Services & Resources / Wolfram Forums
-----
 /
MathGroup Archive
2005
*January
*February
*March
*April
*May
*June
*July
*August
*September
*October
*November
*December
*Archive Index
*Ask about this page
*Print this page
*Give us feedback
*Sign up for the Wolfram Insider

MathGroup Archive 2005

[Date Index] [Thread Index] [Author Index]

Search the Archive

Re: Sum

  • To: mathgroup at smc.vnet.net
  • Subject: [mg55210] Re: Sum
  • From: dh <dh at metrohm.ch>
  • Date: Thu, 17 Mar 2005 03:28:29 -0500 (EST)
  • References: <d192qe$ng4$1@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

Hi mjumbo,
Auxilliary variables like k in Sum[..,{k,..}] are handled apart from 
ordinary variables. Otherwise the following example would give an error:
k=2;
Sum[k,{k,1,2}]
Mathematica does this by taking the unevaluated expression in: 
Sum[expr,{k,1,2}] and literally replace all "external" k against 
"internal" k's. In your casae it takes x (there is nothing to replace). 
Then it calculates the Sum, giving x times Infinity, then it replaces x 
by Exp[-k] giving Exp[-k] times Infinity.
If you want Mathematica to evaulate x first before replacing variables, 
you have to tell Mathematica by:

x=Exp[-k];
Sum[ Evaluate[x] ,{k,0,Infinity}]

Daniel

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.
> 


  • Prev by Date: Re: a simplification problem
  • Next by Date: Re: bug in series expansion routines.
  • Previous by thread: Re: Re: Sum
  • Next by thread: Re: Sum