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Type | Label | Description |
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Statement | ||
In this section, we state the axiom scheme of bounded separation, which is part of CZF set theory. | ||
Axiom | ax-bdsep 13501* | Axiom scheme of bounded (or restricted, or Δ0) separation. It is stated with all possible disjoint variable conditions, to show that this weak form is sufficient. For the full axiom of separation, see ax-sep 4083. (Contributed by BJ, 5-Oct-2019.) |
⊢ BOUNDED 𝜑 ⇒ ⊢ ∀𝑎∃𝑏∀𝑥(𝑥 ∈ 𝑏 ↔ (𝑥 ∈ 𝑎 ∧ 𝜑)) | ||
Theorem | bdsep1 13502* | Version of ax-bdsep 13501 without initial universal quantifier. (Contributed by BJ, 5-Oct-2019.) |
⊢ BOUNDED 𝜑 ⇒ ⊢ ∃𝑏∀𝑥(𝑥 ∈ 𝑏 ↔ (𝑥 ∈ 𝑎 ∧ 𝜑)) | ||
Theorem | bdsep2 13503* | Version of ax-bdsep 13501 with one disjoint variable condition removed and without initial universal quantifier. Use bdsep1 13502 when sufficient. (Contributed by BJ, 5-Oct-2019.) |
⊢ BOUNDED 𝜑 ⇒ ⊢ ∃𝑏∀𝑥(𝑥 ∈ 𝑏 ↔ (𝑥 ∈ 𝑎 ∧ 𝜑)) | ||
Theorem | bdsepnft 13504* | Closed form of bdsepnf 13505. Version of ax-bdsep 13501 with one disjoint variable condition removed, the other disjoint variable condition replaced by a nonfreeness antecedent, and without initial universal quantifier. Use bdsep1 13502 when sufficient. (Contributed by BJ, 19-Oct-2019.) |
⊢ BOUNDED 𝜑 ⇒ ⊢ (∀𝑥Ⅎ𝑏𝜑 → ∃𝑏∀𝑥(𝑥 ∈ 𝑏 ↔ (𝑥 ∈ 𝑎 ∧ 𝜑))) | ||
Theorem | bdsepnf 13505* | Version of ax-bdsep 13501 with one disjoint variable condition removed, the other disjoint variable condition replaced by a nonfreeness hypothesis, and without initial universal quantifier. See also bdsepnfALT 13506. Use bdsep1 13502 when sufficient. (Contributed by BJ, 5-Oct-2019.) |
⊢ Ⅎ𝑏𝜑 & ⊢ BOUNDED 𝜑 ⇒ ⊢ ∃𝑏∀𝑥(𝑥 ∈ 𝑏 ↔ (𝑥 ∈ 𝑎 ∧ 𝜑)) | ||
Theorem | bdsepnfALT 13506* | Alternate proof of bdsepnf 13505, not using bdsepnft 13504. (Contributed by BJ, 5-Oct-2019.) (Proof modification is discouraged.) (New usage is discouraged.) |
⊢ Ⅎ𝑏𝜑 & ⊢ BOUNDED 𝜑 ⇒ ⊢ ∃𝑏∀𝑥(𝑥 ∈ 𝑏 ↔ (𝑥 ∈ 𝑎 ∧ 𝜑)) | ||
Theorem | bdzfauscl 13507* | Closed form of the version of zfauscl 4085 for bounded formulas using bounded separation. (Contributed by BJ, 13-Nov-2019.) |
⊢ BOUNDED 𝜑 ⇒ ⊢ (𝐴 ∈ 𝑉 → ∃𝑦∀𝑥(𝑥 ∈ 𝑦 ↔ (𝑥 ∈ 𝐴 ∧ 𝜑))) | ||
Theorem | bdbm1.3ii 13508* | Bounded version of bm1.3ii 4086. (Contributed by BJ, 5-Oct-2019.) (Proof modification is discouraged.) |
⊢ BOUNDED 𝜑 & ⊢ ∃𝑥∀𝑦(𝜑 → 𝑦 ∈ 𝑥) ⇒ ⊢ ∃𝑥∀𝑦(𝑦 ∈ 𝑥 ↔ 𝜑) | ||
Theorem | bj-axemptylem 13509* | Lemma for bj-axempty 13510 and bj-axempty2 13511. (Contributed by BJ, 25-Oct-2020.) (Proof modification is discouraged.) Use ax-nul 4091 instead. (New usage is discouraged.) |
⊢ ∃𝑥∀𝑦(𝑦 ∈ 𝑥 → ⊥) | ||
Theorem | bj-axempty 13510* | Axiom of the empty set from bounded separation. It is provable from bounded separation since the intuitionistic FOL used in iset.mm assumes a nonempty universe. See axnul 4090. (Contributed by BJ, 25-Oct-2020.) (Proof modification is discouraged.) Use ax-nul 4091 instead. (New usage is discouraged.) |
⊢ ∃𝑥∀𝑦 ∈ 𝑥 ⊥ | ||
Theorem | bj-axempty2 13511* | Axiom of the empty set from bounded separation, alternate version to bj-axempty 13510. (Contributed by BJ, 27-Oct-2020.) (Proof modification is discouraged.) Use ax-nul 4091 instead. (New usage is discouraged.) |
⊢ ∃𝑥∀𝑦 ¬ 𝑦 ∈ 𝑥 | ||
Theorem | bj-nalset 13512* | nalset 4095 from bounded separation. (Contributed by BJ, 18-Nov-2019.) (Proof modification is discouraged.) |
⊢ ¬ ∃𝑥∀𝑦 𝑦 ∈ 𝑥 | ||
Theorem | bj-vprc 13513 | vprc 4097 from bounded separation. (Contributed by BJ, 18-Nov-2019.) (Proof modification is discouraged.) |
⊢ ¬ V ∈ V | ||
Theorem | bj-nvel 13514 | nvel 4098 from bounded separation. (Contributed by BJ, 18-Nov-2019.) (Proof modification is discouraged.) |
⊢ ¬ V ∈ 𝐴 | ||
Theorem | bj-vnex 13515 | vnex 4096 from bounded separation. (Contributed by BJ, 18-Nov-2019.) (Proof modification is discouraged.) |
⊢ ¬ ∃𝑥 𝑥 = V | ||
Theorem | bdinex1 13516 | Bounded version of inex1 4099. (Contributed by BJ, 13-Nov-2019.) (Proof modification is discouraged.) |
⊢ BOUNDED 𝐵 & ⊢ 𝐴 ∈ V ⇒ ⊢ (𝐴 ∩ 𝐵) ∈ V | ||
Theorem | bdinex2 13517 | Bounded version of inex2 4100. (Contributed by BJ, 13-Nov-2019.) (Proof modification is discouraged.) |
⊢ BOUNDED 𝐵 & ⊢ 𝐴 ∈ V ⇒ ⊢ (𝐵 ∩ 𝐴) ∈ V | ||
Theorem | bdinex1g 13518 | Bounded version of inex1g 4101. (Contributed by BJ, 13-Nov-2019.) (Proof modification is discouraged.) |
⊢ BOUNDED 𝐵 ⇒ ⊢ (𝐴 ∈ 𝑉 → (𝐴 ∩ 𝐵) ∈ V) | ||
Theorem | bdssex 13519 | Bounded version of ssex 4102. (Contributed by BJ, 13-Nov-2019.) (Proof modification is discouraged.) |
⊢ BOUNDED 𝐴 & ⊢ 𝐵 ∈ V ⇒ ⊢ (𝐴 ⊆ 𝐵 → 𝐴 ∈ V) | ||
Theorem | bdssexi 13520 | Bounded version of ssexi 4103. (Contributed by BJ, 13-Nov-2019.) (Proof modification is discouraged.) |
⊢ BOUNDED 𝐴 & ⊢ 𝐵 ∈ V & ⊢ 𝐴 ⊆ 𝐵 ⇒ ⊢ 𝐴 ∈ V | ||
Theorem | bdssexg 13521 | Bounded version of ssexg 4104. (Contributed by BJ, 13-Nov-2019.) (Proof modification is discouraged.) |
⊢ BOUNDED 𝐴 ⇒ ⊢ ((𝐴 ⊆ 𝐵 ∧ 𝐵 ∈ 𝐶) → 𝐴 ∈ V) | ||
Theorem | bdssexd 13522 | Bounded version of ssexd 4105. (Contributed by BJ, 13-Nov-2019.) (Proof modification is discouraged.) |
⊢ (𝜑 → 𝐵 ∈ 𝐶) & ⊢ (𝜑 → 𝐴 ⊆ 𝐵) & ⊢ BOUNDED 𝐴 ⇒ ⊢ (𝜑 → 𝐴 ∈ V) | ||
Theorem | bdrabexg 13523* | Bounded version of rabexg 4108. (Contributed by BJ, 19-Nov-2019.) (Proof modification is discouraged.) |
⊢ BOUNDED 𝜑 & ⊢ BOUNDED 𝐴 ⇒ ⊢ (𝐴 ∈ 𝑉 → {𝑥 ∈ 𝐴 ∣ 𝜑} ∈ V) | ||
Theorem | bj-inex 13524 | The intersection of two sets is a set, from bounded separation. (Contributed by BJ, 19-Nov-2019.) (Proof modification is discouraged.) |
⊢ ((𝐴 ∈ 𝑉 ∧ 𝐵 ∈ 𝑊) → (𝐴 ∩ 𝐵) ∈ V) | ||
Theorem | bj-intexr 13525 | intexr 4112 from bounded separation. (Contributed by BJ, 18-Nov-2019.) (Proof modification is discouraged.) |
⊢ (∩ 𝐴 ∈ V → 𝐴 ≠ ∅) | ||
Theorem | bj-intnexr 13526 | intnexr 4113 from bounded separation. (Contributed by BJ, 18-Nov-2019.) (Proof modification is discouraged.) |
⊢ (∩ 𝐴 = V → ¬ ∩ 𝐴 ∈ V) | ||
Theorem | bj-zfpair2 13527 | Proof of zfpair2 4171 using only bounded separation. (Contributed by BJ, 5-Oct-2019.) (Proof modification is discouraged.) |
⊢ {𝑥, 𝑦} ∈ V | ||
Theorem | bj-prexg 13528 | Proof of prexg 4172 using only bounded separation. (Contributed by BJ, 5-Oct-2019.) (Proof modification is discouraged.) |
⊢ ((𝐴 ∈ 𝑉 ∧ 𝐵 ∈ 𝑊) → {𝐴, 𝐵} ∈ V) | ||
Theorem | bj-snexg 13529 | snexg 4146 from bounded separation. (Contributed by BJ, 5-Oct-2019.) (Proof modification is discouraged.) |
⊢ (𝐴 ∈ 𝑉 → {𝐴} ∈ V) | ||
Theorem | bj-snex 13530 | snex 4147 from bounded separation. (Contributed by BJ, 5-Oct-2019.) (Proof modification is discouraged.) |
⊢ 𝐴 ∈ V ⇒ ⊢ {𝐴} ∈ V | ||
Theorem | bj-sels 13531* | If a class is a set, then it is a member of a set. (Copied from set.mm.) (Contributed by BJ, 3-Apr-2019.) |
⊢ (𝐴 ∈ 𝑉 → ∃𝑥 𝐴 ∈ 𝑥) | ||
