The [[Radius of Convergence]] is the radius $R \in \R$ where any value $x \in \C$ given to some [[Series]] $f(x): \C \to \C$ convergences when $|x-a| < R$ and [[Divergent Series|Diverges]] when $|x-a| > R$, where $a$ is some constant of the center of the disc of [[Convergent Series|Convergence]]. To find the [[Radius of Convergence]], you can use the [[Ratio Test]]