Re: Re: Sum

*To*: mathgroup at smc.vnet.net*Subject*: [mg55276] Re: [mg55232] Re: [mg55175] Sum*From*: yehuda ben-shimol <bsyehuda at gmail.com>*Date*: Fri, 18 Mar 2005 05:33:52 -0500 (EST)*References*: <200503161035.FAA23742@smc.vnet.net>*Reply-to*: yehuda ben-shimol <bsyehuda at gmail.com>*Sender*: owner-wri-mathgroup at wolfram.com

As said before, the iterator of Sum is a LOCAL variable to the Sum function as can be seen in the following example Sum[(Print[i]; 1/i^2), {i, Infinity}] and the functions prints K$18 and then returns the result Pi^2/6 K$18 is a local naming (as is used to in a Module[] for local variables) so x must be evaluated with the local variables before the summation takes place yehuda On Thu, 17 Mar 2005 03:29:08 -0500 (EST), DrBob <drbob at bigfoot.com> wrote: > >> If the result weren't infinite, that would be a good idea. > > Oops, I take that back. It would almost always be a bad idea, so I don't know why Evaluate isn't automatic. > > Bobby > > On Wed, 16 Mar 2005 10:10:40 -0600, DrBob <drbob at bigfoot.com> wrote: > > > Sum tried to simplify BEFORE substituting the value of x. If the result weren't infinite, that would be a good idea. But in this case, it's not. But there's a simple fix: > > > > x = Exp[-k] > > Sum[Evaluate@x, {k, 0, Infinity}] > > > > E^(-k) > > E/(-1 + E) > > > > Bobby > > > > On Wed, 16 Mar 2005 05:35:51 -0500 (EST), mjumbo <mjumbo at nm.ru> wrote: > > > >> Can sombody explain me why > >> > >> Sum[Exp[- k], {k, 0, Infinity}] > >> > >> gives the correct result, while the following (imho equivalent) statements > >> > >> x = Exp[-k] > >> Sum[x, {k, 0, Infinity}] > >> > >> don't? What should I do to make it work? It is rather vexing to copy/paste long expressions instead of using short placeholders like x. > >> __________ > >> www.newmail.ru -- Íîâàÿ Ïî÷òà: âñå ïî-íîâîìó. > >> > >> > >> > >> > > > > > > > > -- > DrBob at bigfoot.com > >

**Follow-Ups**:**Re: Re: Re: Sum***From:*DrBob <drbob@bigfoot.com>

**References**:**Sum***From:*mjumbo <mjumbo@nm.ru>