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Re: Working with indefinite number of variables

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  • Subject: [mg129136] Re: Working with indefinite number of variables
  • From: David Bailey <dave at>
  • Date: Mon, 17 Dec 2012 02:59:40 -0500 (EST)
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  • References: <kahkfe$d6j$>

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:




The problem is, of course, that the Sum command can't evaluate if n is 

Possibly it would help if you supplied a bit more context regarding what 
you are trying to do.

David Bailey

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