Is a Monotone Sequence with a Bounded Subsequence Always Bounded?

In summary, the task is to prove that a monotone sequence with a bounded subsequence is also bounded. The attempt at a solution involved using the definition of a monotone sequence and showing that the subsequence is both monotone and bounded, thus converging to a number. Then, it was suggested to take the supremum of the limits of all convergent subsequences and claim that the original sequence converges to this supremum.
  • #1
zachsdado
8
0

Homework Statement



prove that a monotone sequence which has a bounded subsequence is bounded

Homework Equations





The Attempt at a Solution

 
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  • #2
did you even try to solve this?
 
  • #3
Of course

I tried using the def. of a monotone sequence to show that the subsequence was monotone and bounded hence it converged to some number and then tried to prove that the sequence was convergent thus it was bounded
 
  • #4
The problem is of course that there could be a lot of convergent subsequences. Try taking the sup of the limits over all of the convergent subsequences. Claim: the sequence converges to this sup.

I think this should do it.
 

1. What does it mean for a sequence to be bounded?

A sequence is considered bounded if all of its values fall within a certain range. This means that there is a maximum and minimum value for the sequence, and all values in between are also included.

2. How do you prove that a sequence is bounded?

To prove that a sequence is bounded, you can either show that the sequence is increasing and has an upper bound, or that the sequence is decreasing and has a lower bound. Another way to prove boundedness is by using the squeeze theorem, which compares the sequence to another sequence with known bounds.

3. Can a sequence be bounded if it has infinite terms?

Yes, a sequence can still be bounded even if it has an infinite number of terms. As long as all the terms fall within a certain range, the sequence is considered bounded.

4. Is it possible for a sequence to be both increasing and decreasing?

No, a sequence cannot be both increasing and decreasing. An increasing sequence has values that are getting larger, while a decreasing sequence has values that are getting smaller. Therefore, a sequence can only have one of these properties at a time.

5. Does a bounded sequence always converge?

No, a bounded sequence does not always converge. A sequence can be bounded but still have values that fluctuate and do not approach a certain value. Convergence also depends on the behavior of the terms in the sequence, not just the bounds.

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