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MathGroup Archive 2007

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Re: Differentiation w.r.t. elements of lists

  • To: mathgroup at smc.vnet.net
  • Subject: [mg79762] Re: Differentiation w.r.t. elements of lists
  • From: Daniel Hornung <ds.mpg.de.thispartismydomain.daniel.hornung at insertdomainhere.com>
  • Date: Fri, 3 Aug 2007 06:38:04 -0400 (EDT)
  • Organization: GWDG, Goettingen
  • References: <f8pi48$1l4$1@smc.vnet.net> <f8s3e2$38u$1@smc.vnet.net>
  • Reply-to: ds.mpg.de.thispartismydomain.daniel.hornung at insertdomainhere.com

Jens-Peer Kuska wrote:

> Hi,
>
> and you are shure that
>
>  > Another, even shorter test case would be
>  >
>  > D[x[[i]], x[[j]]]
>  >
>  > which "should", IMHO, return KroneckerDelta[i,j].
>
> ??
>
> Than
> D[x[[1]],x[[2]]]
>
> should be zero ? right ?? but what is
>
> with
>
> x = {Sin[y], y}
>
> D[x[[1]], x[[2]]]
>
> the result is not 0 ..
>
> may be that you are wrong.
>
> Regards
>    Jens
>

OK, I think I had thought wrongly there.  But sadly that still doesn't
answer my main question:
Why does the second line result in 0, and what could I do better?

>> h[x_]:=Sum[x[[j]]^2,{j,1,Length[x]}]
>> D[h[x],x[[i]]]

--
Daniel Hornung
Max Planck Institute for Dynamics and Self-Organization
G=C3=B6ttingen, Germany


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