Re: Error when working with a derivative

*To*: mathgroup at smc.vnet.net*Subject*: [mg104971] Re: [mg104947] Error when working with a derivative*From*: Richard Hofler <rhofler at bus.ucf.edu>*Date*: Sun, 15 Nov 2009 05:57:20 -0500 (EST)*References*: <200911140657.BAA18027@smc.vnet.net>

Hi Vicent, You'll likely receive many answers to your question. Here are three methods. In[1]:= MuVaF[ex_,ey_]:=ex^2+ey^2+ex*ey+ex+ey; In[9]:= D[MuVaF[ex,ey], ex] (* so you can the symbolic expression *) % /. {ex->1,ey->2} Out[9]= 1 + 2 ex + ey Out[10]= 5 In[11]:= deriv1 = D[MuVaF[ex,ey], ex] (* so you can the symbolic expression *) deriv1 /. {ex->1,ey->2} Out[11]= 1 + 2 ex + ey Out[12]= 5 In[7]:= DerivMuVaF[ex_,ey_]:=Evaluate[D[MuVaF[ex,ey],ex]] In[8]:= DerivMuVaF[1,2] Out[8]= 5 Does this give you what you want? Richard -----Original Message----- From: Vicent [mailto:vginer at gmail.com] Sent: Saturday, November 14, 2009 1:57 AM To: mathgroup at smc.vnet.net Subject: [mg104971] [mg104947] Error when working with a derivative 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

**References**:**Error when working with a derivative***From:*Vicent <vginer@gmail.com>