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.