Wednesday 29 June 2016

real analysis - Studying the character of $sum_{n=3}^infty frac{1}{n(log(log n))^{alpha}}$




I have to study the character of this series
$$\sum_{n=3}^\infty \frac{1}{n(\log(\log n))^{\alpha}}$$



with $\alpha$ a real parameter.



Considering the Cauchy condensation test, the equivalent series is:
$$\sum_{n=1}^\infty 2^n\frac{1}{2^n[\log(\log 2^n)]^{\alpha}}=\sum_{n=1}^\infty \frac{1}{[\log(n\log 2)]^{\alpha}}$$



Using the ratio test:




$$\lim_{n\rightarrow \infty} \frac{[\log(n\log 2)]^{\alpha}}{[\log((n+1)\log 2)]^{\alpha}}=\lim_{n\rightarrow \infty} \frac{1}{[\log(\log 2)]^{\alpha}}= \frac{1}{[\log(\log 2)]^{\alpha}}\sim \frac{1}{(-0,36)^{\alpha}}$$



Then when $(-0,36)^{\alpha}>1$ the given series converges, otherwise it diverges



if $\alpha =1$ ,$ -0,36<1$ , diverges



if $\alpha =0$ ,$ 1<1$ , diverges



if $\alpha =-1$ ,$ (-0,36)^{-1}=-2,77<1$ , converges




if $\alpha >1$ ,$ (-0,36)^{\alpha}<1$ , converges



if $\alpha <-1$ ,$ (-0,36)^{\alpha}<1$ , converges



if $|\alpha| <1 , \ne 0$ sometimes it doen't exist $ (-0,36)^{\alpha}$
Can someone help me?


Answer



HINT




Let use limit comparison test with



$$\sum \frac{1}{n}$$


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