Answer
Diverges
Work Step by Step
Here, we have $|a_n|=\Sigma_{n=1}^{\infty}(2\sqrt[n] n+1)^n$
Root Test states that when $\Sigma a_n$ is an infinite series with positive terms and, then $r=\lim\limits_{n \to \infty}\sqrt[n] |a_n|$
a) When $0 \leq r \lt 1$, the series converges. (b) When $r \gt 1$, or, $\infty$, so the series diverges. (c) When $r=1$, the ratio test is inconclusive.
Now, $r=\lim\limits_{n \to \infty}\sqrt[n] {(2\sqrt[n] n+1)^n}\\=\lim\limits_{n \to \infty} 2\sqrt[n] n+1 \\=\lim\limits_{n \to \infty} 2 n^{1/n}+1\\=3 \gt 1$
Therefore, the given series diverges by the root test.
