https://ja.wikipedia.org/wiki/%E7%84%A1%E9%99%90%E5%85%AC%E7%90%86 無限公理（英: axiom of infinity）とは公理的集合論におけるZF公理系を構成する公理の一つで、「無限集合の存在」を主張するものである。 定義 ZF公理系における公式な定義は次の通りである。 空集合を要素とし、任意の要素 x に対して x ∪ {x} を要素に持つ集合が存在する： ∃
903 名前：A,∅∈A∧∀x∈A, x∪{x}∈A
(上記の英版) https://en.wikipedia.org/wiki/Axiom_of_infinity Axiom of infinity Formal statement If the notations of both set-builder and empty set are allowed: ∃A(∅∈A∧∀x(x∈A→（x∪{x}）∈A)) （注：英原文では AのところにIを使っているが、和文に合わせた） []
>>727より再録 >”∩{x⊂A|{}∈x∧∀y[y∈x→y∪{y}∈x]}” この式は、下記（ja.ipedia）のペアノの公理 自然数の集合論的構成 の式だが 上記の通り、∩のIterated binary operation の意味が不明確（この説明を求められると詰まるだろう） なので、∩を使わない 別の工夫がある（下記） 例えば en.wikipedia Axiom of infinity, Extracting the natural numbers from the infinite set, Alternative method あるいは fr.wikipedia Axiome de l'infini あるいは、>>569 筑波大 坪井明人 PDF P9 https://www.math.tsukuba.ac.jp/~tsuboi/und/14logic3.pdf 数理論理学II あるいは、>>677 渕野昌 P10（無限公理）https://fuchino.ddo.jp/books/intro-to-set-theory-and-constructibility.pdf 「ゲーデルと20世紀の論理学第４巻」（東京大学出版会，2007）の，渕野 昌の執筆した第I部 以上 (引用終り)
繰り返すが、∩のIterated binary operation の意味が不明確
さらに、wikipedia Axiom of infinity 記述を引用する >>630-631 より https://en.wikipedia.org/wiki/Axiom_of_infinity Axiom of infinity Extracting the natural numbers from the infinite set The infinite set I is a superset of the natural numbers. To show that the natural numbers themselves constitute a set, the axiom schema of specification can be applied to remove unwanted elements, leaving the set N of all natural numbers. This set is unique by the axiom of extensionality. To extract the natural numbers, we need a definition of which sets are natural numbers. The natural numbers can be defined in a way that does not assume any axioms except the axiom of extensionality and the axiom of induction—a natural number is either zero or a successor and each of its elements is either zero or a successor of another of its elements. In formal language, the definition says: ∀n(n∈N⟺([n=∅∨∃k(n=k∪{k})]∧∀m∈n[m=∅∨∃k∈n(m=k∪{k})])). Or, even more formally: ∀n(n∈N⟺([∀k(¬k∈n)∨∃k∀j(j∈n⟺(j∈k∨j=k))]∧ ∀m(m∈n⇒[∀k(¬k∈m)∨∃k(k∈n∧∀j(j∈m⟺(j∈k∨j=k)))]))).
Alternative method An alternative method is the following. Let Φ(x) be the formula that says "x is inductive"; i.e. Φ(x)=(∅∈x∧∀y(y∈x→(y∪{y}∈x))). Informally, what we will do is take the intersection of all inductive sets. More formally, we wish to prove the existence of a unique set W such that ∀x(x∈W↔∀I(Φ(I)→x∈I)). (*) For existence, we will use the Axiom of Infinity combined with the Axiom schema of specification. Let I be an inductive set guaranteed by the Axiom of Infinity. Then we use the axiom schema of specification to define our set W={x∈I:∀J(Φ(J)→x∈J)} – i.e. W is the set of all elements of I, which also happen to be elements of every other inductive set. This clearly satisfies the hypothesis of (*), since if x∈W, then x is in every inductive set, and if x is in every inductive set, it is in particular in I, so it must also be in W. For uniqueness, first note that any set that satisfies (*) is itself inductive, since 0 is in all inductive sets, and if an element x is in all inductive sets, then by the inductive property so is its successor. Thus if there were another set W′ that satisfied (*) we would have that W′⊆W since W is inductive, and W⊆W′since W′is inductive. Thus W=W′. Let ω denote this unique element. This definition is convenient because the principle of induction immediately follows: If I⊆ω is inductive, then also ω⊆I, so that I=ω.■ (引用終り)
さて >>852-853より https://en.wikipedia.org/wiki/Axiom_of_infinity Axiom of infinity Extracting the natural numbers from the infinite set
Φ(x) be the formula that says "x is inductive"; i.e. Φ(x)=(∅∈x∧∀y(y∈x→(y∪{y}∈x))). Informally, what we will do is take the intersection of all inductive sets. More formally, we wish to prove the existence of a unique set W such that ∀x(x∈W↔∀I(Φ(I)→x∈I)). (*) For existence, we will use the Axiom of Infinity combined with the Axiom schema of specification. Let I be an inductive set guaranteed by the Axiom of Infinity. Then we use the axiom schema of specification to define our set W={x∈I:∀J(Φ(J)→x∈J)} – i.e. W is the set of all elements of I, which also happen to be elements of every other inductive set. This clearly satisfies the hypothesis of (*), since if x∈W, then x is in every inductive set, and if x is in every inductive set, it is in particular in I, so it must also be in W. For uniqueness, first note that any set that satisfies (*) is itself inductive, since 0 is in all inductive sets, and if an element x is in all inductive sets, then by the inductive property so is its successor. Thus if there were another set W′ that satisfied (*) we would have that W′⊆W since W is inductive, and W⊆W′since W′is inductive. Thus W=W′. Let ω denote this unique element. This definition is convenient because the principle of induction immediately follows: If I⊆ω is inductive, then also ω⊆I, so that I=ω.■ (引用終り)
