Error when working with a derivative

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

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

**Follow-Ups**:**Re: Error when working with a derivative***From:*Murray Eisenberg <murray@math.umass.edu>

**Re: Error when working with a derivative***From:*Leonid Shifrin <lshifr@gmail.com>

**Re: Error when working with a derivative***From:*Richard Hofler <rhofler@bus.ucf.edu>

**Re: Error when working with a derivative***From:*Tomas Garza <tgarza10@msn.com>