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.