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. :-( Thank you in advance for your answers! -- Vicent Giner-Bosch