Re: Skipping Elements in Sum
- To: mathgroup at smc.vnet.net
- Subject: [mg61149] Re: Skipping Elements in Sum
- From: "Richard J. Fateman" <fateman at eecs.berkeley.edu>
- Date: Tue, 11 Oct 2005 03:22:20 -0400 (EDT)
- Organization: UC Berkeley
- References: <did35k$qh9$1@smc.vnet.net>
- Sender: owner-wri-mathgroup at wolfram.com
Sum[a[i],{i,low,j-1}]+Sum[a[i],{i,j+1,high}] may be far more
useful in many ways, since you can tell how many elements there
are: if low<j<high then high-low else high-low+1 and
other useful symbolic info.
Symbolically manipulating objects with holes shot in them with
Delta functions is not so easy or reliable.
If all you want to do is add them, you can use a For loop and
subtract the excluded elements, but that's not what you asked.
You could try asking a more specific question about what you
really want to do with Sum.
RJF
Bill Rowe wrote:
> On 10/9/05 at 1:36 AM, qcadesigner at gmail.com wrote:
>
>
>>Does anyone know how to skip elements using the mathematica sum?
>>e.g. take the sum of all i, where i not equal to j.
>
>
> Yes.
>
> One way to do this with the Sum function would be to use the KroneckerDelta function as follows:
>
> Sum[Subscript[a, n](1 - KroneckerDelta[n, 3]), {n, 5}]
>
> Another way to create the same sum would be
>
> Total[Table[Subscript[a, n], {n, 5}][[Complement[
> Range[5], {3}]]]]
>
> and there are many other ways to achieve the same result. Which is best depends on exactly what you are trying to accomplish.
> --
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>
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