Convergence Tests: Test for Convergence of Series

In summary, the conversation discusses how to test for convergence of the series \sum sin(1/n) / \sqrt{ln(n)} from 2 to infinity using the limit comparison test. The experts suggest using the integral test with a comparison series, such as 1/(2n), to prove that the original series is divergent. They also mention that Wolfram Alpha's result of the sum converging to 962 is most likely incorrect.
  • #1
craig16
21
0

Homework Statement



Test for convergence

[tex]\sum[/tex] sin(1/n) / [tex]\sqrt{ln(n)}[/tex]

sum from 2 to inf

Homework Equations



Limit comparison test

if lim an / bn to inf
doesn't equal 1 you know if it converges or diverges
by limit comparison test

The Attempt at a Solution



I've tried a lot of different things for bn, I think the only way to test this series for convergence is from the limit comparison test.

I know the series converges I just can't prove it
 
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  • #2
How does sin(1/n) compare with 1/n? How about with 1/(2n)? Think integral test with comparison series like that.
 
Last edited:
  • #3
So , 1/n is always greater than sin(1/n)

If I just replace sin(1/n) with 1/n , I have a "an" that is less than the original and the new function diverges by the integral test.

So the sum is divergent by the comparison test?
 
  • #4
craig16 said:
So , 1/n is always greater than sin(1/n)

If I just replace sin(1/n) with 1/n , I have a "an" that is less than the original and the new function diverges by the integral test.

So the sum is divergent by the comparison test?

You've got a divergent series that's greater than the original. That doesn't prove divergence. You need one that's less than the original. That's why I also suggested 1/(2n).
 
  • #6
Yes, Wolfram Alpha is quite wrong. Unless you believe 962 is the same as infinity.
 
  • #7
Awesome :)

Thank you, I've spent way too much time thinking about that sum, lol.
 
  • #8
craig16 said:
Okay, I see how the test would work replacing sin(1/n) with 1/2n making the new equation always less than the original. Then the integral test works and the sum is divergent.

Does that mean that wolfram is wrong though?

http://www.wolframalpha.com/input/?i=sum from 2 to inf (sin(1/n))/(sqrt(ln(n)))&t=macw01

When I plug the sum in it tells me it converges to 962 or so.

A good way to test if Wolfram-Alpha is lying is to click the More Digits button. If the number changes when you do that, W-A is probably lying to you.
 

1. What is the purpose of convergence tests for series?

Convergence tests for series are used to determine whether an infinite series, which is a sum of infinitely many terms, converges to a finite value or not. This helps us determine the behavior and properties of the series and whether it can be used in calculations or not.

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

Absolute convergence means that a series converges to a finite value regardless of the order in which the terms are added. Conditional convergence means that a series only converges if the terms are added in a specific order. For example, the alternating harmonic series is conditionally convergent, while the regular harmonic series is absolutely convergent.

3. What is the most commonly used convergence test for series?

The most commonly used test is the ratio test, which compares the ratio of consecutive terms in a series to a limit to determine whether the series converges or diverges.

4. How do you determine the convergence of a power series?

The convergence of a power series can be determined by using the root test, which compares the nth root of the absolute value of the series to a limit to determine convergence or divergence. If the limit is less than 1, the series converges, and if it is greater than 1, the series diverges.

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

No, a series can only converge to one value. If a series converges, it means that the sum of its terms approaches a finite value as the number of terms increases. If a series were to converge to more than one value, it would imply that the sum of its terms is both finite and infinite, which is not possible.

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