Wednesday, January 9, 2019

calculus - The series $sum_{n=2}^{infty }frac{1}{log (n!)}$



I'm trying to find out whether this Series

$\sum_{n=2}^{\infty } a_{n}$ converges or not when
$$a_{n}=\frac{1}{\log (n!)}$$



I tried couple of methods, among them: d'Alembert $\frac{a_{n+1}}{a_{n}}$, Cauchy condensation test $\sum_{n=2}^{\infty } 2^{n}a_{2^n}$, and they both didn't work for me.



Edit: I can't use stirling, and integral.



Thank you


Answer



Hint: You can use $$a_n = \frac{1}{\log n!} = \frac{1}{\sum_{k=1}^n \log k} \geq

\frac{1}{n \log n}.$$
Then use the Cauchy condensation test...


No comments:

Post a Comment

analysis - Injection, making bijection

I have injection $f \colon A \rightarrow B$ and I want to get bijection. Can I just resting codomain to $f(A)$? I know that every function i...