Does an Increasing Sequence Bounded by a Convergent Sequence Converge?

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In summary, if $a_n$ is increasing, $a_n\le b_n$ for all $n$, and $b_n\to M$, it is sufficient to prove that $a_n$ is bounded above. By choosing an appropriate value for $\epsilon$, we can show that $a_n$ is bounded above by $M+1$ for $n\gg 1$. Therefore, $a_n$ is bounded above for all $n$ and by the corollary, it must also be convergent.
  • #1
alexmahone
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Suppose $a_n$ is increasing, $a_n\le b_n$ for all $n$, and $b_n\to M$. Prove that $a_n$ converges.

My attempt:

It is sufficient to prove that $a_n$ is bounded above.

Given any $\epsilon>0$,

$|b_n-M|<\epsilon$ for $n\gg 1$

$-\epsilon<b_n-M<\epsilon$ for $n\gg 1$

$M-\epsilon<b_n<M+\epsilon$ for $n\gg 1$

$a_n\le b_n<M+\epsilon$ for $n\gg 1$

$a_n<M+\epsilon$ for $n\gg 1$

Taking $\epsilon=1$ (say) we get $a_n<M+1$ for $n\gg 1$.

So, $a_n$ is bounded above by $M+1$ for $n\gg 1$. Since $a_n$ is increasing, it is bounded above by $M+1$ for all $n$.

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Could someone please check the above proof for me?
 
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  • #2
It seems good, the only thing I do not like is the fact that you write " for $n \gg 1$ " instead of saying "there exists $n_{\epsilon} \in \mathbb{N}$ such that... " or "for large enough $n$". :p
 
  • #3
Here is a similar way, but this idea is useful in its own right.

Theorem: If $(b_n)$ is a convergent sequence then it is bounded.

Proof: Left as exercise ---> you basically proved it above.

Corollary: If $(a_n)$ is increasing and $(b_n)$ is convergent with $a_n\leq b_n$ then $(a_n)$ is convergent.

Proof: There is $A$ such that $b_n \leq A$ by theorem but then $a_n \leq A$ as well and so ... QED

Corollary: If $(a_n)$ is decreasing and $(b_n)$ is convergent with $a_n \geq b_n$ then $(b_n)$ is convergent.

Proof: There is a $B$ such that $b_n \geq B$ by theorem but then $a_n \geq B$ as well and so ... QED
 

FAQ: Does an Increasing Sequence Bounded by a Convergent Sequence Converge?

1. What does it mean for a sequence to converge?

Convergence in a sequence means that the values in the sequence are getting closer and closer to a single fixed value as we move further along in the sequence. This fixed value is known as the limit of the sequence.

2. How do you prove that a sequence converges?

To prove that a sequence converges, we need to show that as we go further along in the sequence, the terms are getting closer and closer to a single fixed value. This can be done by showing that the difference between consecutive terms in the sequence becomes smaller and smaller as we move further along.

3. What is the difference between a convergent and a divergent sequence?

A convergent sequence approaches a single fixed value as we move further along in the sequence, while a divergent sequence does not have a fixed value and can either increase or decrease without bound.

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

No, a sequence can only converge to a single limit. If a sequence converges to more than one limit, it would not have a fixed value and would be considered a divergent sequence.

5. Is it possible for a sequence to have a limit but not converge?

No, if a sequence has a limit, it must converge. This is because the definition of convergence requires the terms in the sequence to get closer and closer to a single fixed value, which is the limit.

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