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Re: Manipulating differential equations

• To: mathgroup at smc.vnet.net
• Subject: [mg18593] Re: [mg18552] Manipulating differential equations
• From: BobHanlon at aol.com
• Date: Tue, 13 Jul 1999 01:01:26 -0400
• Sender: owner-wri-mathgroup at wolfram.com

Martin,

dg1 = c''[x] + g c'[x];

dg1 /. NestList[D[#, x] &, c[x] -> Exp[-a*x], 2] // Simplify

(a*(a - g))/E^(a*x)

Bob Hanlon

In a message dated 7/10/99 7:16:39 AM, martin.heimann at bgc-jena.mpg.de writes:

>I would like to insert test functions into an expression that contains
>differentials:
>
>dg1 = c''[x] + g c'[x]
>
>I would like e.g. to replace c[x] by Exp[-a x]:
>
>dg1/.c[x]->Exp[-a x]
>
>but this doesn't work; one has to put substitution rules for each of the
>derivatives explicitly. Is there a more elegant way to achieve this?
>