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MathGroup Archive 2005

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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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