Convergence of Series with Cosine Terms

  • Thread starter de1irious
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In summary, to show that the series \sum_{n=0}^{\infty} \frac{\cos n}{1+n} does not converge absolutely, you can use the limit comparison test with the series 1/n and show that |cos n| does not tend to a limit as n--> infinity. However, the series does converge using the alternating series test. There may be some confusion about whether it is cos(n) or cos(n*pi), but regardless, the series does not converge absolutely.
  • #1
de1irious
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How would I show that the series whose terms are given by
(cos n)/(1+n) does not converge absolutely? Thanks so much!
 
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  • #2
[tex] \sum_{n=0}^{\infty} \frac{\cos n}{1+n} [/tex]

So [tex] \sum_{n=0}^{\infty} \frac{\cos n}{1+n} \sim \frac{1}{n} [/tex]
 
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  • #3
Hi sorry, I'm having trouble understanding that. How am I supposed to compare that?
 
  • #4
de1irious said:
How would I show that the series whose terms are given by
(cos n)/(1+n) does not converge absolutely? Thanks so much!

Is it cos(n) or cos(n*pi)? If it is the first then the limit comparison test should wor fairly well with the series 1/n.
 
  • #6
de1irious said:
You mean this limit comparison test? http://mathworld.wolfram.com/LimitComparisonTest.html

But what limit does it tend to? I thought |cos n| didn't tend to a limit as n--> infinity.

Yea that test, cos(n) doesn't but it is bounded so I think if you use that fact and maybe the squeeze theorem you should be able to show that the series doesn't converge absolutely. It shouldn't be very hard to show that the series does converge as is using the alternating series test, but I'm not sure if it is cos(n) as opposed to cos(pi*n).
 

Related to Convergence of Series with Cosine Terms

1. What is a basic convergence question?

A basic convergence question refers to a problem or scenario in which a series or sequence of numbers is being studied to determine whether it converges or diverges. This is an important concept in mathematics and is commonly encountered in various scientific fields.

2. How is convergence determined?

Convergence is determined by looking at the behavior of a series or sequence as the number of terms or iterations increases. If the terms or values of the series approach a specific number or value, then the series is said to converge. If the terms become increasingly larger or smaller without approaching a specific value, then the series is said to diverge.

3. What is the difference between absolute and conditional convergence?

Absolute convergence refers to a series in which the absolute values of the terms converge, while conditional convergence refers to a series in which the terms themselves converge. In other words, absolute convergence is a stricter condition than conditional convergence.

4. Can a series converge to more than one value?

No, a series can only converge to one value. The behavior of a series as the number of terms increases should be consistent and approach a specific value. If a series converges to multiple values, it is considered to be divergent.

5. How is convergence important in science?

Convergence is important in science because it allows us to make predictions and analyze data. Many real-world phenomena can be modeled using series or sequences, and determining their convergence can help us understand and make predictions about these phenomena. It is also a fundamental concept in many scientific fields, including physics, engineering, and economics.

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