Re: Sum

*To*: mathgroup at smc.vnet.net*Subject*: [mg55210] Re: Sum*From*: dh <dh at metrohm.ch>*Date*: Thu, 17 Mar 2005 03:28:29 -0500 (EST)*References*: <d192qe$ng4$1@smc.vnet.net>*Sender*: owner-wri-mathgroup at wolfram.com

Hi mjumbo, Auxilliary variables like k in Sum[..,{k,..}] are handled apart from ordinary variables. Otherwise the following example would give an error: k=2; Sum[k,{k,1,2}] Mathematica does this by taking the unevaluated expression in: Sum[expr,{k,1,2}] and literally replace all "external" k against "internal" k's. In your casae it takes x (there is nothing to replace). Then it calculates the Sum, giving x times Infinity, then it replaces x by Exp[-k] giving Exp[-k] times Infinity. If you want Mathematica to evaulate x first before replacing variables, you have to tell Mathematica by: x=Exp[-k]; Sum[ Evaluate[x] ,{k,0,Infinity}] Daniel mjumbo 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. >