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補足 英文 Class (set theory) wikipedia
URLﾘﾝｸ(en.wikipedia.org)(set_theory)
Class (set theory)
(抜粋)
Examples
The collection of all algebraic structures of a given type will usually be a proper class. Examples include the class of all groups, the class of all vector spaces, and many others. I
n category theory, a category whose collection of objects forms a proper class (or whose collection of morphisms forms a proper class) is called a large category.
Classes in formal set theories
Another approach is taken by the von Neumann?Bernays?Godel axioms (NBG); classes are the basic objects in this theory, and a set is then defined to be a class that is an element of some other class.
However, the class existence axioms of NBG are restricted so that they only quantify over sets, rather than over all classes.
This causes NBG to be a conservative extension of ZF.
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