Derivative, Sum and KroneckerDelta symbolically

*To*: mathgroup at smc.vnet.net*Subject*: [mg25040] Derivative, Sum and KroneckerDelta symbolically*From*: Stefan Eder <stefan.eder at eunet.at>*Date*: Fri, 1 Sep 2000 21:57:49 -0400 (EDT)*Sender*: owner-wri-mathgroup at wolfram.com

Hi, does anybody know how to tell Mathematica to solve "Sum[D[Sum[x[n], {n, 1, k}], x[i]], {i, 1, k}]" correctly? In my opinion the expression should give "k" as result, Mathematica says "0". Introducing concrete values instead of k the result is correct: "Sum[D[Sum[x[n], {n, 1, 5}], x[i]], {i, 1, 5}]" = "5". Is there a possibility to tell Mathematica that the expressions above lead to a "KroneckerDelta": Sum[KroneckerDelta[n, i], {n, 1, k}, {i, 1, k}] Thanks, Stefan