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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 &);