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Re: Problem with ->

To: mathgroup@smc.vnet.net
Subject: [mg11417] Re: [mg11379] Problem with ->
From: jpk@max.mpae.gwdg.de
Date: Thu, 12 Mar 1998 01:32:24 -0500


> 
> I want to substitute R[s] by k s.
> I tried:
> R[s] / R'[s] /. R[s] -> k s
> and got
> k s / R'[s]
> instead of the expected
> s
> 
> What am I doing wrong?
> 
> Hans Friedrich Steffani
> --
> Hans Friedrich Steffani
> Institut fuer Elektrische Maschinen und Antriebe, TU Chemnitz
> mailto:hans.steffani@e-technik.tu-chemnitz.de
> http://www.tu-chemnitz.de/~hfst/
> 

The full form of R'[s] is Derivative[1][R][s]  The correct way to do the
replacement is 

R[s]/R'[s] /. Thread[{#,D[#,s]} & /@ (R[s] ->k s) ]

Hope that helps
  Jens

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