TITLE: Does the series $\sum\limits_{n=1}^\infty \frac{1}{n\sqrt[n]{n}}$ converge?
QUESTION [10 upvotes]: Does the following series converge? $$\sum_{n=1}^\infty \frac{1}{n\sqrt[n]{n}}$$

As $$\frac{1}{n\sqrt[n]{n}}=\frac{1}{n^{1+\frac{1}{n}}},$$
I was thinking that you may consider this as a p-series with $p>1$. But I'm not sure if this is correct, as with p-series, p is a fixed number, right ? On the other hand, $1+\frac{1}{n}>1$ for all $n$. Any hints ?

REPLY [2 votes]: AM-GM gives
$$
\sqrt[n]{n}\leq\frac{1}{n}(n+\underbrace{1+\cdots+1}_{n-1})=\frac{1}{n}(2n-1)<\frac{1}{n}2n\implies\frac{1}{n\sqrt[n]{n}}>\frac{1}{2}\frac{1}{n}
$$
so
$$
\sum_{n=1}^M\frac{1}{n\sqrt[n]{n}}>\frac{1}{2}\left(\sum_{n=1}^M\frac{1}{n}\right)\cdot
$$