Re: Error when working with a derivative

• To: mathgroup at smc.vnet.net
• Subject: [mg104956] Re: [mg104947] Error when working with a derivative
• From: Tomas Garza <tgarza10 at msn.com>
• Date: Sun, 15 Nov 2009 05:54:18 -0500 (EST)
• References: <200911140657.BAA18027@smc.vnet.net>

```Try this definition instead:

DerivMuVaF[ex_, ey_] := D[MuVaF[x, y], x] /. {x -> ex, y -> ey}

Then you'll get, for example,

In[13]:= DerivMuVaF[1, 2]

Out[13]= 5

Tomas

> Date: Sat, 14 Nov 2009 01:57:05 -0500
> From: vginer at gmail.com
> Subject: [mg104947] Error when working with a derivative
> To: mathgroup at smc.vnet.net
>
> 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.  :-(
>