Assertion: set's boundary | /r/HasProperty | closed
Facets:
value: always
type: DEGREE
Source sentences:
text: Since the boundary of a set is closed, ∂∂S = ∂∂∂S for any set S.
text: These last two examples illustrate the fact that the boundary of a dense set with empty interior is its closure.
text: The interior and exterior are always open while the boundary is always closed.
text: The boundary of every open set is closed and nowhere dense.