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. -- To reply via email subtract one hundred and four