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MathGroup Archive 2005

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Re: Sum

  • To: mathgroup at smc.vnet.net
  • Subject: [mg55232] Re: [mg55175] Sum
  • From: DrBob <drbob at bigfoot.com>
  • Date: Thu, 17 Mar 2005 03:29:08 -0500 (EST)
  • References: <200503161035.FAA23742@smc.vnet.net> <opsnqm72iyiz9bcq@monster.ma.dl.cox.net>
  • Reply-to: drbob at bigfoot.com
  • Sender: owner-wri-mathgroup at wolfram.com

>> 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: Sum
      • From: Andrzej Kozlowski <akoz@mimuw.edu.pl>
  • References:
    • Sum
      • From: mjumbo <mjumbo@nm.ru>
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