現代数学の系譜　カントル　超限集合論

[過去ﾛｸﾞ] 現代数学の系譜　カントル　超限集合論 (1002ﾚｽ)
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713: 現代数学の系譜 雑談 ◆e.a0E5TtKE  [] 2019/12/13(金) 07:56:11.71 ID:ljJF0g2A

これが分り易いかも
Foundation and epsilon-induction
おサルでも読めるだろう
正則性公理が理解出来ていないんだよね（＾＾；

http://web.mat.bham.ac.uk/R.W.Kaye/logic/foundation.html
Foundation and epsilon-induction
(抜粋)
1. Introduction
Either by examining the sets created in the first few levels of the cumulative hierarchy or from other means, via considering the idea of constructions of sets perhaps, we conclude that we do not expect sets to have infinite descending sequences
x0∋x1∋x2∋x3∋x4∋…
at least for sets in the cumulative hierarchy of constructed sets. The axioms of Zermelo-Fraenkel set theory are intended to represent axioms true in this hierarchy, so we expect to have an axiom stating there can be no such descending sequence.
Unfortunately, the statement that there is no such descending sequence is not first order, but second order. This is analogous to the fact that there are nonstandard structures satisfying all first order sentences of arithmetic true in N.
However, the example of arithmetic provides at least one clue as to a powerful axiom scheme true in all structures without infinite descending chains: induction. Applied to set theory we have the axiom scheme of ∈-induction.
Axiom Scheme of ∈-Induction: For all first order formulas ?(x,a??) of the language L∈, ∀a???(∀x?(∀y∈x??(y)→?(x))→∀x??(x,a??)).
We are not going to adopt this as an axiom scheme for Zermelo Fraekel because it will follow from other axioms, and it will be instructive to see how that happens. We will, however, adopt the following special case of ∈-Induction.
Axiom of Foundation: ∀x?(∃y?y∈x→∃y?(y∈x∧￢∃z?(z∈x∧z∈y))).
Other ways of saying this include: if x is nonepty there is a set y∈x such that y∩x=?; and if x is nonepty there is an ∈-minimal y∈x i.e. one with no z∈x having z∈y.
Proposition. The axiom of fountation follows from the axiom scheme of ∈-induction.
Proof.

2. Applications

http://rio2016.5ch.net/test/read.cgi/math/1570237031/713

722: 現代数学の系譜 雑談 ◆e.a0E5TtKE  [] 2019/12/14(土) 07:47:25.56 ID:s6Tab8iq

>>713
文字化けを直して、再引用しよう
http://web.mat.bham.ac.uk/R.W.Kaye/logic/foundation.html
Foundation and epsilon-induction
(抜粋)
1. Introduction
Either by examining the sets created in the first few levels of the cumulative hierarchy or from other means, via considering the idea of constructions of sets perhaps, we conclude that we do not expect sets to have infinite descending sequences
x0∋x1∋x2∋x3∋x4∋…
at least for sets in the cumulative hierarchy of constructed sets.
The axioms of Zermelo-Fraenkel set theory are intended to represent axioms true in this hierarchy, so we expect to have an axiom stating there can be no such descending sequence.
Unfortunately, the statement that there is no such descending sequence is not first order, but second order.
This is analogous to the fact that there are nonstandard structures satisfying all first order sentences of arithmetic true in N.
However, the example of arithmetic provides at least one clue as to a powerful axiom scheme true in all structures without infinite descending chains: induction.
Applied to set theory we have the axiom scheme of ∈-induction.
Axiom Scheme of ∈-Induction:
For all first order formulas Φ(x,a￣) of the language L∈, ∀a￣(∀x(∀y∈xΦ(y)→Φ(x))→∀xΦ(x,a￣)).
We are not going to adopt this as an axiom scheme for Zermelo Fraekel because it will follow from other axioms, and it will be instructive to see how that happens. We will, however, adopt the following special case of ∈-Induction.
Axiom of Foundation: ∀x(∃y y∈x→∃y(y∈x∧￢∃z(z∈x∧z∈y))).

http://rio2016.5ch.net/test/read.cgi/math/1570237031/722

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