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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)
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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
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