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Topic: Working with indefinite number of variables
Replies: 2   Last Post: Dec 18, 2012 2:35 AM

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 David Bailey Posts: 714 Registered: 11/7/08
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 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

Date Subject Author
12/15/12 Benjamin Hell
12/17/12 David Bailey
12/18/12 Cisco Lane