Let XXX be a topological space. The boundary ∂A\partial A∂A of a set A⊂XA \subset XA⊂X is the intersection of the closure of AAA and the closure of the complement of AAA. That is, ∂A=A‾∩(X∖A)‾.\partial A = \overline A \cap \overline{(X\setminus A)}.∂A=A∩(X∖A).
Wikidata ID: Q875399