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 formalized by Kazimierz Kuratowski,^{[1]} and the idea was further studied by mathematicians such as Wacław Sierpiński and António Monteiro,^{[2]} among others.
A similar set of axioms can be used to define a topological structure using only the dual notion of interior operator.^{[3]}
Definition[edit]
Kuratowski closure operators and weakenings[edit]
Let be an arbitrary set and its power set. A Kuratowski closure operator is a unary operation with the following properties:
[K1] It preserves the empty set: ;[K2] It is extensive: for all , ;
[K3] It is idempotent: for all , ;
[K4] It preserves/distributes over binary unions: for all , .
A consequence of preserving binary unions is the following condition:^{[4]}
[K4'] It is isotonic: .
In fact if we rewrite the equality in [K4] as an inclusion, giving the weaker axiom [K4''] (subadditivity):
[K4''] It is subadditive: for all , ,
then it is easy to see that axioms [K4'] and [K4''] together are equivalent to [K4] (see the nexttolast paragraph of Proof 2 below).
Kuratowski (1966) includes a fifth (optional) axiom requiring that singleton sets should be stable under closure: for all , . He refers to topological spaces which satisfy all five axioms as T_{1}spaces in contrast to the more general spaces which only satisfy the four listed axioms. Indeed, these spaces correspond exactly to the topological T_{1}spaces via the usual correspondence (see below).^{[5]}
If requirement [K3] is omitted, then the axioms define a Čech closure operator.^{[6]} If [K1] is omitted instead, then an operator satisfying [K2], [K3] and [K4'] is said to be a Moore closure operator.^{[7]} A pair is called Kuratowski, Čech or Moore closure space depending on the axioms satisfied by .
Alternative axiomatizations[edit]
The four Kuratowski closure axioms can be replaced by a single condition, given by Pervin:^{[8]}
[P] For all , .
Axioms [K1]—[K4] can be derived as a consequence of this requirement:
 Choose . Then , or . This immediately implies [K1].
 Choose an arbitrary and . Then, applying axiom [K1], , implying [K2].
 Choose and an arbitrary . Then, applying axiom [K1], , which is [K3].
 Choose arbitrary . Applying axioms [K1]—[K3], one derives [K4].
Alternatively, Monteiro (1945) had proposed a weaker axiom that only entails [K2]—[K4]:^{[9]}
[M] For all , .
Requirement [K1] is independent of [M] : indeed, if , the operator defined by the constant assignment satisfies [M] but does not preserve the empty set, since . Notice that, by definition, any operator satisfying [M] is a Moore closure operator.
A more symmetric alternative to [M] was also proven by M. O. Botelho and M. H. Teixeira to imply axioms [K2]—[K4]:^{[2]}
[BT] For all , .
Analogous structures[edit]
Interior, exterior and boundary operators[edit]
A dual notion to Kuratowski closure operators is that of Kuratowski interior operator, which is a map satisfying the following similar requirements:^{[3]}
[I1] It preserves the total space: ;[I2] It is intensive: for all , ;
[I3] It is idempotent: for all , ;
[I4] It preserves binary intersections: for all , .
For these operators, one can reach conclusions that are completely analogous to what was inferred for Kuratowski closures. For example, all Kuratowski interior operators are isotonic, i.e. they satisfy [K4'], and because of intensivity [I2], it is possible to weaken the equality in [I3] to a simple inclusion.
The duality between Kuratowski closures and interiors is provided by the natural complement operator on , the map sending . This map is an orthocomplementation on the power set lattice, meaning it satisfies De Morgan's laws: if is an arbitrary set of indices and ,
Pervin (1964) further provides analogous axioms for Kuratowski exterior operators^{[3]} and Kuratowski boundary operators,^{[10]} which also induce Kuratowski closures via the relations and .
Abstract operators[edit]
Notice that axioms [K1]—[K4] may be adapted to define an abstract unary operation on a general bounded lattice , by formally substituting settheoretic inclusion with the partial order associated to the lattice, settheoretic union with the join operation, and settheoretic intersections with the meet operation; similarly for axioms [I1]—[I4]. If the lattice is orthocomplemented, these two abstract operations induce one another in the usual way. Abstract closure or interior operators can be used to define a generalized topology on the lattice.
Since neither unions nor the empty set appear in the requirement for a Moore closure operator, the definition may be adapted to define an abstract unary operator on an arbitrary poset .
