Friday 19 June 2015

sequences and series - Is this :$sum_{n=1}^{infty } frac{tan(frac{1}{n!})}{arctan (n!)}$ convergent sum?



How do I evaluate this sum :$$\displaystyle\sum_{n=1}^{\infty } \ \frac{\tan(\frac{1}{n!})}{\arctan ({n!})}$$ if it is convergent ?.



Note: I think the limit of it's general term is $0$ as shown here in WA.

and i will surprised if it is convergent



Note: I edited the question beacuse i meant $ arctan(n!)$ in the denominator


Answer



One has, as $n \to \infty$,
$$
\frac{\tan \frac1{n!}}{\arctan (n!)}=\frac{\tan \frac1{n!}}{\frac{\pi}2-\arctan \frac1{n!}}\sim \frac2{\pi} \cdot \frac1{n!}
$$ then by the comparison test the series $ \displaystyle \sum_{n\ge1}\frac{\tan \frac1{n!}}{\arctan n!}$ is convergent.


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