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>
• 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

```

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