# Definition:Complex Disk/Closed

## Definition

Let $a \in \C$ be a complex number.

Let $R>0$ be a real number.

The closed (complex) disk of center $a$ and radius $R$ is the set:

$B(a, R) = \{z\in \C : |z-a| \leq R\}$

where $|\cdot|$ denotes complex modulus.