これで尽きている １）”Informally, what we will do is take the intersection of all inductive sets.” intersection：共通部分 英: intersection（下記）ね ２）で、これ ”Informally”とあるよね。つまり、 ”∩{x⊂A|{}∈x∧∀y[y∈x→y∪{y}∈x]}”>>727 は、”Informally”なんだよ ここを勘違いした人が ja.wikipediaに >>847の”ペアノの公理”を 書いたんじゃないの？ ３）さて、Formallyには ”Let I be an inductive set guaranteed by the Axiom of Infinity. Then we use the axiom schema of specification to define our set W={x∈I:∀J(Φ(J)→x∈J)} – i.e. W is the set of all elements of I, which also happen to be elements of every other inductive set.” だよね。ここに、”∩”は 使われない
>>867 >１）”Informally, what we will do is take the intersection of all inductive sets.” >２）で、これ ”Informally”とあるよね。つまり、 > ”∩{x⊂A|{}∈x∧∀y[y∈x→y∪{y}∈x]}”>>727 は、”Informally”なんだよ はいまた勝手読み。 Informallyである所以は >all inductive sets これを上手く定義できない（内包公理を使えば定義できるがZFには無い）から、任意のひとつのinductive set Aの部分集合族の共通部分で定義している。
959 名前：s mich so schwer drückt, ist das Verhältnis zu dem Taugenichts in Amerika, der meinen Namen entehrt. Sie wissen, welche Nachricht ich vor 4 Monaten von ihm erhalten hatte. Ich sehe, daß es wohl gut gewesen wäre, wenn ich ihm damals in dem Sinne geantwortet hätte, wie Sie rieten, um ihm sofort jede Erwartung abzuschneiden: aber ich vermochte nicht, überhaupt zu antworten.
えーと >>867 より再録 >>852-853より https://en.wikipedia.org/wiki/Axiom_of_infinity Axiom of infinity Extracting the natural numbers from the infinite set Φ(x) be the formula that says "x is inductive"; i.e. Φ(x)=(∅∈x∧∀y(y∈x→(y∪{y}∈x))). Informally, what we will do is take the intersection of all inductive sets. More formally, we wish to prove the existence of a unique set W such that ∀x(x∈W↔∀I(Φ(I)→x∈I)). (*) For existence, we will use the Axiom of Infinity combined with the Axiom schema of specification. Let I be an inductive set guaranteed by the Axiom of Infinity. Then we use the axiom schema of specification to define our set W={x∈I:∀J(Φ(J)→x∈J)} – i.e. W is the set of all elements of I, which also happen to be elements of every other inductive set. This clearly satisfies the hypothesis of (*), since if x∈W, then x is in every inductive set, and if x is in every inductive set, it is in particular in I, so it must also be in W. For uniqueness, first note that any set that satisfies (*) is itself inductive, since 0 is in all inductive sets, and if an element x is in all inductive sets, then by the inductive property so is its successor. Thus if there were another set W′ that satisfied (*) we would have that W′⊆W since W is inductive, and W⊆W′since W′is inductive. Thus W=W′. Let ω denote this unique element. This definition is convenient because the principle of induction immediately follows: If I⊆ω is inductive, then also ω⊆I, so that I=ω.■ (引用終り) １）”Informally, what we will do is take the intersection of all inductive sets.” intersection：共通部分 英: intersection（下記）ね ２）で、これ ”Informally”とあるよね。つまり、 ”∩{x⊂A|{}∈x∧∀y[y∈x→y∪{y}∈x]}”>>727 は、”Informally”なんだよ ここを勘違いした人が ja.wikipediaに >>847の”ペアノの公理”を 書いたんじゃないの？ ３）さて、Formallyには ”Let I be an inductive set guaranteed by the Axiom of Infinity. Then we use the axiom schema of specification to define our set W={x∈I:∀J(Φ(J)→x∈J)} – i.e. W is the set of all elements of I, which also happen to be elements of every other inductive set.” だよね。ここに、”∩”は 使われない (引用終り)
∩は、集合の積で intersection 上記の Axiom of infinity ”Informally, what we will do is take the intersection of all inductive sets. More formally, we wish to prove the existence of a unique set W such that ∀x(x∈W↔∀I(Φ(I)→x∈I)). (*)” における Informally ”take the intersection of all inductive sets.”を なんか勘違いして だれかが書いたと思うんだよね ところが、この『N:=∩{x⊂A∣∅∈x∧∀y[y∈x→y∪{y}∈x]}』を 必死で擁護するやつが いるんだ 自分が書いた式でもないし
繰り返すが en.wikipedia Axiom of infinity ”Extracting the natural numbers from the infinite set”では ”More formally, we wish to prove the existence of a unique set W such that・・”と ”∩”を 使ってないよと指摘したら、発狂する人がいるんだｗ 自分で書いた式でもないだろうし、intersection は en.wikipedia では ”Informally”なのに・・ｗｗ（＾＾ 以上
>>920 >１）”Informally, what we will do is take the intersection of all inductive sets.” > intersection：共通部分 英: intersection（下記）ね Informally である理由は all inductive sets を上手く定義できないから。
