       Re: Error when working with a derivative

• To: mathgroup at smc.vnet.net
• Subject: [mg104972] Re: [mg104947] Error when working with a derivative
• From: "David Park" <djmpark at comcast.net>
• Date: Sun, 15 Nov 2009 05:57:32 -0500 (EST)
• References: <8310136.1258183233535.JavaMail.root@n11>

```Look up Derivative in Help.

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

DerivMuVaF[ex_, ey_] := Derivative[1, 0][MuVaF][ex, ey]

DerivMuVaF[ex, ey]
1 + 2 ex + ey

DerivMuVaF[1, 2]
5

David Park
djmpark at comcast.net
http://home.comcast.net/~djmpark/

From: Vicent [mailto:vginer at gmail.com]

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.  :-(