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Re: Cancel[] and finite symbolic sums ?


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.



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