ガロア第一論文と乗数イデアル他関連資料スレ13 (825ﾚｽ)
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367(2): 現代数学の系譜 雑談 ◆yH25M02vWFhP 02/08(土)11:19 ID:23ITt7NX(3/8) AAS
>>360
>>順序数全体の集まりは集合でない。
>順序数全体のクラスOを集合と仮定する。
>このときOも順序数だからO∈O。正則性公理に反するから仮定は偽、すなわちOは集合でない。
アホなおサルと>>7-10、 10分議論をする暇があったら
下記のen.wikipedia Ordinal number を、3分黙読する方が、よほど有益だわｗ ；ｐ）
（日wikipediaには、順序数のクラスの記述はないけどね （＾＾）
(参考)
外部ﾘﾝｸ:ja.wikipedia.org
順序数（じゅんじょすう、英: ordinal number）とは、整列集合同士の“長さ”を比較するために、自然数[1]を拡張させた概念である。
外部ﾘﾝｸ:en.wikipedia.org
Ordinal number
In set theory, an ordinal number, or ordinal, is a generalization of ordinal numerals (first, second, nth, etc.) aimed to extend enumeration to infinite sets.[1]
Definitions
Well-ordered sets
Essentially, an ordinal is intended to be defined as an isomorphism class of well-ordered sets: that is, as an equivalence class for the equivalence relation of "being order-isomorphic". There is a technical difficulty involved, however, in the fact that the equivalence class is too large to be a set in the usual Zermelo–Fraenkel (ZF) formalization of set theory. But this is not a serious difficulty. The ordinal can be said to be the order type of any set in the class.
Definition of an ordinal as an equivalence class
The original definition of ordinal numbers, found for example in the Principia Mathematica, defines the order type of a well-ordering as the set of all well-orderings similar (order-isomorphic) to that well-ordering: in other words, an ordinal number is genuinely an equivalence class of well-ordered sets. This definition must be abandoned in ZF and related systems of axiomatic set theory because these equivalence classes are too large to form a set. However, this definition still can be used in type theory and in Quine's axiomatic set theory New Foundations and related systems (where it affords a rather surprising alternative solution to the Burali-Forti paradox of the largest ordinal).
Von Neumann definition of ordinals
See also: Set-theoretic definition of natural numbers and Zermelo ordinals
Rather than defining an ordinal as an equivalence class of well-ordered sets, it will be defined as a particular well-ordered set that (canonically) represents the class. Thus, an ordinal number will be a well-ordered set; and every well-ordered set will be order-isomorphic to exactly one ordinal number.
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