Closure
From Conservapedia
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:
-
(augmentation)
- If
then 
(monotonicity)
-
(idempotence)
In the category of topological spaces, this operator is isomorphic to the standard topological one.
