Definition 13.2.2. Open Disk, Boundary and Interior Points, Open and Closed Sets, Bounded Sets.
Let be a set of points in A point in is a boundary point of if all open disks centered at contain both points in and points not in
A point in is an interior point of if there is an open disk centered at that contains only points in
A set is closed if it contains all of its boundary points.
A set is bounded if there is an such that the open disk, centered at the origin with radius contains A set that is not bounded is unbounded.