       substitution of functions in derivatives

• Subject: [mg3384] substitution of functions in derivatives
• From: pyl at ccr.jussieu.fr (lagrée)
• Date: 3 Mar 1996 11:52:17 -0600
• Approved: usenet@wri.com
• Distribution: local
• Newsgroups: wri.mathgroup
• Organization: cnrs
• Sender: daemon at wri.com

```I have a problem with mathematica and the rules:
suppose that I use a function in an expression like:

f=a u[x,y]

next I derivate it:

df=D[f,x]
now, I'd like to substitute u[x,y] and to give it the good value say Cos[x,y]:

df/.u[x_,y_]->Cos[x y]

the result that I want is of course  -(a*y*Sin[x*y])
but what I obtain is only:
a*Derivative[1, 0][u][x, y]

Of course is it possible to write directly:
f=a Cos[x,y]
and the result of df=D[f,x] is what expected, but I want to do some
mainipulations on the expression df before doing the substitution.

If somebody may be kind to give me an idea I would be very grateful.

sincerly

```

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