Boundary (topology) - Wikipedia
https://en.wikipedia.org/wiki/Boundary_(topology)In topology and mathematics in general, the boundary of a subset S of a topological space X is the set of points which can be approached both from S and from the outside of S. More precisely, it is the set of points in the closure of not belonging to the interior of An element of the boundary of is called a boundary point of The term boundary operation refers to finding or taking the boundary o…
Bounded set - Wikipedia
https://en.wikipedia.org/wiki/Bounded_setAn artist's impression of a bounded set (top) and of an unbounded set (bottom). The set at the bottom continues forever towards the right. "Bounded" and "boundary" are distinct concepts; for the latter see boundary (topology). A circle in isolation is a boundaryless bounded set, while the half plane is unbounded yet has a boundary.
Boundary Point of a Set | eMathZone
www.emathzone.com › boundary-point-of-a-setLet A be a subset of a topological space X, a point x ∈ X is said to be boundary point or frontier point of A if each open set containing at x intersects both A and A c. The set of all boundary points of a set A is called the boundary of A or the frontier of A. It is denoted by F r ( A). Since, by definition, each boundary point of A is also a boundary point of A c and vice versa, so the boundary of A is the same as that of A c, i.e. F r ( A) = F r ( A c).