# Integral Test for Limits clarification

• Aug 20th 2012, 09:21 PM
hayjude99
Integral Test for Limits clarification
It is my understanding that for the integral test to apply, the series a must be decreasing, positive, and continuous over [n, ∞) but one of the problems I've encountered seems to break the first rule, yet my textbook still says it converges. I don't know what I am doing incorrectly.

Summation of [ln(n)]/(n2) starting at n=1, counting to infinity
f(x) = [ln(n)]/(n2)
f'(x) = [1 - 2ln(n)]/(n3)

0 = 1 - 2ln(n)
n = e.5 = 1.645

When you do a sign test on either side, you discover that after, to the right, f'(x) is negative, which to the left it is positive. This would leave me to believe that you could not apply the integral test here, yet the book I'm using put this in with the section about the integral test and says it diverges, which leads me to believe I'm missing something.

Thanks

PS sorry for the lack of formatting, I tried to get it to work, but it wasn't cooperating.
• Aug 21st 2012, 12:19 AM
zaidalyafey
Re: Integral Test for Limits clarification
it doesn't have to be decreasing from the beginning I mean from n=1 all the way to infinity ,,, but it is sufficient to do so for n sufficiently large ...

in your example it is increasing ,, then decreasing , but as you stated it has to be decreasing from (n,infinity )
• Aug 21st 2012, 02:01 PM
hayjude99
Re: Integral Test for Limits clarification
Can you clarify?
• Aug 21st 2012, 11:11 PM
zaidalyafey
Re: Integral Test for Limits clarification
Quote:

Originally Posted by hayjude99
Can you clarify?

if you have the function

$\displaystyle f(x)=\frac{ln(x)}{x^2}$ on the interval$\displaystyle \, [2,\infty)$

Can you tell me whether the integral test conditions can be applied to this function?
• Aug 22nd 2012, 12:29 AM
hayjude99
Re: Integral Test for Limits clarification
Yes, they can. Are you saying that basically the first few terms don't matter, so as long as it eventually decreases, the integral test applies?

Also in the OP, I meant to say that the book says it converges. xP sorry. Though I suppose the question is still valid of whether or not I can use the integral test.
• Aug 22nd 2012, 01:02 AM
zaidalyafey
Re: Integral Test for Limits clarification
Quote:

Originally Posted by hayjude99
Are you saying that basically the first few terms don't matter, so as long as it eventually decreases, the integral test applies?

absolutely correct !

Quote:

Originally Posted by hayjude99
It is my understanding that for the integral test to apply, the series a must be decreasing, positive, and continuous over [n, ∞)

See that the condition [n, ∞) states that , so we have to find n such that all the conditions apply .

Best regards ...
• Aug 22nd 2012, 01:05 AM
zaidalyafey
Re: Integral Test for Limits clarification
Quote:

Originally Posted by hayjude99
PS sorry for the lack of formatting, I tried to get it to work, but it wasn't cooperating.

You can quote my reply to see how it works (Happy)
• Aug 22nd 2012, 08:24 AM
hayjude99
Re: Integral Test for Limits clarification
Oh alright, thanks :)
I shall!