Re: Cancel[] and finite symbolic sums ?

*To*: mathgroup at smc.vnet.net*Subject*: [mg102799] Re: Cancel[] and finite symbolic sums ?*From*: pfalloon <pfalloon at gmail.com>*Date*: Thu, 27 Aug 2009 06:37:01 -0400 (EDT)*References*: <h7377b$b84$1@smc.vnet.net>

On Aug 26, 9:44 pm, Andre Holzner <andre.holz... at gmail.com> wrote: > Dear experts, > > I wonder what I'm doing wrong here... > > I try to simplify the following expression: [Sum[W * Subscript[b, k], > {k, 1, N}]/W > by doing > > Cancel[Sum[W * Subscript[b, k], {k, 1, N}]/W] > > it looks like Mathematica just returns me the original expression: Sum > [W*Subscript[b, k], {k, 1, N}]/W > > When I try a sum over b instead of Subscript[b, k], it seems to work > as expected: > > Cancel[Sum[W * b, {k, 1, N}]/W] > > gives: > > b*N > > Any ideas what I'm doing wrong ? > > thanks, > > Andre The difference is that the Sum[..] expression is evaluating in the second case but not in the first. In other words, it doesn't know how to do the first sum, because the arguments are symbolic, so it leaves it as an unevaluated Sum[...]. The W is stuck inside and so cannot be cancelled.