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MathGroup Archive 1999

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Re: Manipulating differential equations

  • To: mathgroup at smc.vnet.net
  • Subject: [mg18640] Re: Manipulating differential equations
  • From: Eckhard Hennig <hennig at itwm.uni-kl.de>
  • Date: Thu, 15 Jul 1999 01:45:37 -0400
  • Organization: ITWM
  • References: <7m6jti$2mc@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

>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?


In[1]:= dg1 = c''[x] + g c'[x];

In[2]:= dg1 /. c -> (Exp[-a #]&)

Out[2]=
    2
   a         a g
-------- - --------
 -(-a x)    -(-a x)
E          E


-- Eckhard

-----------------------------------------------------------
Dipl.-Ing. Eckhard Hennig      mailto:hennig at itwm.uni-kl.de
Institut fuer Techno- und Wirtschaftsmathematik e.V. (ITWM)
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  Voice: +49-(0)631-205-3126    Fax: +49-(0)631-205-4139
    http://www.itwm.uni-kl.de/as/employees/hennig.html

     ITWM - Makers of Analog Insydes for Mathematica
        http://www.itwm.uni-kl.de/as/products/ai
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