Re: [sage-devel] An issue with the .coefficient() function

[Vincent Delecroix]

Dear Eyo,

We use several mailing lists. Please ask these kind of questions on
sage-support. Now the question is forwarded and continue the discussion
there.

Best
Vincent

On 25/06/2017 16:51, baliza Eyo wrote:
> Dear Sage developers,
>
> Here is a quick question about the output of the .coefficient() for an SR
> object.
> having set up the variables as follows
>
> sage: var('x,y')
> (x, y)
>
> Shouldn't the output of the following commands be the same:
>
> sage: ((x^2 - 2)^2 - y^2).coefficient(x^4)
> 0
> sage: expand(((x^2 - 2)^2 - y^2)).coefficient(x^4)
> 1
> sage: ((x^2 - 2)^2 - y^2).diff(x,4).subs(x==0)/factorial(4)
> 1
>
> The last two outputs are correct but why is 0 the intended output for
> ((x^2 - 2)^2 - y^2).coefficient(x^4) ?
> Thanks in advance,
> Best regards,
> Eyo,
>

[Kwankyu]

On 25/06/2017 16:51, baliza Eyo wrote:
> Dear Sage developers,
>
> Here is a quick question about the output of the .coefficient() for an SR
> object.
> having set up the variables as follows
>
> sage: var('x,y')
> (x, y)
>
> Shouldn't the output of the following commands be the same:
>
> sage: ((x^2 - 2)^2 - y^2).coefficient(x^4)
> 0
> sage: expand(((x^2 - 2)^2 - y^2)).coefficient(x^4)
> 1
> sage: ((x^2 - 2)^2 - y^2).diff(x,4).subs(x==0)/factorial(4)
> 1
>
> The last two outputs are correct but why is 0 the intended output for
>   ((x^2 - 2)^2 - y^2).coefficient(x^4)   ?

I agree that this is unintuitive, but this is a well-documented behavior. Check this out.

sage: f=((x^2 - 2)^2 - y^2)

sage: f.coefficient?