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Re: Skipping Elements in Sum
 To: mathgroup at smc.vnet.net
 Subject: [mg61160] Re: Skipping Elements in Sum
 From: "JensPeer Kuska" <kuska at informatik.unileipzig.de>
 Date: Tue, 11 Oct 2005 04:49:56 0400 (EDT)
 Organization: Uni Leipzig
 References: <did35k$qh9$1@smc.vnet.net> <difr6v$fe8$1@smc.vnet.net>
 Sender: ownerwrimathgroup at wolfram.com
Hi,
the main problem with
Sum[(1KroneckerDelta[i,j])*a[i,j],{i,1,10},{j,1,10}]
ist that a[i,j] in many physical applications
contain
a singularity and Mathematica try to evaluate
a[i,i]
before it find out that the term
(1KroneckerDelta[i,j]).
In the worst case it get 0*Infinity and don't know
what
to do with it.
Regards
Jens
"Richard J. Fateman" <fateman at eecs.berkeley.edu>
schrieb im Newsbeitrag
news:difr6v$fe8$1 at smc.vnet.net...
 Sum[a[i],{i,low,j1}]+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 highlow else
highlow+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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