Convergence of a Challenging Series with an Additional Factor

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Homework Help Overview

The discussion revolves around the convergence of the series \(\sum_{n=2}^{\infty} \frac{n}{(n^2-5)(\ln n)^2}\), focusing on the implications of an additional factor in the series.

Discussion Character

  • Exploratory, Assumption checking, Mathematical reasoning

Approaches and Questions Raised

  • Participants discuss various convergence tests, including Limit Comparison and the Ratio Test, noting challenges with these methods. There is an attempt to simplify the problem by comparing the series to \(\frac{2}{n}\) for large \(n\). Some participants express confusion about the implications of this comparison and the role of the additional factor \(\frac{1}{(\ln n)^2}\).

Discussion Status

The discussion is ongoing, with participants exploring different approaches to determine convergence. Some guidance has been offered regarding the use of the Integral Test, but there is no consensus on the best method or the convergence of the series itself.

Contextual Notes

Participants are considering the impact of the additional factor \(\frac{1}{(\ln n)^2}\) on convergence, and there is a mention of the need to check the existence of an integral related to the series.

simba924
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Homework Statement



[tex]\sum[/tex] from n=2 to [tex]\infty[/tex] of n/((n2-5)*(ln n)2)

Homework Equations





The Attempt at a Solution


I've tried Limit Comparison but I always get a limit of 0 which will not work. Ratio test doesn't help. I don't think a direct comparison can be made but that seems to be the only other option...
 
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Hi simba924!

To simplify the problem a bit, show that

[tex]\frac{n}{n^2-5}\le\frac{2}{n}[/tex]

for large n. Then apply the http://en.wikipedia.org/wiki/Integral_test_for_convergence" .
 
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Ahh thanks. Can you explain that in a little more detail though, I don't really get it.
 
Hey I still need some help with this one. I'm pretty sure that [tex]\sum[/tex] 2/n is not convergent because it is in form integer/np where p=1

Can anyone help?
 
simba924 said:
Hey I still need some help with this one. I'm pretty sure that [tex]\sum[/tex] 2/n is not convergent because it is in form integer/np where p=1

Can anyone help?

Remember, you still have the additional factor 1/(ln n)^2 (without which it would diverge). To do the integral test you have to check that

[tex]\int_2^{\infty}\frac{1}{x(\log(x))^2}[/tex]

exists (you can find an explicit antiderivative, Hint: substitution).
 

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