Connection to other axiomatizations of topology[edit]
Induction of topology from closure[edit]
A closure operator naturally induces a topology as follows. Let be an arbitrary set. We shall say that a subset is closed with respect to a Kuratowski closure operator if and only if it is a fixed point of said operator, or in other words it is stable under , i.e. . The claim is that the family of all subsets of the total space that are complements of closed sets satisfies the three usual requirements for a topology, or equivalently, the family of all closed sets satisfies the following:
[T1] It is a bounded sublattice of , i.e. ;[T2] It is complete under arbitrary intersections, i.e. if is an arbitrary set of indices and , then ;
[T3] It is complete under finite unions, i.e. if is a finite set of indices and , then .
Notice that, by idempotency [K3], one may succinctly write .
Proof 1.


[T1] By extensivity [K2], and since closure maps the power set of into itself (that is, the image of any subset is a subset of ), we have . Thus . The preservation of the empty set [K1] readily implies . [T2] Next, let be an arbitrary set of indices and let be closed for every . By extensivity [K2], . Also, by isotonicity [K4'], if for all indices , then for all , which implies . Therefore, , meaning . [T3] Finally, let be a finite set of indices and let be closed for every . From the preservation of binary unions [K4], and using induction on the number of subsets of which we take the union, we have . Thus, . 
Induction of closure from topology[edit]
Conversely, given a family satisfying axioms [T1]—[T3], it is possible to construct a Kuratowski closure operator in the following way: if and is the inclusion upset of , then
Proof 2.


[K1] Since , reduces to the intersection of all sets in the family ; but by axiom [T1], so the intersection collapses to the null set and [K1] follows. [K2] By definition of , we have that for all , and thus must be contained in the intersection of all such sets. Hence follows extensivity [K2]. [K3] Notice that, for all , the family contains itself as a minimal element w.r.t. inclusion. Hence , which is idempotence [K3]. [K4’] Let : then , and thus . Since the latter family may contain more elements than the former, we find , which is isotonicity [K4']. Notice that isotonicity implies and , which together imply . [K4] Finally, fix . Axiom [T2] implies ; furthermore, axiom [T2] implies that . By extensivity [K2] one has and , so that . But , so that all in all . Since then is a minimal element of w.r.t. inclusion, we find . Point 4. ensures additivity [K4]. 
Exact correspondence between the two structures[edit]
In fact, these two complementary constructions are inverse to one another: if is the collection of all Kuratowski closure operators on , and is the collection of all families consisting of complements of all sets in a topology, i.e. the collection of all families satisfying [T1]—[T3], then such that is a bijection, whose inverse is given by the assignment .
Proof 3.


First we prove that , the identity operator on . For a given Kuratowski closure , define ; then if its primed closure is the intersection of all stable sets that contain . Its nonprimed closure satisfies this description: by extensivity [K2] we have , and by idempotence [K3] we have , and thus . Now, let such that : by isotonicity [K4'] we have , and since we conclude that . Hence is the minimal element of w.r.t. inclusion, implying . Now we prove that . If and is the family of all sets that are stable under , the result follows if both and . Let : hence . Since is the intersection of an arbitrary subfamily of , and the latter is complete under arbitrary intersections by [T2], then . Conversely, if , then is the minimal superset of that is contained in . But that is trivially itself, implying . 
We observe that one may also extend the bijection to the collection of all Čech closure operators, which strictly contains ; this extension is also surjective, which signifies that all Čech closure operators on also induce a topology on .^{[11]} However, this means that is no longer a bijection.
Examples[edit]
This section needs expansion. You can help by adding to it. (August 2019) 
 As discussed above, given a topological space we may define the closure of any subset to be the set , i.e. the intersection of all closed sets of which contain . The set is the smallest closed set of containing , and the operator is a Kuratowski closure operator.
 If is any set, the operators such that are Kuratowski closures. The first induces the indiscrete topology , while the second induces the discrete topology .
 Fix an arbitrary , and let be such that for all . Then defines a Kuratowski closure; the corresponding family of closed sets coincides with , the family of all subsets that contain . When , we once again retrieve the discrete topology (i.e. , as can be seen from the definitions).
 If is a cardinal number such that , then the operator such thatsatisfies all four Kuratowski axioms.^{[12]} In the case where , if , this operator induces the cofinite topology on ; if , it induces the cocountable topology.
