You can try to compute over ZZ with p added as a generator, because

   J in Z^n/<p> ~= J + <p> in Z^n

sage: p = next_prime(2^100)
sage: R.<x,y,z> = ZZ[]
sage: I = Ideal(R.random_element() for _ in range(R.ngens()))    
sage: I += [p]

sage: I.groebner_basis()
[x^2 + 380295180068468820449010961696*x*z + 253530120045645880299340641111*z^2 
+ 1014120480182583521197362564522*z, y^2 + x*z + 6*y*z + 2*y, 
1267650600228229401496703205653]

It won't be efficient but better than the toy implementation I guess.

On Thursday 13 Dec 2012, Santanu Sarkar wrote:
> Thanks for your information. No, I need large
> prime field.
> 
> On 13 December 2012 12:19, Simon King <[email protected]> wrote:
> > Hi!
> > 
> > On 2012-12-13, Santanu Sarkar <[email protected]> wrote:
> > > When I want to calculate
> > > Groebner basis, I have following error.
> > > 
> > > 
> > > verbose 0 (3292: multi_polynomial_ideal.py, groebner_basis) Warning:
> > > falling back to very slow toy implementation.
> > 
> > This is not an error but a warning.
> > 
> > > P1=next_prime(2^100)
> > > R.<x,y,z>=GF(P1)[]
> > 
> > The warning tells you that your finite field is so big that it can not
> > be dealt with in (lib)Singular. Hence, if you want to compute a Gröbner
> > basis over such a big finite prime field, Sage can not use the fast
> > implementation in (lib)Singular; instead, Sage will try to use a generic
> > implementation of Buchberger's algorithm (that's what is called "toy
> > implementation" in the warning message). If you are lucky, the
> > computation will succeed soon, if you aren't lucky, then there is not
> > much one can do about it.
> > 
> > Are you sure that you need this particularly large finite field? Would a
> > smaller finite field do? Would QQ do?
> > 
> > Best regards,
> > Simon
> > 
> > 
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Cheers,
Martin

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