Advanced Calculus Infinite Series

Click For Summary
SUMMARY

The discussion focuses on proving that for a monotone decreasing sequence of positive numbers {an}, if the series an converges, then the limit of n*an approaches 0. The proof begins with the established fact that lim an = 0 due to the convergence of the series. A critical insight is that if n*an does not converge to 0, it implies the existence of a positive constant e such that n*an > e for infinitely many n, which contradicts the monotonicity and convergence of {an}.

PREREQUISITES
  • Understanding of monotone sequences
  • Familiarity with convergence of series
  • Knowledge of limits in calculus
  • Basic proof techniques in mathematical analysis
NEXT STEPS
  • Study the properties of monotone decreasing sequences
  • Learn about convergence tests for series in calculus
  • Explore the concept of limits and their applications in proofs
  • Investigate the harmonic series and its divergence
USEFUL FOR

Students of advanced calculus, mathematicians focusing on series convergence, and anyone interested in mathematical proofs involving limits and sequences.

jtn2007
Messages
3
Reaction score
0

Homework Statement



Suppose that {an} is a monotone decreasing sequence of positive numbers. Show that if the series an converges, then the lim(nan)=0.

Homework Equations


The Attempt at a Solution



I started the proof with the fact that since I know the sequence is monotone decreasing and the the series converges then the lim an=0 but I am not sure how to show that the lim nan=o.

any help would be greatly appreciated
 
Physics news on Phys.org
It's really a lot like the proof the harmonic series diverges. If n*a_n does not converge to 0, then there is a positive constant e such that n*a_n>e for an infinite number of n, right? Use that with the monotonicity to show a_n must diverge.
 

Similar threads

  • · Replies 1 ·
Replies
1
Views
2K
  • · Replies 5 ·
Replies
5
Views
2K
Replies
3
Views
2K
  • · Replies 2 ·
Replies
2
Views
2K
Replies
7
Views
2K
  • · Replies 4 ·
Replies
4
Views
2K
Replies
8
Views
2K
Replies
3
Views
3K
  • · Replies 6 ·
Replies
6
Views
2K
Replies
6
Views
3K