On Tuesday, May 10, 2016 at 10:55:20 PM UTC-7, Ralf Stephan wrote:
>
> On Tuesday, May 10, 2016 at 10:48:10 PM UTC+2, mmarco wrote:
>>
>> Thanks for the answer. So you propose that Expression.polynomial() should
>> return either a polynomial or a laurent polynomial depending on the
>> expression?
>>
>
> Depending on expression and ring argument:
>
I think people missed the double definition of `x`, particularly because
the reassignment of `x` gets hidden by doing in the RHS of an assignment.
To be clearer:
>
> sage: R = PolynomialRing(QQ,'x') #to avoid confusion about what x is
> sage: S = LaurentPolynomialRing(QQ, 'y')
> sage: var('x,y,z')
> sage: parent((z).polynomial(QQ))
> Univariate Polynomial Ring in z over Rational Field
> sage: parent((1/z+z).polynomial(QQ))
> Univariate Laurent Polynomial Ring in z over Rational Field
>
I would say this is an error, because 1/z+z is not a polynomial over QQ in
most normal senses of the word. Where would you stop? what would
(1/(z+1) + z).polynomial(QQ)
do? Will that return an answer in QQ[z,U]/((z+1)*U-1) ?
I know that in symbolics you have to read off return types from properties
of the input, not just from its type, but I think this going a little too
far. "SR.polynomial(...)" should return polynomials in variables that are
algebraically independent over the base ring. So no laurent polynomials.
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