Re: Delayed Derivative Operator

*To*: mathgroup at smc.vnet.net*Subject*: [mg81784] Re: Delayed Derivative Operator*From*: Norbert Marxer <marxer at mec.li>*Date*: Wed, 3 Oct 2007 06:08:23 -0400 (EDT)*References*: <fdvd73$so5$1@smc.vnet.net>

On 3 Okt., 08:35, 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 Hello Looking at the FullForm of your expression "eq" shows: Equal[Derivative[1][f][x],5] This means you cannot use your replacement rule. But the following rules both work: rule = f -> Function[{x}, x^3]; rule = f -> (#1^3 & ); That is the commands eq := D[f[x], x] == 5; rule = f -> (#1^3 & ); eq /. rule will give the following result 3 x^2==5 I hope this helps. Best Regards Norbert Marxer