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. > -- > To reply via email subtract one hundred and four >

**Follow-Ups**:**Re: Re: Skipping Elements in Sum***From:*Andrzej Kozlowski <akoz@mimuw.edu.pl>