On Thursday, May 12, 2016 at 4:56:54 AM UTC-7, Ralf Stephan wrote:
>
> On Thu, May 12, 2016 at 10:48 AM mmarco <mma...@unizar.es <javascript:>> 
> wrote:
>
>> But even if we implement expression.laurent_polynomial(), it wouldn't 
>> automatically allow conversion from SR to LaurentPolynomialRing, would it?
>>
>
> Right, it's conversion on demand.
>
 
The thing that should probably be and probably is in place is:

var('x')
R=LaurentPolynomialRing(QQ,'x')
R(1+1/x) 

That way the user knows exactly what to expect.

The interface that <symbolic expression>.laurent_polynomial(QQ) would 
implement would be more of the type "convert into a laurent polynomial in 
some suitable ring". That's very prone to unpredictable results, e.g.:

var('x,y')

(1/x+1/y).laurent_polynomial()
(1/x+1/y-1/y).laurent_polynomial()
( (x+y)/(x*y) -1/y ).laurent_polynomial()

so I would be hesitant to offer the routine in the first place. 

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