16/11/19 14:32:00.99 0Q0Vh9CE.net
>>333
Scottのトリック補足
URLﾘﾝｸ(alg-d.com)
本当は怖い濃度の話 : 選択公理 | 壱大整域 2013年10月27日更新
(抜粋)
この定義は明らかに選択公理に依存しています．では選択公理を使わずに濃度が定義できるのかというと，Scottのトリックというものを使い定義することができます．
URLﾘﾝｸ(en.wikipedia.org)
Scott's trick
(抜粋)
In set theory, Scott's trick is a method for giving a definition of equivalence classes for equivalence relations on a proper class (Jech 2003:65). The method relies on the axiom of regularity but not on the axiom of choice.
It can be used to define representatives for ordinal numbers in ZF, Zermelo?Fraenkel set theory without the axiom of choice (Forster 2003:182). The method was introduced by Dana Scott (1955).
Beyond the problem of defining set representatives for ordinal numbers, Scott's trick can be used to obtain representatives for cardinal numbers and more generally for isomorphism types, for example, order types of linearly ordered sets (Jech 2003:65).
It is credited to be indispensable (even in the presence of the axiom of choice) when taking ultrapowers of proper classes in model theory. (Kanamori 1994:47)