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 | > |

**Follow-Ups**:**Re: Re: Skipping Elements in Sum***From:*Andrzej Kozlowski <andrzej@yhc.att.ne.jp>