On Friday, June 12, 2015 at 11:17:37 AM UTC+2, Néstor wrote:

> Hello,
>
> I've got a rational expression in sage and I would like to convert it to a 
> polynomial with coefficients in some fraction field. 
>
> More precisely, I've got something like this:
>
> a , x = var( 'a , x' ) ;
> P = x/a ;
>
> and I would like to see it like a polynomial in the polynomial ring in the 
> variable x with coefficients on the fraction field Q(a) of rational 
> expressions in the variable a:
>
> C.<a> = PolynomialRing(QQ) ; 
> B = FractionField(C) ;
> A.<x> = PolynomialRing(B) ;
>
> But when I try
>
> P.polynomial(A)
>
> I get the following error
>
> TypeError: unable to coerce since the denominator is not 1
>
>
try

    sage: A(P)
    1/a*x
    sage: _.parent()
    Univariate Polynomial Ring in x over Fraction Field of Univariate 
Polynomial Ring in a over Rational Field

Sometimes, when Sage is not smart enough to perform a conversion between 
complicated rings, I where able to perform the conversion with the ugly 
hack:

    sage: A(str(P))

 

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