taking partial derivatives with respect to vector elements: need help with syntax
- To: mathgroup at smc.vnet.net
- Subject: [mg72859] taking partial derivatives with respect to vector elements: need help with syntax
- From: kai_spam at web.de
- Date: Tue, 23 Jan 2007 04:27:05 -0500 (EST)
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