$\sum_{n=1}^{\infty} \frac{a_n}{n}$

$\sum_{n=1}^{\infty} (a_n)^2$ (assuming $a_n > 0$)

$\sum_{n=1}^{\infty} 2a_n$

$\sum_{n=1}^{\infty} a_n^{1/2}$ (assuming $a_n \geq 0$)