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Converting polynomials between rings

asked 2014-09-19 07:44:21 -0500

Alasdair gravatar image

I have two polynomials, each one explicitly created as members of different multivariate polynomial rings. So for example I might have

R1.<a,b,c,t> = PolynomialRing(QQ) 
L = (a*t^2+b*t+c).subs(a=2,b=3,c=4)

R2.<A,B,C,D,t> = PolynomialRing(QQ)
p = (A*t+B)^2+(C*t+D)^2

Now I want to compare the coefficients of the two polynomials, which means converting L to be a polynomial in the ring R2.

However, I'm not sure (or more to the point, can't remember) how to do this. Is there some simple, canonical way to do this? Thanks!

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answered 2014-09-19 09:39:45 -0500

In your example, having defined

sage: R1.<a,b,c,t> = PolynomialRing(QQ)
sage: L = (a*t^2+b*t+c).subs(a=2,b=3,c=4)

we get

sage: L
2*t^2 + 3*t + 4

(did you mean L = a*t^2+b*t+c without substituting values for a, b, c?)

Having then defined

sage: R2.<A,B,C,D,t> = PolynomialRing(QQ)
sage: p = (A*t+B)^2+(C*t+D)^2

you can convert polynomials from one ring to the other

sage: R2(L)
2*D^2 + 3*D + 4

Here, the order of the variables matters, more than their names: t, the fourth variable in R1, is mapped to D, the fourth variable in R2.

If you want a, b,c, t to be mapped to A, B, C, t, you might want to include an extra variable d in R1 and not use it.

Or you could compare string representations of your polynomials, applying string replacements as necessary. Or you could use p.monomials() and p.coefficients().

You could also define a ring homomorphism from R1 to R2 mapping the variables to the variables of your choice, see the reference manual.

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Thank you very much. It seems as long as I have the same number of variables in each ring, then when I move from one to the other, the variables will be mapped in the order I've defined them. This makes perfect sense, not that I see how it works!

Alasdair gravatar imageAlasdair ( 2014-09-21 04:11:12 -0500 )edit

The variables for a polynomial ring `R` are just `R.0`, `R.1`, etc, (and `a`, `b`, `t`, `x`, `y`, or other, are just display names), so it's easy to map them to the variables `S.0`, `S.1` of another polynomial ring `S`.

slelievre gravatar imageslelievre ( 2014-09-21 04:24:23 -0500 )edit

answered 2014-09-19 09:25:39 -0500

Luca gravatar image

Because of the order you've defined the objects, L(t=t) will be an element of R2. Slightly more robustly, in your example you can do any of the following.

sage: L(t=R2.gen(4)).parent()
Multivariate Polynomial Ring in A, B, C, D, t over Rational Field
sage: L(t=R2.4).parent()
Multivariate Polynomial Ring in A, B, C, D, t over Rational Field
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Thank you - this should work, and I'll check it later.

Alasdair gravatar imageAlasdair ( 2014-09-21 04:11:35 -0500 )edit

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Asked: 2014-09-19 07:44:21 -0500

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Last updated: Sep 19 '14