Friday, 22 July 2016

Alternating series: $sumlimits_{n= 1}^{infty} (-1)^{n-1} frac{ln(n)}{n}$ convergence?




Determine whether the series converges absolutely, conditionally or diverges?



$$\sum\limits_{n= 1}^{\infty} (-1)^{n-1} \frac{\ln(n)}{n}$$



I know that $\sum|a_{n}|$ diverges by using the comparison test:



$$\frac{\ln(n)}{n} > \frac{1}{n}$$ and the smaller, r.h.s being the divergent harmonic series.



So, should my conclusion for the alternating series be divergent or convergent conditionally*?







* How to estimate whether the alternating series terms are cancelling?


Answer



Let $f(x)=\frac{\ln x}{x}$ so $f'(x)=\frac{1-\ln x}{x^2}\le 0$ for $x\ge e$ and so the sequence $\left(\frac{\ln n}{n}\right)_{n\ge3}$ is decreasing to $0$. Apply now the alternating series criteria to conclude the convergence of the series.


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