[過去ﾛｸﾞ] 純粋・応用数学・数学隣接分野（含むガロア理論）20 (1002ﾚｽ)
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631(1): 現代数学の系譜 雑談 ◆yH25M02vWFhP 06/18(水)17:04 ID:1ZjEJMOG(4/4) AAS
つづき
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)
以上
852(3): 現代数学の系譜 雑談 ◆yH25M02vWFhP 07/10(木)07:08 ID:J4CWtGen(1/3) AAS
>>848-851
ふっふ、ほっほ
もう詰んだのか？ｗ ；ｐ）
>>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 外部ﾘﾝｸ[pdf]:www.math.tsukuba.ac.jp 数理論理学II
あるいは、>>677 渕野昌 P10（無限公理）外部ﾘﾝｸ[pdf]:fuchino.ddo.jp 「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
外部ﾘﾝｸ:en.wikipedia.org
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)))]))).
つづく
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