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Re: Error when working with a derivative

  • To: mathgroup at
  • Subject: [mg104959] Re: Error when working with a derivative
  • From: Helen Read <hpr at>
  • Date: Sun, 15 Nov 2009 05:54:55 -0500 (EST)
  • References: <hdlk4k$hcb$>
  • Reply-to: HPR <read at>

Vicent 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??

Use set instead of set-delayed when you define your derivative function. 
That is, use an equal sign without the colon.

derivMuVaF[ex_, ey_] = D[MuVaF[ex, ey], ex]

By the way, it's a good habit to begin your function names with a lower 
case letter, in order to avoid inadvertent conflicts with built-in names 
and symbols.

Helen Read
University of Vermont

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