Re: [eclipse-clp-users] Large integers and delayed goals

From: Joachim Schimpf <jschimpf_at_...311...>
Date: Sat, 17 Mar 2012 00:29:23 +0100
Sergey Dymchenko wrote:
> Hi,
> 
> I want to find positive natural numbers A, B, C, D, such that A^4 +
> B^4 + C^4 = D^4.
> My program do this:
> 
>     A #> 0, B #> 0, C #> 0, D #> 0,
>     A #=< B, B #=< C, C #=< D,
>     A*A*A*A + B*B*B*B + C*C*C*C #= D*D*D*D,
>     labeling([A, B, C, D])
> 
> But I get incorrect result [1, 1, 9742, 9742] with 10 delayed goals.
> As far as I understand, the system finds that the result is imprecise,
> but correct enough because 9742^4 is a large number.
> Probably floating arithmetic is used, not big integers...
> Is there a way to force precise arithmetic for large integers?

Hi Sergey,

you are right, library(ic) uses floating point arithmetic, which
gives you 53 bits of precision.  Now, 9742^4 is just above 2^53,
that's why you get the first pseudo-result for this magnitude.

A simple, brute force way to avoid the pseudo-results is to add
a test after search:

     A #> 0, B #> 0, C #> 0, D #> 0, A #=< B, B #=< C, C #=< D,
     A^4 + B^4 + C^4 #= D^4,
     labeling([A, B, C, D]),
     A^4 + B^4 + C^4 =:= D^4.   %%% HERE %%%

A somewhat more elegant way is the following:

     A #> 0, B #> 0, C #> 0, D #> 0, A #=< B, B #=< C, C #=< D,
     [ic,suspend]:(A^4 + B^4 + C^4 #= D^4),  %%% HERE %%%
     labeling([A, B, C, D]).

By annotating the critical constraint with [ic,suspend], we set up
two versions of the constraint: the 'ic' version (as before), plus
the 'suspend' version (which delays until ground and then tests
the equality using integer arithmetic).  If you have multiple
constraints, you only need to do this for those that are at
risk of imprecision due to the large numbers.


While this solves the precision problem, I'm afraid I can't help
with actually finding solutions - are there any?
There is no magic in the propagation that finds [A,B,C] triplets
whose 4th powers sum up to a 4th power, so it's mostly search.
With your labeling order of A,B,C,D, you get stuck at

?- A #> 0, B #> 0, C #> 0, D #> 0, A #=< B, B #=< C, C #=< D,
    [ic, suspend] : (A ^ 4 + B ^ 4 + C ^ 4 #= D ^ 4),
    A = 1,
    B = 1.

A = 1
B = 1
C = C{85 .. 1.0Inf}
D = D{85 .. 1.0Inf}
There are 9 delayed goals.
Yes (0.00s cpu)

and then you keep trying values for C from 85 up to infinity.
Unless there really is a solution with A=B=1, you will look for
larger Cs forever, never going beyond the initial values for A and B.

It may be better to label D first, beccause this immediately leads
to bounds on the other variables:

?- A #> 0, B #> 0, C #> 0, D #> 0, A #=< B, B #=< C, C #=< D,
    [ic, suspend] : (A ^ 4 + B ^ 4 + C ^ 4 #= D ^ 4),
    D = 6.

A = A{3 .. 5}
B = B{3 .. 5}
C = C{3 .. 5}
D = 6
There are 11 delayed goals.
Yes (0.00s cpu)

Now the remaining search space for [A,B,C] is bounded and can
be exhausted in finite time.  So you can do:

?- A #> 0, B #> 0, C #> 0, D #> 0, A #=< B, B #=< C, C #=< D,
    [ic, suspend] : (A ^ 4 + B ^ 4 + C ^ 4 #= D ^ 4),
    labeling([D]),
    writeln(d = D),
    labeling([A, B, C]).

d = 6
d = 7
d = 8
d = 9
d = 10
...

and at least have the satisfaction of seeing something happening ;)

[By the way, I have used X^4 instead of X*X*X*X in all the
constraints, because that generally gives better propagation]

Cheers,
Joachim
Received on Fri Mar 16 2012 - 23:29:31 CET

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