Theorem | bj-axun2 13532* | axun2 4396 from bounded separation. (Contributed by BJ, 15-Oct-2019.) (Proof modification is discouraged.) |
⊢ ∃𝑦∀𝑧(𝑧 ∈ 𝑦 ↔ ∃𝑤(𝑧 ∈ 𝑤 ∧ 𝑤 ∈ 𝑥)) | ||
Theorem | bj-uniex2 13533* | uniex2 4397 from bounded separation. (Contributed by BJ, 15-Oct-2019.) (Proof modification is discouraged.) |
⊢ ∃𝑦 𝑦 = ∪ 𝑥 | ||
Theorem | bj-uniex 13534 | uniex 4398 from bounded separation. (Contributed by BJ, 13-Nov-2019.) (Proof modification is discouraged.) |
⊢ 𝐴 ∈ V ⇒ ⊢ ∪ 𝐴 ∈ V | ||
Theorem | bj-uniexg 13535 | uniexg 4400 from bounded separation. (Contributed by BJ, 13-Nov-2019.) (Proof modification is discouraged.) |
⊢ (𝐴 ∈ 𝑉 → ∪ 𝐴 ∈ V) | ||
Theorem | bj-unex 13536 | unex 4402 from bounded separation. (Contributed by BJ, 13-Nov-2019.) (Proof modification is discouraged.) |
⊢ 𝐴 ∈ V & ⊢ 𝐵 ∈ V ⇒ ⊢ (𝐴 ∪ 𝐵) ∈ V | ||
Theorem | bdunexb 13537 | Bounded version of unexb 4403. (Contributed by BJ, 13-Nov-2019.) (Proof modification is discouraged.) |
⊢ BOUNDED 𝐴 & ⊢ BOUNDED 𝐵 ⇒ ⊢ ((𝐴 ∈ V ∧ 𝐵 ∈ V) ↔ (𝐴 ∪ 𝐵) ∈ V) | ||
Theorem | bj-unexg 13538 | unexg 4404 from bounded separation. (Contributed by BJ, 13-Nov-2019.) (Proof modification is discouraged.) |
⊢ ((𝐴 ∈ 𝑉 ∧ 𝐵 ∈ 𝑊) → (𝐴 ∪ 𝐵) ∈ V) | ||
Theorem | bj-sucexg 13539 | sucexg 4458 from bounded separation. (Contributed by BJ, 13-Nov-2019.) (Proof modification is discouraged.) |
⊢ (𝐴 ∈ 𝑉 → suc 𝐴 ∈ V) | ||
Theorem | bj-sucex 13540 | sucex 4459 from bounded separation. (Contributed by BJ, 13-Nov-2019.) (Proof modification is discouraged.) |
⊢ 𝐴 ∈ V ⇒ ⊢ suc 𝐴 ∈ V | ||
Axiom | ax-bj-d0cl 13541 | Axiom for Δ0-classical logic. (Contributed by BJ, 2-Jan-2020.) |
⊢ BOUNDED 𝜑 ⇒ ⊢ DECID 𝜑 | ||
Theorem | bj-d0clsepcl 13542 | Δ0-classical logic and separation implies classical logic. (Contributed by BJ, 2-Jan-2020.) (Proof modification is discouraged.) |
⊢ DECID 𝜑 | ||
Syntax | wind 13543 | Syntax for inductive classes. |
wff Ind 𝐴 | ||
Definition | df-bj-ind 13544* | Define the property of being an inductive class. (Contributed by BJ, 30-Nov-2019.) |
⊢ (Ind 𝐴 ↔ (∅ ∈ 𝐴 ∧ ∀𝑥 ∈ 𝐴 suc 𝑥 ∈ 𝐴)) | ||
Theorem | bj-indsuc 13545 | A direct consequence of the definition of Ind. (Contributed by BJ, 30-Nov-2019.) |
⊢ (Ind 𝐴 → (𝐵 ∈ 𝐴 → suc 𝐵 ∈ 𝐴)) | ||
Theorem | bj-indeq 13546 | Equality property for Ind. (Contributed by BJ, 30-Nov-2019.) |
⊢ (𝐴 = 𝐵 → (Ind 𝐴 ↔ Ind 𝐵)) | ||
Theorem | bj-bdind 13547 | Boundedness of the formula "the setvar 𝑥 is an inductive class". (Contributed by BJ, 30-Nov-2019.) |
⊢ BOUNDED Ind 𝑥 | ||
Theorem | bj-indint 13548* | The property of being an inductive class is closed under intersections. (Contributed by BJ, 30-Nov-2019.) |
⊢ Ind ∩ {𝑥 ∈ 𝐴 ∣ Ind 𝑥} | ||
Theorem | bj-indind 13549* | If 𝐴 is inductive and 𝐵 is "inductive in 𝐴", then (𝐴 ∩ 𝐵) is inductive. (Contributed by BJ, 25-Oct-2020.) |
⊢ ((Ind 𝐴 ∧ (∅ ∈ 𝐵 ∧ ∀𝑥 ∈ 𝐴 (𝑥 ∈ 𝐵 → suc 𝑥 ∈ 𝐵))) → Ind (𝐴 ∩ 𝐵)) | ||
Theorem | bj-dfom 13550 | Alternate definition of ω, as the intersection of all the inductive sets. Proposal: make this the definition. (Contributed by BJ, 30-Nov-2019.) |
⊢ ω = ∩ {𝑥 ∣ Ind 𝑥} | ||
Theorem | bj-omind 13551 | ω is an inductive class. (Contributed by BJ, 30-Nov-2019.) |
⊢ Ind ω | ||
Theorem | bj-omssind 13552 | ω is included in all the inductive sets (but for the moment, we cannot prove that it is included in all the inductive classes). (Contributed by BJ, 30-Nov-2019.) (Proof modification is discouraged.) |
⊢ (𝐴 ∈ 𝑉 → (Ind 𝐴 → ω ⊆ 𝐴)) | ||
