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

```

• Prev by Date: Re: Recurrence Equation RSolve, no error shown, no solution?
• Next by Date: Re: silly question on formatting tables
• Previous by thread: Working with indefinite number of variables
• Next by thread: Re: Working with indefinite number of variables