Every subsequence converges to L implies a_n -> L

  • Thread starter Mr Davis 97
  • Start date
  • Tags
    Subsequence
In summary, the problem states that in a bounded sequence ##\{a_n\}##, every convergent subsequence has the limit ##L##. The task is to prove that ##\lim_{n\to\infty}a_n = L##. By considering the limits of the subsequence and using the definitions of ##\liminf## and ##\limsup##, it can be shown that ##\lim_{n\to\infty} a_n = L##.
  • #1
Mr Davis 97
1,462
44

Homework Statement


Let ##\{a_n\}## be a bounded sequence such that every convergent subsequence has limit ##L##. Prove that ##\lim_{n\to\infty}a_n = L##.

Homework Equations

The Attempt at a Solution


I'm not really understanding this problem. Isn't ##\{a_n\}## a subsequence of itself? So isn't it immediately the case that ##\lim_{n\to\infty}a_n = L## by the hypothesis?

EDIT: Nevermind. I notice now that it says every convergent subsequence.
 
Physics news on Phys.org
  • #2
Hint: This is easy considering ##\liminf_{n \to \infty}a_n## and ## \limsup_{n \to \infty} a_n##
 
  • #3
Math_QED said:
Hint: This is easy considering ##\liminf_{n \to \infty}a_n## and ## \limsup_{n \to \infty} a_n##
Could we just say that since the set of subsequential limits is just ##\{L\}##, we have that ##\limsup a_n = \sup \{L\} = L = \inf \{L\} = \liminf a_n##. So ##\lim_{n\to\infty} a_n = L##?
 
  • #4
Mr Davis 97 said:
Could we just say that since the set of subsequential limits is just ##\{L\}##, we have that ##\limsup a_n = \sup \{L\} = L = \inf \{L\} = \liminf a_n##. So ##\lim_{n\to\infty} a_n = L##?

Exactly. Well done!
 

1. What is the definition of a subsequence?

A subsequence is a sequence that is obtained by removing some elements from a given sequence while maintaining the relative order of the remaining elements.

2. How is the convergence of a subsequence related to the convergence of the original sequence?

If every subsequence of a sequence converges to a specific limit, then the original sequence also converges to the same limit. This is known as the "subsequence convergence theorem."

3. What does it mean for a sequence to converge to a limit?

A sequence converges to a limit L if, for any small positive number ε, there exists a positive integer N such that for all n greater than or equal to N, the terms of the sequence are within ε of the limit L.

4. Can a sequence converge to more than one limit?

No, a sequence can only converge to one limit. If a sequence converges to more than one limit, then it is said to be a divergent sequence.

5. How can I prove that a sequence converges to a specific limit?

To prove that a sequence converges to a specific limit L, you must show that for any small positive number ε, there exists a positive integer N such that for all n greater than or equal to N, the terms of the sequence are within ε of the limit L. This can be done using the definition of convergence or by using the subsequence convergence theorem.

Similar threads

  • Calculus and Beyond Homework Help
Replies
4
Views
885
  • Calculus and Beyond Homework Help
Replies
1
Views
265
  • Calculus and Beyond Homework Help
Replies
2
Views
713
  • Calculus and Beyond Homework Help
Replies
34
Views
2K
  • Calculus and Beyond Homework Help
Replies
6
Views
2K
  • Calculus and Beyond Homework Help
Replies
4
Views
1K
  • Calculus and Beyond Homework Help
Replies
2
Views
1K
  • Calculus and Beyond Homework Help
Replies
7
Views
1K
  • Calculus and Beyond Homework Help
Replies
1
Views
783
  • Calculus and Beyond Homework Help
Replies
3
Views
845
Back
Top