Theorem | bj-ssom 13553* | A characterization of subclasses of ω. (Contributed by BJ, 30-Nov-2019.) (Proof modification is discouraged.) |
⊢ (∀𝑥(Ind 𝑥 → 𝐴 ⊆ 𝑥) ↔ 𝐴 ⊆ ω) | ||
Theorem | bj-om 13554* | A set is equal to ω if and only if it is the smallest inductive set. (Contributed by BJ, 30-Nov-2019.) (Proof modification is discouraged.) |
⊢ (𝐴 ∈ 𝑉 → (𝐴 = ω ↔ (Ind 𝐴 ∧ ∀𝑥(Ind 𝑥 → 𝐴 ⊆ 𝑥)))) | ||
Theorem | bj-2inf 13555* | Two formulations of the axiom of infinity (see ax-infvn 13558 and bj-omex 13559) . (Contributed by BJ, 30-Nov-2019.) (Proof modification is discouraged.) |
⊢ (ω ∈ V ↔ ∃𝑥(Ind 𝑥 ∧ ∀𝑦(Ind 𝑦 → 𝑥 ⊆ 𝑦))) | ||
The first three Peano postulates follow from constructive set theory (actually, from its core axioms). The proofs peano1 4554 and peano3 4556 already show this. In this section, we prove bj-peano2 13556 to complete this program. We also prove a preliminary version of the fifth Peano postulate from the core axioms. | ||
Theorem | bj-peano2 13556 | Constructive proof of peano2 4555. Temporary note: another possibility is to simply replace sucexg 4458 with bj-sucexg 13539 in the proof of peano2 4555. (Contributed by BJ, 18-Nov-2019.) (Proof modification is discouraged.) |
⊢ (𝐴 ∈ ω → suc 𝐴 ∈ ω) | ||
Theorem | peano5set 13557* | Version of peano5 4558 when ω ∩ 𝐴 is assumed to be a set, allowing a proof from the core axioms of CZF. (Contributed by BJ, 19-Nov-2019.) (Proof modification is discouraged.) |
⊢ ((ω ∩ 𝐴) ∈ 𝑉 → ((∅ ∈ 𝐴 ∧ ∀𝑥 ∈ ω (𝑥 ∈ 𝐴 → suc 𝑥 ∈ 𝐴)) → ω ⊆ 𝐴)) | ||
In the absence of full separation, the axiom of infinity has to be stated more precisely, as the existence of the smallest class containing the empty set and the successor of each of its elements. | ||
In this section, we introduce the axiom of infinity in a constructive setting (ax-infvn 13558) and deduce that the class ω of natural number ordinals is a set (bj-omex 13559). | ||
Axiom | ax-infvn 13558* | Axiom of infinity in a constructive setting. This asserts the existence of the special set we want (the set of natural numbers), instead of the existence of a set with some properties (ax-iinf 4548) from which one then proves, using full separation, that the wanted set exists (omex 4553). "vn" is for "von Neumann". (Contributed by BJ, 14-Nov-2019.) |
⊢ ∃𝑥(Ind 𝑥 ∧ ∀𝑦(Ind 𝑦 → 𝑥 ⊆ 𝑦)) | ||
Theorem | bj-omex 13559 | Proof of omex 4553 from ax-infvn 13558. (Contributed by BJ, 14-Nov-2019.) (Proof modification is discouraged.) |
⊢ ω ∈ V | ||
In this section, we give constructive proofs of two versions of Peano's fifth postulate. | ||
Theorem | bdpeano5 13560* | Bounded version of peano5 4558. (Contributed by BJ, 19-Nov-2019.) (Proof modification is discouraged.) |
⊢ BOUNDED 𝐴 ⇒ ⊢ ((∅ ∈ 𝐴 ∧ ∀𝑥 ∈ ω (𝑥 ∈ 𝐴 → suc 𝑥 ∈ 𝐴)) → ω ⊆ 𝐴) | ||
Theorem | speano5 13561* | Version of peano5 4558 when 𝐴 is assumed to be a set, allowing a proof from the core axioms of CZF. (Contributed by BJ, 19-Nov-2019.) (Proof modification is discouraged.) |
⊢ ((𝐴 ∈ 𝑉 ∧ ∅ ∈ 𝐴 ∧ ∀𝑥 ∈ ω (𝑥 ∈ 𝐴 → suc 𝑥 ∈ 𝐴)) → ω ⊆ 𝐴) | ||
In this section, we prove various versions of bounded induction from the basic axioms of CZF (in particular, without the axiom of set induction). We also prove Peano's fourth postulate. Together with the results from the previous sections, this proves from the core axioms of CZF (with infinity) that the set of natural number ordinals satisfies the five Peano postulates and thus provides a model for the set of natural numbers. | ||
