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MathGroup Archive 2007

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Re: Working with D, definition of a function

  • To: mathgroup at smc.vnet.net
  • Subject: [mg75961] Re: Working with D, definition of a function
  • From: Norbert Marxer <marxer at mec.li>
  • Date: Mon, 14 May 2007 03:35:48 -0400 (EDT)
  • References: <f26nfr$450$1@smc.vnet.net>

On 13 Mai, 11:58, "Thomas Schmelzer" <thomas.... at balliol.ox.ac.uk>
wrote:
> Hi,
>
> I have defined a map
>
> ComPath[w_, mu_, bias_] := mu*(\[ImaginaryI]*w + 1)^2 + bias;
> Now, I would like to use the derivative with respect to w as a further
> function.
>
> D[ComPath[w, mu, 0], w]
>
> gives the answer I expect, but
>
> ComPathPrime[w_, mu_] := D[ComPath[w, mu, 0], w];
>
> doesn't seem to make sense
>
> ComPathPrime[2, 1]
>
> results in
>
> General::ivar : 2 is  not a valid variable.
>
> If it seems I have a lack of knowledge about the internals of Mathmatica -
> that's right.
>
> Can you briefly explain why this should not work?
>
> Best,
>
> Thomas

Hello

You should take the derivative with symbols (not with numbers): e.g.

ComPath[w_, mu_, bias_] := mu*(I*w + 1)^2 + bias
ComPathPrime[w_, mu_] := Block[{w0, mu0},
     D[ComPath[w0, mu0, 0], w0] /. {w0 -> w, mu0 -> mu}]
ComPathPrime[2, 1]

will give

-4 + 2*I

Best Regards
Norbert Marxer



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