\sum_{n=0}^\infty a_n(x - x_0)^n
for more, see semester 2
l = \lim_{n \to \infty} |\frac{a_{n+1}}{a_n}| or l = \lim_{n \to \infty} \sqrt[n]{|a_n|}
(Note: here l \ne l)
then:
R = \begin{cases} \frac{1}{l} & \text{if}\ l > 0 \\ \infty & \text{if}\ l = 0 \\ 0 & \text{if}\ l = \infty \\ \end{cases}