The set of all points that satisfy the inequality lz 20 e, where e is S a positive real number, is called an open disk centered at 20 with radius and denoted as B (20, €). It is also called the e-neighborhood of zo, or simply a neighborhood of 20. In Figure 4.1, the dashed boundary curve means that the boundary points do not belong to the set. The neighborhood lzl 1 is called the open unit disk.