Sunday, 8 April 2018

sequences and series - Is sumin=3nftyfrac1nlogn absolutely convergent, conditionally convergent or divergent?



Classify
n=31nlog(n)
as absolutely convergent, conditionally convergent or divergent.



Is it,
n=31n is a divergent p-series as p=1, and

lim by comparison test. And this converges to 0.



So,
\sum_{n=3}^\infty \frac{1}{n\log (n)}
is conditionally convergence?



I'm not sure if I'm doing right or not. Could you guide me?



Thanks in advance! :)


Answer



Note that if your given series is convergent then it's also absolutely convergent since \frac{1}{n\log n}\geq 0\quad \forall n\geq 3.




Now, since the sequence (\frac{1}{n\log n}) is decreasing to 0 so by the integral test your series has the same nature that the improper integral
\int_3^\infty\frac{dx}{x\log x}=\left[\log(\log(x))\right]_3^{\to\infty}=\infty
hence the series is divergent.


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