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MathGroup Archive 2005

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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


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