# Math Help - prove this result please

2. ## result

i know this result, alpha <= 1, it is then divergent...but how to prove that?

3. When $\alpha > 1$, use the integral test.

4. ## cheers

cheers mate!!!

5. Originally Posted by szpengchao
I

$\sum\frac{1}{n\ln(n)}$

$\int_3^{\infty}\frac{dn}{n\ln(n)}=\ln(\ln(n))\bigg |_3^{\infty}=\infty$

divergent

ii

$\sum_{n=3}^{\infty}\frac{1}{n\ln(n)(\ln\ln(n)))^2}$

$\int\frac{dn}{n\ln(n)(\ln\ln(n)))^2}=\frac{-1}{\ln\ln(n))}\bigg|_3^{\infty}=\frac{1}{\ln(\ln(3 ))}$

convergent

iii

$\sum_{n=3}^{\infty}\frac{1}{n^{1+\frac{1}{n}}\ln(n )}$

Limit comparison test with $\frac{1}{n\ln(n)}$

$\lim_{n\to\infty}\frac{\frac{1}{n\ln(n)}}{\frac{1} {n^{1+\frac{1}{n}}\ln(n)}}=\lim_{n\to\infty}n^{\fr ac{1}{n}}\cdot\lim_{n\to\infty}\frac{n\ln(n)}{n\ln (n)}$

Side note $L=\lim_{n\to\infty}n^{\frac{1}{n}}\Rightarrow{ln(L )=\lim_{n\to\infty}\frac{\ln(n)}{n}=0}$

so $\ln(L)=0\Rightarrow{L=e^0=1}$

so $\lim_{n\to\infty}n^{\frac{1}{n}}\cdot\lim_{n\to\in fty}\frac{n\ln(n)}{n\ln(n)}=1\cdot{1}=1$

(EDIT: I should rephrase so you dont misinterpret) Since the limit converges to a finite value and the comparison series diverges, this series diverges