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