Solution
= Solution
The radius is $R=\sup\{r\geq0:\sum a_nz^n\text{ converges whenever }|z|\lt r\}$; equivalently $R^{-1}=\limsup|a_n|^{1/n}$. The same definition applies over $\mathbb R$ and $\mathbb C$.
= Solution
The radius is $R=\sup\{r\geq0:\sum a_nz^n\text{ converges whenever }|z|\lt r\}$; equivalently $R^{-1}=\limsup|a_n|^{1/n}$. The same definition applies over $\mathbb R$ and $\mathbb C$.