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Kuratowski closure axioms
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Kuratowski closure axioms : ウィキペディア英語版
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.
== Definition ==

Let X be a set and \mathcal(X) its power set.

A Kuratowski Closure Operator is an assignment \operatorname:\mathcal(X) \to \mathcal(X) with the following properties:〔 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.〕
# \operatorname(\varnothing) = \varnothing (Preservation of Nullary Union)
# A \subseteq \operatorname(A) \textA \subseteq X (Extensivity)
# \operatorname(A \cup B) = \operatorname(A) \cup \operatorname(B) \textA,B \subseteq X (Preservation of Binary Union)
# \operatorname(\operatorname(A)) = \operatorname(A) \textA \subseteq X (Idempotence)
If the last axiom, idempotence, is omitted, then the axioms define a preclosure operator.
A consequence of the third axiom is: A \subseteq B \Rightarrow \operatorname(A) \subseteq \operatorname(B) (Preservation of Inclusion).
The four Kuratowski closure axioms can be replaced by a single condition, namely,
:A \cup \operatorname(A) \cup \operatorname(\operatorname(B)) = \operatorname(A \cup B) \setminus \operatorname(\varnothing) \textA, B \subseteq X.

抄文引用元・出典: フリー百科事典『 ウィキペディア(Wikipedia)
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