ところで、本題 Zermelo ordinalsな、下記”Unlike von Neumann's construction, the Zermelo ordinals do not account for infinite ordinals.” ってあるよね。機械翻訳に手を入れると「フォンノイマンの構成とは異なり、ゼルメロの序数は無限の序数を説明しません。」となる この解釈は、 １）後者の繰り返しではω＝Nに到達できない、 ２）可算無限で欲しいのは自然数全体から成るN＝ωだが、Zermeloの後者関数とはアンマッチ だってこと
ところで、下記”A countable non-standard model of arithmetic satisfying the Peano Arithmetic (that is, the first-order Peano axioms) was developed by Skolem in 1933. The hypernatural numbers are an uncountable model that ca
827 名前：n be constructed from the ordinary natural numbers via the ultrapower construction.” とあるよね ここから、レーヴェンハイム?スコーレムの定理 「いくらでも大きな有限のモデルを持つ理論は無限のモデルを持たねばならない」と続く
(参考) https://en.wikipedia.org/wiki/Natural_number Natural number Contents 2.7 Infinity 3 Generalizations 4 Formal definitions 4.1 Peano axioms 4.2 Constructions based on set theory 4.2.1 Von Neumann ordinals 4.2.2 Zermelo ordinals
Generalizations Two important generalizations of natural numbers arise from the two uses of counting and ordering: cardinal numbers and ordinal numbers.
For finite well-ordered sets, there is a one-to-one correspondence between ordinal and cardinal numbers; therefore they can both be expressed by the same natural number, the number of elements of the set. This number can also be used to describe the position of an element in a larger finite, or an infinite, sequence.
A countable non-standard model of arithmetic satisfying the Peano Arithmetic (that is, the first-order Peano axioms) was developed by Skolem in 1933. The hypernatural numbers are an uncountable model that can be constructed from the ordinary natural numbers via the ultrapower construction.
Constructions based on set theory Von Neumann ordinals The standard definition, now called definition of von Neumann ordinals, is: "each ordinal is the well-ordered set of all smaller ordinals."
Zermelo ordinals Although the standard construction is useful, it is not the only possible construction. Ernst Zermelo's construction goes as follows:[38] Set 0 = { } Define S(a) = {a}, It then follows that 0 = { }, 1 = {0} = {{ }}, 2 = {1} = {{{ }}}, n = {n?1} = {{{...}}}, etc. Each natural number is then equal to the set containing just the natural number preceding it. This is the definition of Zermelo ordinals. Unlike von Neumann's construction, the Zermelo ordinals do not account for infinite ordinals.
https://ja.wikipedia.org/wiki/%E3%83%AC%E3%83%BC%E3%83%B4%E3%82%A7%E3%83%B3%E3%83%8F%E3%82%A4%E3%83%A0%E2%80%93%E3%82%B9%E3%82%B3%E3%83%BC%E3%83%AC%E3%83%A0%E3%81%AE%E5%AE%9A%E7%90%86 レーヴェンハイム?スコーレムの定理 冒頭の簡単な言明の場合、理論の無限のモデルとは、ここでいう M である。定理の上方部分の証明は、いくらでも大きな有限のモデルを持つ理論は無限のモデルを持たねばならないことをも示す。この事実を定理の一部とする場合もある。
（念のため英文） https://en.wikipedia.org/wiki/L%C3%B6wenheim%E2%80%93Skolem_theorem Lowenheim-Skolem theorem Consequences The statement given in the introduction follows immediately by taking M to be an infinite model of the theory. The proof of the upward part of the theorem also shows that a theory with arbitrarily large finite models must have an infinite model; sometimes this is considered to be part of the theorem.
説明するよ １．いま、簡単のために、ノイマンがやったように、後者suc(a)=a∪{a}として、空集合φから出発して、自然数の集合Nを作るとする ２．いわゆる、（下記）遺伝的有限集合、Hereditarily finite setができる ３．で、Hereditarily finite setを全部集めると、”all finite von Neumann ordinals are in H_aleph_0”、 ”the class of sets representing the natural numbers, i.e it includes each element in the standard model of natural numbers.” となるわけだ ４．上記3のNは、一階述語論理では示せない。表現力弱いから（レーヴェンハイム-スコーレムの定理） ５．「natural numbers Nが出来ている」と言いたい。そのために、「無限公理を置いた」ってことね（細かい技術的な話があるが省く） ６．つまり、自然数の集合Nの元∀n達は、後者suc(a)=a∪{a}で尽くせると、考えて良い。というか、そう考えるべきなのだ で、1,2,3,・・n・・(→∞) が、数直線の上に並んで、ずっと無限に続く 一方で、y=1/xで、逆数を作ると、1,1/2,1/3,・・1/n・・→0 に写せる 1,1/2,1/3,・・1/n・・ 達は、全て有限で、列全体の長さは可算無限、つまり自然数Nの元を並べた列と同じ長さになる （ここで、逆数を使ったのは、https://encyclopediaofmath.org/wiki/Ordinal_number Ordinal number の記述を参考にした。有理数Ｑの稠密性を使う議論は分かり易いね）
（参考） https://ja.wikipedia.org/wiki/%E9%81%BA%E4%BC%9D%E7%9A%84%E6%9C%89%E9%99%90%E9%9B%86%E5%90%88 遺伝的有限集合（英: hereditarily finite set）は有限個の遺伝的有限集合からなる有限集合と定義される。この定義は帰納的である。遺伝的という名称は遺伝的有限という性質がその元に遺伝することによる。 https://en.wikipedia.org/wiki/Hereditarily_finite_set Hereditarily finite set Contents 3 Axiomatizations 3.1 Theories of finite sets 3.2 ZF
Theories of finite sets The set
860 名前：Φ also represents the first von Neumann ordinal number, denoted 0. And indeed all finite von Neumann ordinals are in H_aleph_0 and thus the class of sets representing the natural numbers, i.e it includes each element in the standard model of natural numbers.
