Re: Sum

*To*: mathgroup at smc.vnet.net*Subject*: [mg55206] Re: Sum*From*: David Bailey <dave at Remove_Thisdbailey.co.uk>*Date*: Thu, 17 Mar 2005 03:28:26 -0500 (EST)*References*: <d192qe$ng4$1@smc.vnet.net>*Sender*: owner-wri-mathgroup at wolfram.com

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. > Hi, The variable k is considered local to the Sum construct, and therefore distinct from the global variable k. Here is one way to get what you want: With[{x=x},Sum[x,{k,0,Infinity}]] This splices the value of x into the Sum before it is executed. David Bailey dbaileyconsultancy.co.uk