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In topology, the closure of a set A is the intersection of all closed sets containing A. In metric spaces, this can be also defined as the set of all limit points of the set A.

A closure operator is an abstract (category theory) form of the topological notion of closure which can be applied to any set A. It is a function cl from A to the power set of A satisfying the following conditions:

In the category of topological spaces, this operator is isomorphic to the standard topological one.