ZF The hereditarily finite sets are a subclass of the Von Neumann universe. Here, the class of all well-founded hereditarily finite sets is denoted Vω. Note that this is also a set in this context. (引用終り) 以上 []
(参加) https://en.wikipedia.org/wiki/Zermelo%E2%80%93Fraenkel_set_theory Zermelo?Fraenkel set theory Contents 1 History History The modern study of set theory was initiated by Georg Cantor and Richard Dedekind in the 1870s. However, the discovery of paradoxes in naive set theory, such as Russell's paradox, led to the desire for a more rigorous form of set theory that was free of these paradoxes. In 1922, Fraenkel and Thoralf Skolem independently proposed operationalizing a "definite" property as one that could be formulated as a well-formed formula in a first-order logic whose atomic formulas were limited to set membership and identity. They also independently proposed replacing the axiom schema of specification with the axiom schema of replacement. Appending this schema, as well as the axiom of regularity (first proposed by John von Neumann),[3] to Zermelo set theory yields the theory denoted by ZF. Adding to ZF either the axiom of choice (AC) or a statement that is equivalent to it yields ZFC.
https://encyclopediaofmath.org/index.php?title=ZFC encyclopediaofmath.org ZFC [a17] J. von Neumann, "Eine Axiomatisierung der Mengenlehre" J. Reine Angew. Math. (Crelle's J.) , 154 (1925) pp. 219?240 (引用終り) 以上
１．基本は、下記 Foundations of mathematicsのToward resolution of the crisis にある通り ”In practice, most mathematicians either do not work from axiomatic systems, or if they do, do not doubt the consistency of ZFC, generally their preferred axiomatic system. In most of mathematics as it is practiced, the incompleteness and paradoxes of the underlying formal theories never played a role anyway, and in those branches in which they do or whose formalization attempts would run the risk of forming inconsistent theories (such as logic and category theory), they may be treated carefully.” <上記のgoogle機械訳が下記> ”実際には、ほとんどの数学者は公理システムから作業しないか、または作業する場合は、ZFCの一貫性、一般的には彼らの好ましい公理システムを疑うことはありません。 実践されている数学のほとんどでは、基礎となる形式理論の不完全性とパラドックスがとにかく役割を果たしたことはなく、それらが行われている、または形式化の試みが一貫性のない理論（論理や圏論など）を形成するリスクを冒すブランチでは 理論）、それらは慎重に扱われるかもしれません。” と ２．「形式化の試みが一貫性のない理論（論理や圏論など）を形成するリスクを冒すブランチでは 理論）、それらは慎重に扱われるかもしれません」 は、まさにIUT IVの付録で望月先生が書かれていた ”Set-theoretic Foundations”が当てはまる気がする ３．21世紀の大きな流れは、一つは圏論 IUTもそうだし、拓郎先生の3億円論文も圏論使ったそうな もう一つは、”高階論理”。逆数学は2階算術を使うという 20世紀後半から21世紀のトレンドは、 脱ZFCだと思う。ZFCだけじゃ、狭いし、新しいことは出てこない 脱ZFCの一番の先駆者が、グロタンディークだったかも。ZFCGを考えたり、トポスから景を提唱したり、全く従来の基礎論に捕らわれない発想の人だった
(参考) https://en.wikipedia.org/wiki/Foundations_of_mathematics Foundations of mathematics Contents Toward resolution of the crisis In practice, most mathematicians either do not work from axiomatic systems, or if they do, do not doubt the consistency of ZFC, generally their preferred axiomatic system. In most of mathematics as it is practiced, the incompleteness and paradoxes of the underlying formal theories never played a role anyway, and in those branches in which they do or whose formalization attempts would run the risk of forming inconsistent theories (such as logic and category theory), they may be treated carefully. The development of category theory in the middle of the 20th century showed the usefulness of set theories guaranteeing the existence of larger classes than does ZFC, such as Von Neumann?Bernays?Godel set theory or Tarski?Grothendieck set theory, albeit that in very many cases the use of large cardinal axioms or Grothendieck universes is formally eliminable. One goal of the reverse mathematics program is to identify whether there are areas of "core mathematics" in which foundational issues may again provoke a crisis.
https://en.wikipedia.org/wiki/Topos The Grothendieck topoi find applications in algebraic geometry; the more general elementary topoi are used in logic.
In practice, most mathematicians either do not work from axiomatic systems, or if they do, do not doubt the consistency of ZFC, generally their preferred axiomatic system.
In most of mathematics as it is practiced, the incompleteness and paradoxes of the underlying formal theories never played a role anyway, and in those branches in which they do or whose formalization attempts would run the risk of forming inconsistent theories (such as logic and category theory), they may be treated carefully.
