# [GiNaC-list] Given a formal iterated derivative (GiNaC::fderivative), what actually is it?

Vladimir V. Kisil kisilv at maths.leeds.ac.uk
Sun Apr 10 12:39:14 CEST 2016

```>>>>> On Sun, 10 Apr 2016 11:33:59 +0200, "Richard B. Kreckel" <kreckel at in.terlu.de> said:

RK> On 04/10/2016 12:23 AM, Vladimir V. Kisil wrote:
>>>>>>> On Sat, 9 Apr 2016 23:59:36 +0200, "Richard B. Kreckel"
>>>>>>> <kreckel at in.terlu.de> said:
>> >> f(x,y).diff(x, 3).diff(y, 2) >> which is printed as >>
>> D[0,0,0,1,1](f)(x,y) >> This is fantastic.
>>
RK> Hmmm...  Wouldn't it be more fantastic if it were printed
RK> D[x,x,x,y,y](f)(x,y)?
>>
>> But how to print f(x+y,x-y).diff(x, 3).diff(y, 2) in this case?

RK> Well, I would have supposed like this:
RK> D[x+y,x+y,x+y,x+y,x+y](f)(x+y,x-y)+D[x+y,x+y,x+y,x+y,x-y](f)(x+y,x-y)-2*D[x+y,x+y,x+y,x-y,x-y](f)(x+y,x-y)-2*D[x+y,x+y,x-y,x-y,x-y](f)(x+y,x-y)+D[x+y,x-y,x-y,x-y,x-y](f)(x+y,x-y)+D[x-y,x-y,x-y,x-y,x-y](f)(x+y,x-y)

Yet, from a purist point of view, I have a concern that for
f(x,x).diff(x, 3) the print-out will be ambiguous...
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