Saturday, 29 October 2016

calculus - convergence of $sum_{n=1}^{infty} frac {1}{log(e^n+e^{-n})}$?



Test convergence of $\sum_{n=1}^{\infty} \dfrac {1}{\log(e^n+e^{-n})}$



Attempt: I have tried the integral test, the comparison test ( for which I couldn't find a suitable comparator).



However, I haven't been able to proceed much. Could anyone please give me a direction on how to proceed ahead with this problem.



Thank you very much for your help in this regard.



Answer



$e^{n}+e^{-n}< 2e^{n}$, hence
$$\frac{1}{\log(e^n+e^{-n})}>\frac{1}{n+\log 2}$$
and the series is divergent by direct comparison with the harmonic series.


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