MathGroup Archive 1996

[Date Index] [Thread Index] [Author Index]

Search the Archive

substitution of functions in derivatives

  • To: mathgroup at smc.vnet.net
  • Subject: [mg3384] substitution of functions in derivatives
  • From: pyl at ccr.jussieu.fr (lagrée)
  • Date: Sun, 3 Mar 1996 02:23:30 -0500
  • Organization: cnrs
  • Sender: owner-wri-mathgroup at wolfram.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

==== [MESSAGE SEPARATOR] ====


  • Prev by Date: output_to_input
  • Next by Date: substitution of functions in derivatives
  • Previous by thread: Re: output_to_input
  • Next by thread: substitution of functions in derivatives