Re: Skipping Elements in Sum
- To: mathgroup at smc.vnet.net
- Subject: [mg61160] Re: Skipping Elements in Sum
- From: "Jens-Peer Kuska" <kuska at informatik.uni-leipzig.de>
- Date: Tue, 11 Oct 2005 04:49:56 -0400 (EDT)
- Organization: Uni Leipzig
- References: <did35k$qh9$1@smc.vnet.net> <difr6v$fe8$1@smc.vnet.net>
- Sender: owner-wri-mathgroup at wolfram.com
Hi,
the main problem with
Sum[(1-KroneckerDelta[i,j])*a[i,j],{i,1,10},{j,1,10}]
ist that a[i,j] in many physical applications
contain
a singularity and Mathematica try to evaluate
a[i,i]
before it find out that the term
(1-KroneckerDelta[i,j]).
In the worst case it get 0*Infinity and don't know
what
to do with it.
Regards
Jens
"Richard J. Fateman" <fateman at eecs.berkeley.edu>
schrieb im Newsbeitrag
news:difr6v$fe8$1 at smc.vnet.net...
| 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.
| > --
| > To reply via email subtract one hundred and
four
| >
|
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