Boundary Point of a Set | eMathZone
www.emathzone.com › boundary-point-of-a-setBoundary Point of a Set. Let 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).