Theorem | findset 13562* | Bounded induction (principle of induction when 𝐴 is assumed to be a set) allowing a proof from basic constructive axioms. See find 4559 for a nonconstructive proof of the general case. See bdfind 13563 for a proof when 𝐴 is assumed to be bounded. (Contributed by BJ, 22-Nov-2019.) (Proof modification is discouraged.) |
⊢ (𝐴 ∈ 𝑉 → ((𝐴 ⊆ ω ∧ ∅ ∈ 𝐴 ∧ ∀𝑥 ∈ 𝐴 suc 𝑥 ∈ 𝐴) → 𝐴 = ω)) | ||
Theorem | bdfind 13563* | Bounded induction (principle of induction when 𝐴 is assumed to be bounded), proved from basic constructive axioms. See find 4559 for a nonconstructive proof of the general case. See findset 13562 for a proof when 𝐴 is assumed to be a set. (Contributed by BJ, 22-Nov-2019.) (Proof modification is discouraged.) |
⊢ BOUNDED 𝐴 ⇒ ⊢ ((𝐴 ⊆ ω ∧ ∅ ∈ 𝐴 ∧ ∀𝑥 ∈ 𝐴 suc 𝑥 ∈ 𝐴) → 𝐴 = ω) | ||
Theorem | bj-bdfindis 13564* | Bounded induction (principle of induction for bounded formulas), using implicit substitutions (the biconditional versions of the hypotheses are implicit substitutions, and we have weakened them to implications). Constructive proof (from CZF). See finds 4560 for a proof of full induction in IZF. From this version, it is easy to prove bounded versions of finds 4560, finds2 4561, finds1 4562. (Contributed by BJ, 21-Nov-2019.) (Proof modification is discouraged.) |
⊢ BOUNDED 𝜑 & ⊢ Ⅎ𝑥𝜓 & ⊢ Ⅎ𝑥𝜒 & ⊢ Ⅎ𝑥𝜃 & ⊢ (𝑥 = ∅ → (𝜓 → 𝜑)) & ⊢ (𝑥 = 𝑦 → (𝜑 → 𝜒)) & ⊢ (𝑥 = suc 𝑦 → (𝜃 → 𝜑)) ⇒ ⊢ ((𝜓 ∧ ∀𝑦 ∈ ω (𝜒 → 𝜃)) → ∀𝑥 ∈ ω 𝜑) | ||
Theorem | bj-bdfindisg 13565* | Version of bj-bdfindis 13564 using a class term in the consequent. Constructive proof (from CZF). See the comment of bj-bdfindis 13564 for explanations. (Contributed by BJ, 21-Nov-2019.) (Proof modification is discouraged.) |
⊢ BOUNDED 𝜑 & ⊢ Ⅎ𝑥𝜓 & ⊢ Ⅎ𝑥𝜒 & ⊢ Ⅎ𝑥𝜃 & ⊢ (𝑥 = ∅ → (𝜓 → 𝜑)) & ⊢ (𝑥 = 𝑦 → (𝜑 → 𝜒)) & ⊢ (𝑥 = suc 𝑦 → (𝜃 → 𝜑)) & ⊢ Ⅎ𝑥𝐴 & ⊢ Ⅎ𝑥𝜏 & ⊢ (𝑥 = 𝐴 → (𝜑 → 𝜏)) ⇒ ⊢ ((𝜓 ∧ ∀𝑦 ∈ ω (𝜒 → 𝜃)) → (𝐴 ∈ ω → 𝜏)) | ||
Theorem | bj-bdfindes 13566 | Bounded induction (principle of induction for bounded formulas), using explicit substitutions. Constructive proof (from CZF). See the comment of bj-bdfindis 13564 for explanations. From this version, it is easy to prove the bounded version of findes 4563. (Contributed by BJ, 21-Nov-2019.) (Proof modification is discouraged.) |
⊢ BOUNDED 𝜑 ⇒ ⊢ (([∅ / 𝑥]𝜑 ∧ ∀𝑥 ∈ ω (𝜑 → [suc 𝑥 / 𝑥]𝜑)) → ∀𝑥 ∈ ω 𝜑) | ||
Theorem | bj-nn0suc0 13567* | Constructive proof of a variant of nn0suc 4564. For a constructive proof of nn0suc 4564, see bj-nn0suc 13581. (Contributed by BJ, 19-Nov-2019.) (Proof modification is discouraged.) |
⊢ (𝐴 ∈ ω → (𝐴 = ∅ ∨ ∃𝑥 ∈ 𝐴 𝐴 = suc 𝑥)) | ||
Theorem | bj-nntrans 13568 | A natural number is a transitive set. (Contributed by BJ, 22-Nov-2019.) (Proof modification is discouraged.) |
⊢ (𝐴 ∈ ω → (𝐵 ∈ 𝐴 → 𝐵 ⊆ 𝐴)) | ||
Theorem | bj-nntrans2 13569 | A natural number is a transitive set. (Contributed by BJ, 22-Nov-2019.) (Proof modification is discouraged.) |
⊢ (𝐴 ∈ ω → Tr 𝐴) | ||
Theorem | bj-nnelirr 13570 | A natural number does not belong to itself. Version of elirr 4501 for natural numbers, which does not require ax-setind 4497. (Contributed by BJ, 24-Nov-2019.) (Proof modification is discouraged.) |
⊢ (𝐴 ∈ ω → ¬ 𝐴 ∈ 𝐴) | ||
Theorem | bj-nnen2lp 13571 |
A version of en2lp 4514 for natural numbers, which does not require
ax-setind 4497.
