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Re: Replace in operators

  • To: mathgroup at smc.vnet.net
  • Subject: [mg102940] Re: [mg102869] Replace in operators
  • From: Bob Hanlon <hanlonr at cox.net>
  • Date: Wed, 2 Sep 2009 04:06:57 -0400 (EDT)
  • Reply-to: hanlonr at cox.net

Try this

expr = f[x] + f'[x] + f''[x];

expr /. NestList[D[#, x] &, f[x] -> g[x], 2]

Derivative[2][g][x] + Derivative[1][g][x] + g[x]


Bob Hanlon

---- did <didier.oslo at hotmail.com> wrote: 

=============
I can't figure out how to force Mathematica to replace
f[x] by g[x] in expressions involving operators.
For example:

f[x] + D[f[x],x] /. f[x] -> g[x]

f is not replaced in the derivative. I found
somewhere in the manual that the replacement
does not work on operators, but then it does
not indicate how to do it.

In my real problem, f[x]->g[x] is a (long)
list of complicated transformations resulting
from the resolution of many equations.
Thus, changing f[x]->g[x] by f->g is not
an option.

f[x] + D[f[x],x] is actually a complicated
expression involving many operators.

There are no ways I can do the
substitution by hand, bit by bit.

Any suggestions ?
Thanks

PS: it may help to precise the context.
I'm trying to solve a system of PDEs by
a small parameter expansion. Solving the
system at order N, I want to re-inject the
results into the equations at order N+1.




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