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。
うむ １）その結論は、正しい。下記の独 de.wikipediaの英訳 Infinity axiomで、”The natural numbers are therefore defined as the intersection of all inductive sets, as the smallest inductive set.” とある通りだ ２）ところで 下記の 独 de.wikipedia Infinity axiom では 記号∩ 使ってないよ？ 記号∩ は、使わなくてもいいの？ 記号∩ は、使わなくてもいいのならば、その方がすっきりしてないかな？ｗ ；ｐ）
(参考) https://de.wikipedia.org/wiki/Unendlichkeitsaxiom （google翻訳 独→英） Infinity axiom The axiom of infinity is an axiom of set theory that postulates the existence of an inductive set . It is called the axiom of infinity because inductive sets are also infinite sets .
formulation There are a lot A, which is the empty set ∅ and with each element x∈A also the amount x∪{x}contains. ∃A:(∅∈A∧∀x:(x∈A⇒x∪{x}∈A)) The infinity axiom does not merely postulate, as the name might suggest, the existence of any infinite set. It postulates the existence of an inductive set and thus, consequently, the existence of the set of natural numbers according to John von Neumann's model .
Significance for mathematics Natural numbers By the existence of at least one inductive set I together with the exclusion axiom, the existence of natural numbers as a set is also ensured: N:={x∈I∣∀z(z inductive ⟹ x∈z)} The natural numbers are therefore defined as the intersection of all inductive sets, as the smallest inductive set.
Infinite quantities Without the infinity axiom, ZF would only guarantee the existence of finite sets. No statements could be made about the existence of infinite sets. The infinity axiom, together with the power set axiom , ensures that there are also uncountable sets, such as the real numbers.
(参考) https://fr.wikipedia.org/wiki/Axiome_de_l%27infini （google翻訳 仏→英） Axiom of infinity Statement of the axiom The axiom is therefore written: There exists a set to which the empty set belongs and which is closed by application of the successor x ↦ x ∪ { x }, that is, in the formal language of set theory (the calculus of egalitarian first-order predicates with the only non-logical symbol being that for membership, "∈"): ∃A Cl(A) where Cl( Y ) is the predicate “∅ ∈ Y and ∀ y ( y ∈ Y ⇒ y ∪ { y } ∈ Y )”, expressing “ Y is closed under successor and ∅ belongs to it” (for the abbreviations “∅ ∈ Y ” and “ y ∪ { y } ∈ Y ”, defined from ∈, see Axiom of the empty set , Axiom of the pair and Axiom of the union ).
The set of natural numbers Definition To formalize the "and so on", let us define the predicate Ent(x) as : ∀A (Cl(A)⇒x∈A)
Throughout the following, we will call "natural integers" - or "integers" - the elements x verifying Ent( x ).
With this definition, 0 is an "integer" — formally: we have Ent(0) — and the successor x + of any "integer" x is an "integer" — Ent( x ) ⇒ Ent( x + ), and the axiom of infinity is equivalent to ∃ω ∀x(Ent(x)⇔x∈ω), that's to say : The class of natural numbers is a set . Indeed : ・let A be a set verifying Cl( A ) whose existence is ensured by the axiom of infinity. Then, the existence of the set ω is ensured by the axiom scheme of comprehension and its uniqueness by the axiom of extensionality , by defining ω as the intersection (therefore the smallest in the sense of inclusion) of all sets containing 0 and closed by successor ( A only intervenes to be able to define ω as a set, but ω does not depend on A ): ω = { x ∈ A | Ent( x ) } ;
・conversely, let ω be a set whose elements are the natural numbers. Then, ω verifies Cl(ω). The very definition of the set ω gives a statement of the principle of recurrence on the integers: any set to which 0 belongs and which is closed by successor is a superset of ω. We can give a slightly more familiar statement but equivalent in set theory by the comprehension scheme, we denote x + the successor of x , we then have for an arbitrary property expressed in the language of set theory by the formula P x a 1 … a k (no other free variable ): ∀ a 1 , … , a k { [ P 0 a 1 … a k and ∀ y ∈ ω ( P y a 1 … a k ⇒ P y + a 1 … a k )] ⇒ ∀ x ∈ ω P x a 1 … a k } (any property that is true at 0 and passes to the successor on integers is true for all integers). For example: every element of ω is a finite ordinal .
The recurrence is valid for any property expressed in the language of set theory. This is not trivial: it makes this recurrence a much stronger property than the recurrence of Peano arithmetic (as a first-order theory), the language of set theory being strictly more expressive than that of Peano arithmetic. (引用終り) 以上 []
fr.wikipedia Axiom of infinity（無限公理）より ”let A be a set verifying Cl( A ) whose existence is ensured by the axiom of infinity. Then, the existence of the set ω is ensured by the axiom scheme of comprehension and its uniqueness by the axiom of extensionality , by defining ω as the intersection (therefore the smallest in the sense of inclusion) of all sets containing 0 and closed by successor ( A only intervenes to be able to define ω as a set, but ω does not depend on A ): ω = { x ∈ A | Ent( x ) } ;”
とあるよ ”by defining ω as the intersection (therefore the smallest in the sense of inclusion) of all sets containing 0 and closed by successor ( A only intervenes to be able to define ω as a set, but ω does not depend on A )” とあるよ ”by defining ω as the intersection” とあるよ