Note: using this theorem and bj-nnelirr 13570, one can remove dependency on ax-setind 4497 from nntri2 6442 and nndcel 6448; one can actually remove more dependencies from these. (Contributed by BJ, 28-Nov-2019.) (Proof modification is discouraged.) |
⊢ ((𝐴 ∈ ω ∧ 𝐵 ∈ ω) → ¬ (𝐴 ∈ 𝐵 ∧ 𝐵 ∈ 𝐴)) | ||
Theorem | bj-peano4 13572 | Remove from peano4 4557 dependency on ax-setind 4497. Therefore, it only requires core constructive axioms (albeit more of them). (Contributed by BJ, 28-Nov-2019.) (Proof modification is discouraged.) |
⊢ ((𝐴 ∈ ω ∧ 𝐵 ∈ ω) → (suc 𝐴 = suc 𝐵 ↔ 𝐴 = 𝐵)) | ||
Theorem | bj-omtrans 13573 |
The set ω is transitive. A natural number is
included in
ω. Constructive proof of elnn 4566.
The idea is to use bounded induction with the formula 𝑥 ⊆ ω. This formula, in a logic with terms, is bounded. So in our logic without terms, we need to temporarily replace it with 𝑥 ⊆ 𝑎 and then deduce the original claim. (Contributed by BJ, 29-Dec-2019.) (Proof modification is discouraged.) |
⊢ (𝐴 ∈ ω → 𝐴 ⊆ ω) | ||
Theorem | bj-omtrans2 13574 | The set ω is transitive. (Contributed by BJ, 29-Dec-2019.) (Proof modification is discouraged.) |
⊢ Tr ω | ||
Theorem | bj-nnord 13575 | A natural number is an ordinal class. Constructive proof of nnord 4572. Can also be proved from bj-nnelon 13576 if the latter is proved from bj-omssonALT 13580. (Contributed by BJ, 27-Oct-2020.) (Proof modification is discouraged.) |
⊢ (𝐴 ∈ ω → Ord 𝐴) | ||
Theorem | bj-nnelon 13576 | A natural number is an ordinal. Constructive proof of nnon 4570. Can also be proved from bj-omssonALT 13580. (Contributed by BJ, 27-Oct-2020.) (Proof modification is discouraged.) |
⊢ (𝐴 ∈ ω → 𝐴 ∈ On) | ||
Theorem | bj-omord 13577 | The set ω is an ordinal class. Constructive proof of ordom 4567. (Contributed by BJ, 29-Dec-2019.) (Proof modification is discouraged.) |
⊢ Ord ω | ||
Theorem | bj-omelon 13578 | The set ω is an ordinal. Constructive proof of omelon 4569. (Contributed by BJ, 29-Dec-2019.) (Proof modification is discouraged.) |
⊢ ω ∈ On | ||
Theorem | bj-omsson 13579 | Constructive proof of omsson 4573. See also bj-omssonALT 13580. (Contributed by BJ, 27-Oct-2020.) (Proof modification is discouraged.) (New usage is discouraged. |
⊢ ω ⊆ On | ||
Theorem | bj-omssonALT 13580 | Alternate proof of bj-omsson 13579. (Contributed by BJ, 27-Oct-2020.) (Proof modification is discouraged.) (New usage is discouraged.) |
⊢ ω ⊆ On | ||
Theorem | bj-nn0suc 13581* | Proof of (biconditional form of) nn0suc 4564 from the core axioms of CZF. See also bj-nn0sucALT 13595. As a characterization of the elements of ω, this could be labeled "elom". (Contributed by BJ, 19-Nov-2019.) (Proof modification is discouraged.) |
⊢ (𝐴 ∈ ω ↔ (𝐴 = ∅ ∨ ∃𝑥 ∈ ω 𝐴 = suc 𝑥)) | ||
In this section, we add the axiom of set induction to the core axioms of CZF. | ||
In this section, we prove some variants of the axiom of set induction. | ||
