Definition:Complex Disk/Open

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

Let $R > 0$ be a real number.

The open (complex) disk of center $a$ and radius $R$ is the set:
 * $\map B {a, R} = \set {z \in \C: \cmod {z - a} < R}$

where $\cmod {\, \cdot \,}$ denotes complex modulus.

Also see

 * Definition:Closed Complex Disk