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Manipulating differential equations


Dear colleagues,

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?

Cheers,
Martin Heimann
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Max-Planck-Institut fuer Biogeochemie
Postfach 100164, D-07701 Jena, Germany
(Street Address: Tatzendpromenade 1a)

Tel.:  +49-3641-64-3773/3701,  Fax.: +49-3641-64-3710
Email: martin.heimann at bgc-jena.mpg.de


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