Boundary (topology)

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In topology and mathematics in general, the boundary of a subset S of a topological space X is the set of points in the closure of S not belonging to the interior of S. An element of the boundary of S is called a boundary point of S. The term boundary operation refers to finding or taking the boundary of a set. Notations used for boundary of a set S include bd ⁡ ( S ) , fr ⁡ ( S ) , {\displaystyle \operatorname {bd} (S),\operatorname {fr} (S),} and ∂ S {\displaystyle \partial S} .

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