Theorem | setindft 13582* | Axiom of set-induction with a disjoint variable condition replaced with a nonfreeness hypothesis. (Contributed by BJ, 22-Nov-2019.) |
⊢ (∀𝑥Ⅎ𝑦𝜑 → (∀𝑥(∀𝑦 ∈ 𝑥 [𝑦 / 𝑥]𝜑 → 𝜑) → ∀𝑥𝜑)) | ||
Theorem | setindf 13583* | Axiom of set-induction with a disjoint variable condition replaced with a nonfreeness hypothesis. (Contributed by BJ, 22-Nov-2019.) |
⊢ Ⅎ𝑦𝜑 ⇒ ⊢ (∀𝑥(∀𝑦 ∈ 𝑥 [𝑦 / 𝑥]𝜑 → 𝜑) → ∀𝑥𝜑) | ||
Theorem | setindis 13584* | Axiom of set induction using implicit substitutions. (Contributed by BJ, 22-Nov-2019.) |
⊢ Ⅎ𝑥𝜓 & ⊢ Ⅎ𝑥𝜒 & ⊢ Ⅎ𝑦𝜑 & ⊢ Ⅎ𝑦𝜓 & ⊢ (𝑥 = 𝑧 → (𝜑 → 𝜓)) & ⊢ (𝑥 = 𝑦 → (𝜒 → 𝜑)) ⇒ ⊢ (∀𝑦(∀𝑧 ∈ 𝑦 𝜓 → 𝜒) → ∀𝑥𝜑) | ||
Axiom | ax-bdsetind 13585* | Axiom of bounded set induction. (Contributed by BJ, 28-Nov-2019.) |
⊢ BOUNDED 𝜑 ⇒ ⊢ (∀𝑎(∀𝑦 ∈ 𝑎 [𝑦 / 𝑎]𝜑 → 𝜑) → ∀𝑎𝜑) | ||
Theorem | bdsetindis 13586* | Axiom of bounded set induction using implicit substitutions. (Contributed by BJ, 22-Nov-2019.) (Proof modification is discouraged.) |
⊢ BOUNDED 𝜑 & ⊢ Ⅎ𝑥𝜓 & ⊢ Ⅎ𝑥𝜒 & ⊢ Ⅎ𝑦𝜑 & ⊢ Ⅎ𝑦𝜓 & ⊢ (𝑥 = 𝑧 → (𝜑 → 𝜓)) & ⊢ (𝑥 = 𝑦 → (𝜒 → 𝜑)) ⇒ ⊢ (∀𝑦(∀𝑧 ∈ 𝑦 𝜓 → 𝜒) → ∀𝑥𝜑) | ||
Theorem | bj-inf2vnlem1 13587* | Lemma for bj-inf2vn 13591. Remark: unoptimized proof (have to use more deduction style). (Contributed by BJ, 8-Dec-2019.) (Proof modification is discouraged.) |
⊢ (∀𝑥(𝑥 ∈ 𝐴 ↔ (𝑥 = ∅ ∨ ∃𝑦 ∈ 𝐴 𝑥 = suc 𝑦)) → Ind 𝐴) | ||
Theorem | bj-inf2vnlem2 13588* | Lemma for bj-inf2vnlem3 13589 and bj-inf2vnlem4 13590. Remark: unoptimized proof (have to use more deduction style). (Contributed by BJ, 8-Dec-2019.) (Proof modification is discouraged.) |
⊢ (∀𝑥 ∈ 𝐴 (𝑥 = ∅ ∨ ∃𝑦 ∈ 𝐴 𝑥 = suc 𝑦) → (Ind 𝑍 → ∀𝑢(∀𝑡 ∈ 𝑢 (𝑡 ∈ 𝐴 → 𝑡 ∈ 𝑍) → (𝑢 ∈ 𝐴 → 𝑢 ∈ 𝑍)))) | ||
Theorem | bj-inf2vnlem3 13589* | Lemma for bj-inf2vn 13591. (Contributed by BJ, 8-Dec-2019.) (Proof modification is discouraged.) |
⊢ BOUNDED 𝐴 & ⊢ BOUNDED 𝑍 ⇒ ⊢ (∀𝑥 ∈ 𝐴 (𝑥 = ∅ ∨ ∃𝑦 ∈ 𝐴 𝑥 = suc 𝑦) → (Ind 𝑍 → 𝐴 ⊆ 𝑍)) | ||
Theorem | bj-inf2vnlem4 13590* | Lemma for bj-inf2vn2 13592. (Contributed by BJ, 8-Dec-2019.) (Proof modification is discouraged.) |
⊢ (∀𝑥 ∈ 𝐴 (𝑥 = ∅ ∨ ∃𝑦 ∈ 𝐴 𝑥 = suc 𝑦) → (Ind 𝑍 → 𝐴 ⊆ 𝑍)) | ||
Theorem | bj-inf2vn 13591* | A sufficient condition for ω to be a set. See bj-inf2vn2 13592 for the unbounded version from full set induction. (Contributed by BJ, 8-Dec-2019.) (Proof modification is discouraged.) |
⊢ BOUNDED 𝐴 ⇒ ⊢ (𝐴 ∈ 𝑉 → (∀𝑥(𝑥 ∈ 𝐴 ↔ (𝑥 = ∅ ∨ ∃𝑦 ∈ 𝐴 𝑥 = suc 𝑦)) → 𝐴 = ω)) | ||
Theorem | bj-inf2vn2 13592* | A sufficient condition for ω to be a set; unbounded version of bj-inf2vn 13591. (Contributed by BJ, 8-Dec-2019.) (Proof modification is discouraged.) |
⊢ (𝐴 ∈ 𝑉 → (∀𝑥(𝑥 ∈ 𝐴 ↔ (𝑥 = ∅ ∨ ∃𝑦 ∈ 𝐴 𝑥 = suc 𝑦)) → 𝐴 = ω)) | ||
Axiom | ax-inf2 13593* | Another axiom of infinity in a constructive setting (see ax-infvn 13558). (Contributed by BJ, 14-Nov-2019.) (New usage is discouraged.) |
⊢ ∃𝑎∀𝑥(𝑥 ∈ 𝑎 ↔ (𝑥 = ∅ ∨ ∃𝑦 ∈ 𝑎 𝑥 = suc 𝑦)) | ||
Theorem | bj-omex2 13594 | Using bounded set induction and the strong axiom of infinity, ω is a set, that is, we recover ax-infvn 13558 (see bj-2inf 13555 for the equivalence of the latter with bj-omex 13559). (Contributed by BJ, 8-Dec-2019.) (Proof modification is discouraged.) (New usage is discouraged.) |
