Re: Delayed Derivative Operator

*To*: mathgroup at smc.vnet.net*Subject*: [mg81794] Re: [mg81762] Delayed Derivative Operator*From*: Andrzej Kozlowski <akoz at mimuw.edu.pl>*Date*: Wed, 3 Oct 2007 06:13:35 -0400 (EDT)*References*: <200710030626.CAA28793@smc.vnet.net>

On 3 Oct 2007, at 15:26, ToddSmith 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 > > One way: eq = D[f[x], x] == 5; eq /. f -> (#1^2 & ) 2*x == 5 Andrzej Kozlowski

**References**:**Delayed Derivative Operator***From:*ToddSmith <elliptic1@gmail.com>