Re: Working with D, definition of a function

*To*: mathgroup at smc.vnet.net*Subject*: [mg75995] Re: Working with D, definition of a function*From*: Szabolcs <szhorvat at gmail.com>*Date*: Mon, 14 May 2007 03:53:39 -0400 (EDT)*Organization*: University of Bergen*References*: <f26nfr$450$1@smc.vnet.net>

Thomas Schmelzer 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 This is a common mistake new users make. Look up Set and SetDelayed (= and :=). You don't need ; at the end of definitions here. http://reference.wolfram.com/mathematica/tutorial/ImmediateAndDelayedDefinitions.html You must use ComPathPrime[w_, mu_] = D[ComPath[w, mu, 0], w] to get the result you expect.