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Re: Error when working with a derivative

  • To: mathgroup at smc.vnet.net
  • Subject: [mg104970] Re: Error when working with a derivative
  • From: Norbert Marxer <marxer at mec.li>
  • Date: Sun, 15 Nov 2009 05:57:09 -0500 (EST)
  • References: <hdlk4k$hcb$1@smc.vnet.net>

On Nov 14, 7:53 am, Vicent <vgi... at gmail.com> wrote:
> Hello.
>
> This is my first message to the list.
>
> I work with Mathematica 7.0.0 on Windows.
>
> I work with a multiple-variable function, say "MuVaF", and I want to
> define a function which is the partial derivative with respect to one
> of the variables. I tried with this:
>
>     MuVaF[ex_, ey_] := ex^2 + ey^2 + ex*ey + ex + ey
>
> Then, if I try to derivate it with respect to "ex":
>
>     D[MuVaF[ex, ey], ex]
>
> I get this:
>
>     1 + 2 ex + ey
>
> Which sound OK to me. But if I try this:
>
>     DerivMuVaF[ex_, ey_] := D[MuVaF[ex, ey], ex]
>
> And then this (trying to evaluate the function for a given point):
>
>     DerivMuVaF[1, 2]
>
> I get an error message:
>
>     General::ivar: 1 is not a valid variable. >>
>
> I think that's because Mathematica is understanding I am trying to
> perform the derivative on "1"; it is expecting to get a variable and I
> am giving a number instead. So, what's the right way to tell
> Mathematica I want to work with the derivative function of a
> previously defined function??
>
> I guess the answer should be easy, but I haven't been able to find it
> out by my own.  :-(
>
> Thank you in advance for your answers!
>
> --
> Vicent Giner-Bosch

Hello

     MuVaF[ex_, ey_] := ex^2 + ey^2 + ex*ey + ex + ey

     DerivMuVaF := Derivative[1, 0][MuVaF]
     DerivMuVaF[1, 2]

or directly

     Derivative[1, 0][MuVaF][1, 2]

should work.

Best Regards
Norbert Marxer


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