25/06/18 17:03:29.67 1ZjEJMOG.net
つづき
集合論の研究の内部でも，Cantor とDedekind の集合論について述べたよ
うな，「純粋集合論」と「数学としての集合論」の問の大きな分離は早い時期か
ら見られたが，20 世紀の終りごろから，この2 つの集合論の潮流が合流し，新
しいパラダイムが生れつつあるように見える．
（ついでに下記も）
URLﾘﾝｸ(fuchino.ddo.jp)
RIMS研究集会「数学史の研究」での講演 2010
Kronecker,Dedekind,Hilbert on the Foundation of Arithemetic
渕野昌神戸大学大学院システム情報学研究科
URLﾘﾝｸ(www.jstage.jst.go.jp)
URLﾘﾝｸ(www.jstage.jst.go.jp)
数学 2013 Volume 65 Issue 4 Pages 411-421
特別企画 これから学ぶ人のために
公理的集合論 渕野昌
URL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