Re: Delayed Derivative Operator

*To*: mathgroup at smc.vnet.net*Subject*: [mg81797] Re: Delayed Derivative Operator*From*: Andrew Moylan <andrew.j.moylan at gmail.com>*Date*: Wed, 3 Oct 2007 06:15:08 -0400 (EDT)*References*: <fdvd73$so5$1@smc.vnet.net>

On Oct 3, 4:35 pm, ToddSmith <ellipt... at gmail.com> wrote: > Hi, > I would like to be able to have an equation > > eq := D[f[x],x]==5; > > And a substitution rule > > rule = f[x]->x^3; > > And perform the substitution > > eq/.rule > > And get back the equation 3x^2==5. But Mathematica sees f'[x] and > doesn't do any substitutions. I tried a delayed assignment for eq but > it didn't work. What can I do? > > -Thanks Create a substitution rule for the function f, rather than for the whole expression f[x]: rule = f -> (#^3 &);