Working with indefinite number of variables

*To*: mathgroup at smc.vnet.net*Subject*: [mg129095] Working with indefinite number of variables*From*: Benjamin Hell <hellben at gmx.de>*Date*: Sat, 15 Dec 2012 05:47:49 -0500 (EST)*Delivered-to*: l-mathgroup@mail-archive0.wolfram.com*Delivered-to*: l-mathgroup@wolfram.com*Delivered-to*: mathgroup-newout@smc.vnet.net*Delivered-to*: mathgroup-newsend@smc.vnet.net

Hi, I have been wrapping my head around this for a while now and I have not found a solution so far. I want to work with an indefinite number of variables in mathematica and use some built in functions. To make things more specific for starters I want to do the following: Define a sum with n summands each containing a new variable x[i] (in the i-th summand): sum[n_] = Sum[i*x[i], {i, 1, n}] Then I want to differentiate the expression with respect to some x[i] like: D[sum[n],x[2]] Mathematica return 0 instead of 2. If I supply a specific n like: D[sum[2],x[2]] everything works fine. I thought about using Assumptions for n, but with no success so far. How can I do that right?