>>788
＞保存拡大なわけない

ID:/m0NW9m+ は、ヒキコモリ数学基礎論くんか　；ｐ）
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(参考)
(google検索)：集合論 NBG 保存拡大 pdf
公理的集合論研究書 〜証明論的アプローチ〜 pdf 更新 - kazz の数学 ...
はてなブログ
https://kazz-scw-2010.ｈａｔｅｎａblog.com/entry/2025/08/31/141720
BGE (別名 NBG) が ZFC の保存拡大であることの recursive な証明の部分です. すでに何ヶ月も前に証明は完成しているのですが、読者 ...

強制法セミナー第 1 回：数理論理学の初歩 - konn-san.com
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https://konn-san.com/2024-forcing-seminar-notes/01-basic-mathematical-logic.pdf
2024/06/23 — 集合変数に限った論理式を考えると、NBG は ZFC の保存. 拡大であり ... こうした都合から、集合論の理論としては NBG で. はなく ZFC が広く使 ...
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https://en.wikipedia.org/wiki/Class_(set_theory)
Class (set theory)
Classes in formal set theories
Because classes do not have any formal status in the theory of ZF, the axioms of ZF do not immediately apply to classes. However, if an inaccessible cardinal κ is assumed, then the sets of smaller rank form a model of ZF (a Grothendieck universe), and its subsets can be thought of as "classes".
Another approach is taken by the von Neumann–Bernays–Gödel 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 ZFC.
Morse–Kelley set theory admits proper classes as basic objects, like NBG, but also allows quantification over all proper classes in its class existence axioms. This causes MK to be strictly stronger than both NBG and ZFC.