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