Kuratowski closure axioms
In topology and related branches of mathematics, the Kuratowski closure axioms are a set of axioms that can be used to define a topological structure on a set. They are equivalent to the more commonly used open set definition. They were first introduced by Kazimierz Kuratowski.
A similar set of axioms can be used to define a topological structure using only the dual notion of interior operator.
- (Preservation of Nullary Union)
- (Preservation of Binary Union)
If the last axiom, idempotence, is omitted, then the axioms define a preclosure operator.
A consequence of the third axiom is: (Preservation of Inclusion).
The four Kuratowski closure axioms can be replaced by a single condition, namely,
Connection to other axiomatizations of topology
Induction of Topology
A closure operator naturally induces a topology as follows:
A subset is called closed if and only if .
Empty Set and Entire Space are closed:
By extensitivity, and since closure maps the power set of into itself (that is, the image of any subset is a subset of ), we have . Thus is closed.
The preservation of nullary unions states that . Thus is closed.
Arbitrary intersections of closed sets are closed:
Let be an arbitrary set of indices and closed for every .
Also, by preservation of inclusions,
Therefore, . Thus is closed.
Finite unions of closed sets are closed:
Let be a finite set of indices and let be closed for every .
From the preservation of binary unions and using induction we have . Thus is closed.
Induction of closure
In any induced topology (relative to the subset A) the closed sets induce a new closure operator that is just the original closure operator restricted to A: 
Recovering notions from topology
A point is close to a subset iff .
A function is continuous at a point iff .
- Kuratowski 1966, p. 38
- Kuratowski (1966) has a fifth (optional) axiom stating that singleton sets are their own closures. He refers to topological spaces which satisfy all five axioms as T1 spaces in contrast to the more general spaces which only satisfy the four listed axioms.
- Pervin 1964, p. 43
- Pervin 1964, p. 42
- Pervin 1964, p. 49, Theorem 3.4.3
- Pervin, William J. (1964), Foundations of General Topology, Academic Press