, a win, is not altered. It
is only delayed. Another term suggested as a name for it is "squeeze", but this
never caught on (obviously).
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