⊢ ω ∈ V | ||
Theorem | bj-nn0sucALT 13595* | Alternate proof of bj-nn0suc 13581, also constructive but from ax-inf2 13593, hence requiring ax-bdsetind 13585. (Contributed by BJ, 8-Dec-2019.) (Proof modification is discouraged.) (New usage is discouraged.) |
⊢ (𝐴 ∈ ω ↔ (𝐴 = ∅ ∨ ∃𝑥 ∈ ω 𝐴 = suc 𝑥)) | ||
In this section, using the axiom of set induction, we prove full induction on the set of natural numbers. | ||
Theorem | bj-findis 13596* | Principle of induction, using implicit substitutions (the biconditional versions of the hypotheses are implicit substitutions, and we have weakened them to implications). Constructive proof (from CZF). See bj-bdfindis 13564 for a bounded version not requiring ax-setind 4497. See finds 4560 for a proof in IZF. From this version, it is easy to prove of finds 4560, finds2 4561, finds1 4562. (Contributed by BJ, 22-Dec-2019.) (Proof modification is discouraged.) |
⊢ Ⅎ𝑥𝜓 & ⊢ Ⅎ𝑥𝜒 & ⊢ Ⅎ𝑥𝜃 & ⊢ (𝑥 = ∅ → (𝜓 → 𝜑)) & ⊢ (𝑥 = 𝑦 → (𝜑 → 𝜒)) & ⊢ (𝑥 = suc 𝑦 → (𝜃 → 𝜑)) ⇒ ⊢ ((𝜓 ∧ ∀𝑦 ∈ ω (𝜒 → 𝜃)) → ∀𝑥 ∈ ω 𝜑) | ||
Theorem | bj-findisg 13597* | Version of bj-findis 13596 using a class term in the consequent. Constructive proof (from CZF). See the comment of bj-findis 13596 for explanations. (Contributed by BJ, 21-Nov-2019.) (Proof modification is discouraged.) |
⊢ Ⅎ𝑥𝜓 & ⊢ Ⅎ𝑥𝜒 & ⊢ Ⅎ𝑥𝜃 & ⊢ (𝑥 = ∅ → (𝜓 → 𝜑)) & ⊢ (𝑥 = 𝑦 → (𝜑 → 𝜒)) & ⊢ (𝑥 = suc 𝑦 → (𝜃 → 𝜑)) & ⊢ Ⅎ𝑥𝐴 & ⊢ Ⅎ𝑥𝜏 & ⊢ (𝑥 = 𝐴 → (𝜑 → 𝜏)) ⇒ ⊢ ((𝜓 ∧ ∀𝑦 ∈ ω (𝜒 → 𝜃)) → (𝐴 ∈ ω → 𝜏)) | ||
Theorem | bj-findes 13598 | Principle of induction, using explicit substitutions. Constructive proof (from CZF). See the comment of bj-findis 13596 for explanations. From this version, it is easy to prove findes 4563. (Contributed by BJ, 21-Nov-2019.) (Proof modification is discouraged.) |
⊢ (([∅ / 𝑥]𝜑 ∧ ∀𝑥 ∈ ω (𝜑 → [suc 𝑥 / 𝑥]𝜑)) → ∀𝑥 ∈ ω 𝜑) | ||
In this section, we state the axiom scheme of strong collection, which is part of CZF set theory. | ||
Axiom | ax-strcoll 13599* | Axiom scheme of strong collection. It is stated with all possible disjoint variable conditions, to show that this weak form is sufficient. The antecedent means that 𝜑 represents a multivalued function on 𝑎, or equivalently a collection of nonempty classes indexed by 𝑎, and the axiom asserts the existence of a set 𝑏 which "collects" at least one element in the image of each 𝑥 ∈ 𝑎 and which is made only of such elements. That second conjunct is what makes it "strong", compared to the axiom scheme of collection ax-coll 4080. (Contributed by BJ, 5-Oct-2019.) |
⊢ ∀𝑎(∀𝑥 ∈ 𝑎 ∃𝑦𝜑 → ∃𝑏(∀𝑥 ∈ 𝑎 ∃𝑦 ∈ 𝑏 𝜑 ∧ ∀𝑦 ∈ 𝑏 ∃𝑥 ∈ 𝑎 𝜑)) | ||
Theorem | strcoll2 13600* | Version of ax-strcoll 13599 with one disjoint variable condition removed and without initial universal quantifier. (Contributed by BJ, 5-Oct-2019.) |
⊢ (∀𝑥 ∈ 𝑎 ∃𝑦𝜑 → ∃𝑏(∀𝑥 ∈ 𝑎 ∃𝑦 ∈ 𝑏 𝜑 ∧ ∀𝑦 ∈ 𝑏 ∃𝑥 ∈ 𝑎 𝜑)) |
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