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Multivariate Polynomials over Rational Function Fields

asked 2010-08-25 20:12:25 -0500

Sameer Agarwal gravatar image

updated 2011-04-28 09:48:26 -0500

Kelvin Li gravatar image

Is it possible to define multivariate polynomials where the coefficients lie in a rational function field and do Groebner basis computations on them? Maple, Reduce and Axiom support this. For example I would like to be able to compute the Groebner basis of the polynomials

{v * x^2 + y, u* x * y + y^2}

where the polynomials belong to the ring Q(u,v)[x,y].

I tried the following

B.<u,v> = PolynomialRing(QQ, 'u', 'v')
R.<x,y> = PolynomialRing(B, 'x', 'y')
I = R.ideal(v * x^2 + y, u* x * y + y^2) 
g = I.groebner_basis()

This fails with the error

TypeError: Can only reduce polynomials over fields.
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answered 2010-08-25 20:15:22 -0500

William Stein gravatar image

Make u and v be in the Fraction field:

sage: B.<u,v> = Frac(QQ['u,v'])
sage: R.<x,y> = PolynomialRing(B, 'x', 'y')
sage: I = R.ideal(v * x^2 + y, u* x * y + y^2) 
sage: g = I.groebner_basis()
sage: g
[y^3 + u^2/v*y^2, x^2 + 1/v*y, x*y + 1/u*y^2]
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Comments

Thanks, that works perfectly. Now my quest for the Elimination ideal I2 = I.elimination_ideal([x]) fails with TypeError: Cannot call Singular function 'eliminate' with ring parameter of type '' Sameer Agarwal ( 2010-08-25 21:14:24 -0500 )edit
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answered 2010-08-26 04:24:49 -0500

niles gravatar image

When I try this, the error I get is a little more informative:

sage: I2 = I.elimination_ideal([x])
...
TypeError: Cannot call Singular function 'eliminate' with ring parameter of type 
'<class 'sage.rings.polynomial.multi_polynomial_ring.MPolynomialRing_polydict_domain'>'

The function eliminte comes from

sage: eliminate = sage.libs.singular.ff.eliminate

which is a wrapper for the Singular function and is supposed to convert Sage's rings to rings that Singular understands . . . however it seems that this wrapper does not understand the _polydict_domain rings. Perhaps one could convert R to a ring that the wrapper does understand (or file this as a bug in the wrapper and fix it). Checking, I see that there is a ring type MPolynomialRing_libsingular, but it only allows base rings from a very small list (like ZZ and finite fields).

Maybe someone who knows more about the Singular interface can help here?

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Asked: 2010-08-25 20:12:25 -0500

Seen: 474 times

Last updated: Aug 26 '10