Show ( (-1)^n + (1/n) ) diverges.

  • Thread starter Unassuming
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In summary, the given sequence (-1)^n + (1/n) diverges and does not converge to any specific value. This is shown by separating the sequence into two subsequences and observing that they oscillate between 1 and -1, indicating that the overall sequence does not converge.
  • #1
Unassuming
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Homework Statement


Show that the sequence (-1)^n + (1/n) diverges.


Homework Equations





The Attempt at a Solution



I have assumed it converges and found the difference between consecutive terms to be "1+ (1/ 2n(n+1) ). I have tried many things but I cannot progress.
 
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  • #2
Well, your expression for the difference is wrong.
Think otherwise:
Your sequence is a sum of two other sequences.
What can you say about the convergens/divergence of those two subsequences?
 
  • #3
They both converge to two different numbers.
 
  • #4
We have not gotten to the subsequences yet so I hesitate to say that this violates the thrm saying that subsequences always converge to the same point as the sequence itself.

This problem is two sections before that theorem and any talk of subsequences. Is there any way to prove it without them?
 
  • #5
Unassuming said:
They both converge to two different numbers.
And what are the two numbers you think they converge to?
 
  • #6
1 and -1? I am beginning to doubt myself.
 
  • #7
Unassuming said:
1 and -1? I am beginning to doubt myself.

Try a few different values of n, and see if that helps visualize what is happening to each term. For the (-1)^n part, can i suggest that you see what happens when n=1,2,3,4...Does it really converge to 1?

EDIT: Sorry misunderstood what you were saying, thought that you meant (-1)^n converges to 1 for all even and odd n.
 
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  • #8
I know what it does. I have tried values of n. I have pictured it and I am ready to prove it. Am I missing something here?

For all the even n, it converges to 1 from the right.

For all the odd n, it converges to -1 from the right.

I am trying to prove that it diverges.
 
  • #9
Ok. First - separate them into two separate sequences. It in fact, does NOT converge. You took some partial sums, did the partial sums ever tend to a certain value? The partial sums "oscillates" between 1 and -1 . It doesn't converge. For the purpose of the question, we need only know that is it bounded. I'm sure you can do the rest.
 
  • #10
Unassuming said:
1 and -1? I am beginning to doubt myself.
No, neither of them converges to 1 or -1! I have no idea how you could come to that conclusion.

Look at a few terms: {1/n} starts 1, 1/2, 1/3, 1/4, 1/5, ... what does that converge to?

{(-1)n starts -1, 1, -1, 1, -1, 1... what does that converge to? Does it converge at all?
 
  • #11
O wait sorry, I thought we were doing series instead of sequences. Ignore my comment about the boundedness.
 

What does "Show ( (-1)^n + (1/n) ) diverges" mean?

This notation is asking for a proof or demonstration that the series (-1)^n + (1/n) diverges. In other words, it is asking for evidence that the terms of this series do not approach a finite limit as n approaches infinity.

What is a series?

A series is an infinite sum of terms. It is represented by the notation ∑ (sigma) and can be written in the form ∑(an) = a1 + a2 + a3 + ... + an + ... , where an is each term in the series and n is the index or position of the term in the series.

What does it mean for a series to diverge?

A series diverges if the sum of its terms does not approach a finite limit as the number of terms increases. In other words, the series does not have a defined sum and the terms do not approach a specific value as n approaches infinity.

How do you determine if a series diverges?

There are several tests that can be used to determine if a series diverges, such as the divergence test, comparison test, limit comparison test, and integral test. In this specific case, we can use the divergence test to show that the series (-1)^n + (1/n) diverges.

Can you provide a proof that (-1)^n + (1/n) diverges?

Yes, the proof is as follows:
By the divergence test, a series ∑(an) diverges if lim n→∞ an ≠ 0.
In this case, an = (-1)^n + (1/n).
As n approaches infinity, the terms alternate between 1 and 0, so the limit does not exist and is not equal to 0. Therefore, the series (-1)^n + (1/n) diverges.

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