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Re: Delayed Derivative Operator

• To: mathgroup at smc.vnet.net
• Subject: [mg81792] Re: Delayed Derivative Operator
• From: Peter Breitfeld <phbrf at t-online.de>
• Date: Wed, 3 Oct 2007 06:12:32 -0400 (EDT)
• References: <fdvd73$so5$1@smc.vnet.net>

ToddSmith schrieb:
> 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?

To force Mathematica to handle Derivatives properly you should use
pure functions. So your rule should be:

rule=f->(#^3&)

Gruss Peter
-- 
==-==-==-==-==-==-==-==-==-==-==-==-==-==-==-==-==-==-==-==-==-==
Peter Breitfeld, Bad Saulgau, Germany -- http://www.pBreitfeld.de