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

Richard B. Kreckel kreckel at in.terlu.de
Sun Apr 10 13:03:47 CEST 2016

```On 04/10/2016 12:39 PM, Vladimir V. Kisil wrote:
>>>>>> 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...

That's correct. Thanks for pointing it out!

-richy.
--
Richard B. Kreckel
<http://in.terlu.de/~kreckel/>
```