Re: Working with indefinite number of variables

*To*: mathgroup at smc.vnet.net*Subject*: [mg129136] Re: Working with indefinite number of variables*From*: David Bailey <dave at removedbailey.co.uk>*Date*: Mon, 17 Dec 2012 02:59:40 -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*References*: <kahkfe$d6j$1@smc.vnet.net>

On 15/12/2012 10:47, Benjamin Hell wrote: > 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? > Clearly D[sum[n],x[2]] should ideally return something like: If[n>=2,2,0] not 2 The problem is, of course, that the Sum command can't evaluate if n is undefined! Possibly it would help if you supplied a bit more context regarding what you are trying to do. David Bailey http://www.dbaileyconsultancy.co.uk