       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[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

```

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