Re: taking partial derivatives with respect to vector elements: need

*To*: mathgroup at smc.vnet.net*Subject*: [mg72904] Re: taking partial derivatives with respect to vector elements: need*From*: Jens-Peer Kuska <kuska at informatik.uni-leipzig.de>*Date*: Wed, 24 Jan 2007 17:45:38 -0500 (EST)*Organization*: Uni Leipzig*References*: <ep4jmf$p84$1@smc.vnet.net>*Reply-to*: kuska at informatik.uni-leipzig.de

Hi, a) y = Table[x[i], {i, 1, 8}] D[Plus @@ y, x[3]] should work, b) avoid the symbol N because Mathematica use a function N[] to convert a symbolic value into a floating point number c) Mathematica can't know that in D[Sum[x[i],{i,nn}],x[j]] j is an integer, and that it can have a value in the range Element[j,{1,2,..,nn}] and you have to add the rule to build the derivative by yourself. Regards Jens kai_spam at web.de wrote: > Hi, I'd appreciate any help on the following simply(?) syntax problem: > > I would like to take a partial derivative with respect to a variable > which is indexed within a sum. > Among other things I'm trying the following: > > Sum[x[[i]], {i, N}] > > which works when I specify x={1,2,3} and N=3, but I want to keep x and > N unspecified. In which case it produces the message: > > Part:::pspec : Part specification i is neither an integer nor a list of > integers. > > I am aiming for something similar to the following: > > D[Sum[x[[i]],{i,N}],x[2]] or even D[Sum[x[[i]],{i,N}],x[j]] > > I realize this is quite a rookie-question but I couldn't figure it out > with the mathematica help + book. > Thanks in advance, Kai >