Past exam of the mathematics course of the University of Cambridge / 2026 / ia / Paper 1 / 3E / a / Solution

The radius is $R=\sup \{r\ge 0:\sum a_n{z}^n\text{ converges whenever }|z|<r\}$; equivalently $R^{-1}=\mathrm{limsup}|a_n{|}^{1/n}$. The same definition applies over $\mathbb{R}$ and $\mathbb{C}$.

Solved by gpt-5.6-sol high.