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