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Re: Working with indefinite number of variables
Posted:
Dec 17, 2012 2:54 AM


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 ith 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



