Friday 27 December 2019

real analysis - Does the series $sum_{nge1}frac{lnleft(frac{n+1}nright)}{sqrt n}$ converge?



Could you please give me some hint how to decide about convergence of the series
$\sum_{n\ge1}\frac{ln\left(\frac{n+1}n\right)}{\sqrt n}$ ?




I tried using comparison test:
$\frac{ln\left(\frac{n+1}n\right)}{\sqrt n}\ge \frac {ln\left(\frac1n\right)}{\sqrt n}=-\frac {ln(n)}{\sqrt n}$.
Series $\sum_{n\ge1}\frac{ln(n)}{\sqrt n}$ diverges by integral test, but for comparison test all compared sequenced must be non-negative and $ln\left(\frac1n\right)\le0$ for all n.



Thanks.


Answer



Yes it does using the asymptotic comparison:



$$\frac{\ln\left(\frac{n+1}n\right)}{\sqrt n}\sim_\infty\frac{1}{n\sqrt n}$$




Remark For the comparison test the general term of the series must have an unchanged sign i.e. not alternating series (no matter positive or negative).


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