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Tag: exterior

In topology, the exterior of a subset S of a topological space X is the union of all open sets of X which are disjoint from S. It is itself an open set and is disjoint from S. The exterior of S is denoted by or The exterior is equal to X  S, the complement of the topological closure of S and to the interior of the complement of S in X. Many properties follow in a straightforward way from those of the interior operator, such as the following. Unlike the interior operator, ext is not idempotent, but the following holds: