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).
Boundary (topology) - Wikipedia
https://en.wikipedia.org/wiki/Boundary_(topology)There are several equivalent definitions for the boundary of a subset of a topological space which will be denoted by or simply if is understood: 1. It is the closure of minus the interior of in : ∂ S := S ¯ ∖ int X S {\displaystyle \partial S~:=~{\overline {S}}\setminus \operatorname {int} _{X}S} where denotes the closure of in and denotes the topological interior of in