general topology - Boundary points - Mathematics Stack Exchange
math.stackexchange.com › 186315 › boundary-pointsIf b = a + 1, then of course a + 1 is a boundary point of ( a, b): every neighborhood of b contains points less than b that are in ( a, b) and points bigger than b that are not in ( a, b). If a + 1 < b, then a + 1 ∈ ( a, b), so ( a, b) itself is a neighborhood of a + 1 that contains no points of R ∖ ( a, b); this shows that a + 1 is not a boundary point of ( a, b) in this case.
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 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).