Re: How to deal with "precision too low in forqfvec" ?

[hermann@stamm-wilbrandt.de]

On 2023-12-23 22:48, Bill Allombert wrote:
> On Sat, Dec 23, 2023 at 10:27:02PM +0100, hermann@stamm-wilbrandt.de 
> wrote:
> > Below script results in this error:
> >
> > $ gp -q < w3.gp
> >   ***   at top-level: forqfvec(v,M,X,if(qfeval(M,v)==X,V=v;break(),p
> >   ***                                          ^---------------------
> >   ***   precision too low in forqfvec.
> >
> > How can that fix the loop variable?
>
> You cannot... The large precision variant of forqfvec has not been 
> implemented
> yet. (This is the variant corresponding to qfminim(,,,2)).
>
> We have plan to implement it, though.
>
> Cheers,
> Bill
Thanks,

I used this function as workaround based on you "qfminim(,,,2)" remark.
I get farther than before, but with parisizemax=24G run out of memory 
for bigger numbers:

vfirst(M,x)={
  v=qfminim(M,x,1,2)[3];
  if(qfeval(M,v[,1])==x,return(v[,1]));
  for(e=1,oo,
    [n,m,v]=qfminim(M,x,2^e,2);
    for(i=n/4+1,n/2,
      if(qfeval(M,v[,i])==x,print("i=",i);return(v[,i]))));
}


From qfminim help:

? ?qfminim
qfminim(x,{B},{m},{flag=0}): x being a square and symmetric matrix 
representing
a positive definite quadratic form, this function deals with the vectors 
of x
whose norm is less than or equal to B, enumerated using the Fincke-Pohst
algorithm, storing at most m vectors (no limit if m is omitted).
... 1: ignores m and returns the first vector whose norm is less than B. 
...

?


vfirst() returns the first vector with norm equal to x.
There is no qfminim() equivalent option as flag=1 returns a vector less 
than B.

What I need is large precision variant returning first vector equals B, 
not less.

This is for an application where I don't know any vector v with 
qfeval(M,v)==B unlike Dirichlet construction (but know that such a 
vector exists by construction).

Regards,

Hermann.