Convergence Proof for xn/xn+1: Need Help!

In summary, the statement is true intuitively, but the proof is a bit more difficult. It appears that the proof would involve showing that the sequence is non-decreasing monotone and then proving that it is bounded.
  • #1
bballninja
6
0

Homework Statement



If xn-> ∞ then xn/xn+1 converges.

Homework Equations





The Attempt at a Solution



I can see why the statement is true intuitively, but do not know how to make a rigorous proof. I have looked at the definitions of divergence/convergence but can get any ideas of how to prove this. Do I maybe start by showing that xn/xn+1 is bounded?
 
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  • #2
It's not true. Try to find a counterexample.
 
  • #3
I'm pretty sure it's true, since each successive term of the sequence will be larger or equal to the previous, so xn/xn+1 should always be ≤ 1
 
  • #4
bballninja said:
I'm pretty sure it's true, since each successive term of the sequence will be larger or equal to the previous, so xn/xn+1 should always be ≤ 1

That would be true if the convergence were monotone (i.e. xn is increasing). But even if it were, that wouldn't prove it converges. There are a lot of numbers between 0 and 1.
 
  • #5
Well would I be able to claim that it is non-decreasing monotone, and show that it is bounded which implies convergence?
 
  • #6
bballninja said:
Well would I be able to claim that it is non-decreasing monotone, and show that it is bounded which implies convergence?

(1,1,2,2,4,4,8,8,16,16,...). Does it converge to infinity? What about your ratio?
 
  • #7
Ahh I'm so sorry haha. The instructor just emailed us that there was a typo and the ratio should actually be xn / (xn+1). This makes more sense now. Thanks for your help though
 
  • #8
bballninja said:
Ahh I'm so sorry haha. The instructor just emailed us that there was a typo and the ratio should actually be xn / (xn+1). This makes more sense now. Thanks for your help though

No problem, you're welcome. The correction makes a BIG difference.
 
  • #9
I still can't come up with an answer and my presentation is at 10.

So far I've been able to show that since xn→∞, then 1/xn→0. Then

1/(xn+1) < 1/xn < ε

If anyone is available to help me, that would be very appreciated.
 

1. What is a convergence proof?

A convergence proof is a type of mathematical proof that demonstrates that a sequence of numbers or functions approaches a specific value or limit as its inputs increase. It is often used in calculus and other areas of mathematics to show the behavior of a sequence or series.

2. Why is a convergence proof important?

A convergence proof is important because it provides a rigorous and logical proof of the behavior of a sequence or series. This can help to confirm the validity of mathematical theories and provide a deeper understanding of the behavior of functions.

3. What are the key steps in a convergence proof?

The key steps in a convergence proof typically involve showing that the sequence or series is bounded, monotonic, and that the limit of the sequence or series is the desired value. This is often done using algebraic manipulations and the definition of limits.

4. Are there different types of convergence proofs?

Yes, there are different types of convergence proofs depending on the type of sequence or series being analyzed. Some common types include convergence by comparison, convergence by ratio or root test, and convergence by the squeeze theorem.

5. How can I improve my skills in writing convergence proofs?

To improve your skills in writing convergence proofs, it is important to have a strong understanding of mathematical concepts such as limits and series. Additionally, practice and reviewing examples of convergence proofs can help improve your skills over time.

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