Convergence/Divergence of Series: cos(1/n)

  • Thread starter rcmango
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In summary, the student is asking for help determining if the series given in the link converges absolutely, converges conditionally, or diverges. They suggest starting with the limit comparison test or ratio test. The expert responds by first checking if the limit at infinity is less than 1, which would indicate convergence. However, as the limit is equal to 1, the series is divergent and cannot be absolutely convergent. Additionally, for an alternating series, the last term must also converge to zero, further supporting that the series is divergent. The student thanks the expert for their help.
  • #1
rcmango
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Homework Statement



question is if this series converge absolutely, converge conditionally, or it diverges?

here it is: http://img442.imageshack.us/img442/5899/untitled8tn.jpg

Homework Equations



cos1/n

The Attempt at a Solution



not sure where to start, maybe with the limit comparison test or ratio test?
please help. thanks.
 
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  • #2
[tex]\sum_{n=1}^{\infinity} -1^n \cos \frac{1}{n}[/tex]. First check if the limit at infinity is less than 1. If each term is more than 1, it won't converge. So as we take the limit, Cos 0, its equal to 1. Since its 1, it diverges. So it can't be absolutely convergent.

For an alternating series, the last term has to converge to zero as well, so its divergent as well.
 
Last edited:
  • #3
thanks for your help.
 
  • #4
No problemo :)
 

1. What is the meaning of convergence and divergence of a series?

Convergence and divergence refer to the behavior of a series as the terms approach infinity. A convergent series is one where the terms approach a finite limit, while a divergent series is one where the terms do not approach a finite limit.

2. How can I determine if the series cos(1/n) converges or diverges?

The convergence or divergence of a series can be determined by evaluating the limit of the terms as n approaches infinity. In the case of cos(1/n), the limit will approach 1, indicating that the series converges.

3. Can the convergence of cos(1/n) be proven using the Ratio Test?

Yes, the Ratio Test can be used to prove the convergence of cos(1/n). The ratio of successive terms in the series will approach 1, indicating that the series converges.

4. Is there a specific method for finding the sum of a convergent series like cos(1/n)?

There is no specific method for finding the sum of a series like cos(1/n). However, the sum can be approximated by adding a finite number of terms and using mathematical techniques like the Midpoint Rule.

5. How can the convergence of cos(1/n) be applied in real-world scenarios?

The convergence of cos(1/n) can be applied in fields like physics and engineering to model situations where a quantity approaches a limit. It can also be used in financial mathematics to calculate the present value of an infinite stream of payments.

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