Properties[edit]
 Since any Kuratowski closure is isotonic, and so is obviously any inclusion mapping, one has the (isotonic) Galois connection , provided one views as a poset with respect to inclusion, and as a subposet of . Indeed, it can be easily verified that, for all and , if and only if .
 If is a subfamily of , then
 If , then .
Topological concepts in terms of closure[edit]
Refinements and subspaces[edit]
A pair of Kuratowski closures such that for all induce topologies such that , and vice versa. In other words, dominates if and only if the topology induced by the latter is a refinement of the topology induced by the former, or equivalently .^{[13]} For example, clearly dominates (the latter just being the identity on ). Since the same conclusion can be reached substituting with the family containing the complements of all its members, if is endowed with the partial order for all and is endowed with the refinement order, then we may conclude that is an antitonic mapping between posets.
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: , for all .^{[14]}
Continuous maps, closed maps and homeomorphisms[edit]
A function is continuous at a point iff , and it is continuous everywhere iff
Separation axioms[edit]
Let be a Kuratowski closure space. Then
 is a T_{0}space iff implies ;^{[18]}
 is a T_{1}space iff for all ;^{[19]}
 is a T_{2}space iff implies that there exists a set such that both and , where is the set complement operator.^{[20]}
Closeness and separation[edit]
A point is close to a subset if This can be used to define a proximity relation on the points and subsets of a set.^{[21]}
Two sets are separated iff . The space is connected iff it cannot be written as the union of two separated subsets.^{[22]}
See also[edit]
 Topological space
 Closure operator
 Čech closure operator
 Closure algebra
 Characterizations of the category of topological spaces
Notes[edit]
 ^ Kuratowski (1922).
 ^ ^{a} ^{b} Monteiro (1945), p. 160 .
 ^ ^{a} ^{b} ^{c} Pervin (1964), p. 44.
 ^ Pervin (1964), p. 43, Exercise 6.
 ^ Kuratowski (1966), p. 38.
 ^ Arkhangel'skij & Fedorchuk (1990), p. 25.
 ^ "Moore closure". nLab. March 7, 2015. Retrieved August 19, 2019.
 ^ Pervin (1964), p. 42, Exercise 5.
 ^ Monteiro (1945), p. 158 .
 ^ Pervin (1964), p. 46, Exercise 4.
 ^ Arkhangel'skij & Fedorchuk (1990), p. 26.
 ^ A proof for the case can be found at "Is the following a Kuratowski closure operator?!". Stack Exchange. November 21, 2015.
 ^ Pervin (1964), p. 43, Exercise 10.
 ^ Pervin (1964), p. 49, Theorem 3.4.3.
 ^ Pervin (1964), p. 60, Theorem 4.3.1.
 ^ Pervin (1964), p. 66, Exercise 3.
 ^ Pervin (1964), p. 67, Exercise 5.
 ^ Pervin (1964), p. 69, Theorem 5.1.1.
 ^ Pervin (1964), p. 70, Theorem 5.1.2.
 ^ A proof can be found at this link.
 ^ Pervin (1964), pp. 193–196.
 ^ Pervin (1964), p. 51.
References[edit]
 Kuratowski, Kazimierz (1922) [1920], "Sur l'opération A de l'Analysis Situs" [On the operation A in Analysis Situs] (PDF), Fundamenta Mathematicae (in French), 3, pp. 182–199, lay summary – Translation by Mark Bowron (2010).
 Kuratowski, Kazimierz (1966) [1958], Topology, I, translated by Jaworowski, J., Academic Press, ISBN 0124292011, LCCN 66029221.
 Pervin, William J. (1964), Boas, Ralph P. Jr. (ed.), Foundations of General Topology, Academic Press, ISBN 9781483225159, LCCN 6417796.
 Arkhangel'skij, A.V.; Fedorchuk, V.V. (1990) [1988], Gamkrelidze, R.V.; Arkhangel'skij, A.V.; Pontryagin, L.S. (eds.), General Topology I, Encyclopaedia of Mathematical Sciences, 17, translated by O'Shea, D.B., Berlin: SpringerVerlag, ISBN 9783642647673, LCCN 8926209.
 Monteiro, António (September 1943), "Caractérisation de l'opération de fermeture par un seul axiome" [Characterization of the operation of closure by a single axiom], Portugaliae mathematica (in French) (published 1945), 4 (4), pp. 158–160, Zbl 0060.39406.