Boundary Sets (𝜕·, 𝑑𝑏(·), 𝑓𝑟(·)) the boundary of a set 𝑆 is the set of points in the closure of 𝑆 not belonging to the interior of 𝑆 Boundary Point a boundary point is an element of a boundary set Boundaryless TODO Other Topological Spaces (Boundary - Boundaryless) Totally Boundedness)