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Re: Sum
*To*: mathgroup at smc.vnet.net
*Subject*: [mg55256] Re: Sum
*From*: Bill Rowe <readnewsciv at earthlink.net>
*Date*: Thu, 17 Mar 2005 03:30:52 -0500 (EST)
*Sender*: owner-wri-mathgroup at wolfram.com
On 3/16/05 at 5:35 AM, mjumbo at nm.ru (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?
Use any of
x[k_]:= Exp[-k]
Sum[x[k], {k, 0, Infinity}]
x[k_]= Exp[-k]
Sum[x[k], {k, 0, Infinity}]
x = Exp[-k]
Sum[Evaluate[x], {k, 0, Infinity}]
The difference is when things get evaluated. With you syntax, x is not evaluated before the summation begins. You syntax is equivalent to Sum[y, {k, 0, Infinity}]/.y->Exp[-k]. That is the sum evaluatest to x Infinity then x gets evaluated for a result of Exp[-k] Infinity.
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