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Theorem List for Metamath Proof Explorer - 35701-35800   *Has distinct variable group(s)
TypeLabelDescription
Statement
 
Theorembj-pr2eq 35701 Substitution property for pr2. (Contributed by BJ, 6-Oct-2018.)
(𝐴 = 𝐵 → pr2 𝐴 = pr2 𝐵)
 
Theorembj-pr2un 35702 The second projection preserves unions. (Contributed by BJ, 6-Apr-2019.)
pr2 (𝐴𝐵) = (pr2 𝐴 ∪ pr2 𝐵)
 
Theorembj-pr2val 35703 Value of the second projection. (Contributed by BJ, 6-Apr-2019.)
pr2 ({𝐴} × tag 𝐵) = if(𝐴 = 1o, 𝐵, ∅)
 
Theorembj-pr22val 35704 Value of the second projection of a couple. (Contributed by BJ, 6-Oct-2018.)
pr2𝐴, 𝐵⦆ = 𝐵
 
Theorembj-pr2ex 35705 Sethood of the second projection. (Contributed by BJ, 6-Oct-2018.)
(𝐴𝑉 → pr2 𝐴 ∈ V)
 
Theorembj-2uplth 35706 The characteristic property of couples. Note that this holds without sethood hypotheses (compare opth 5469). (Contributed by BJ, 6-Oct-2018.)
(⦅𝐴, 𝐵⦆ = ⦅𝐶, 𝐷⦆ ↔ (𝐴 = 𝐶𝐵 = 𝐷))
 
Theorembj-2uplex 35707 A couple is a set if and only if its coordinates are sets. For the advantages offered by the reverse closure property, see the section head comment. (Contributed by BJ, 6-Oct-2018.)
(⦅𝐴, 𝐵⦆ ∈ V ↔ (𝐴 ∈ V ∧ 𝐵 ∈ V))
 
Theorembj-2upln0 35708 A couple is nonempty. (Contributed by BJ, 21-Apr-2019.)
𝐴, 𝐵⦆ ≠ ∅
 
Theorembj-2upln1upl 35709 A couple is never equal to a monuple. It is in order to have this "non-clashing" result that tagging was used. Without tagging, we would have 𝐴, ∅⦆ = ⦅𝐴. Note that in the context of Morse tuples, it is natural to define the 0-tuple as the empty set. Therefore, the present theorem together with bj-1upln0 35694 and bj-2upln0 35708 tell us that an m-tuple may equal an n-tuple only when m = n, at least for m, n <= 2, but this result would extend as soon as we define n-tuples for higher values of n. (Contributed by BJ, 21-Apr-2019.)
𝐴, 𝐵⦆ ≠ ⦅𝐶
 
21.15.5.16  Set theory: elementary operations relative to a universe

Some elementary set-theoretic operations "relative to a universe" (by which is merely meant some given class considered as a universe).

 
Theorembj-rcleqf 35710 Relative version of cleqf 2933. (Contributed by BJ, 27-Dec-2023.)
𝑥𝐴    &   𝑥𝐵    &   𝑥𝑉       ((𝑉𝐴) = (𝑉𝐵) ↔ ∀𝑥𝑉 (𝑥𝐴𝑥𝐵))
 
Theorembj-rcleq 35711* Relative version of dfcleq 2724. (Contributed by BJ, 27-Dec-2023.)
((𝑉𝐴) = (𝑉𝐵) ↔ ∀𝑥𝑉 (𝑥𝐴𝑥𝐵))
 
Theorembj-reabeq 35712* Relative form of eqabb 2872. (Contributed by BJ, 27-Dec-2023.)
((𝑉𝐴) = {𝑥𝑉𝜑} ↔ ∀𝑥𝑉 (𝑥𝐴𝜑))
 
Theorembj-disj2r 35713 Relative version of ssdifin0 4479, allowing a biconditional, and of disj2 4453. (Contributed by BJ, 11-Nov-2021.) This proof does not rely, even indirectly, on ssdifin0 4479 nor disj2 4453. (Proof modification is discouraged.)
((𝐴𝑉) ⊆ (𝑉𝐵) ↔ ((𝐴𝐵) ∩ 𝑉) = ∅)
 
Theorembj-sscon 35714 Contraposition law for relative subclasses. Relative and generalized version of ssconb 4133, which it can shorten, as well as conss2 42973. (Contributed by BJ, 11-Nov-2021.) This proof does not rely, even indirectly, on ssconb 4133 nor conss2 42973. (Proof modification is discouraged.)
((𝐴𝑉) ⊆ (𝑉𝐵) ↔ (𝐵𝑉) ⊆ (𝑉𝐴))
 
21.15.5.17  Axioms for finite unions

In this section, we introduce the axiom of singleton ax-bj-sn 35718 and the axiom of binary union ax-bj-bun 35722. Both axioms are implied by the standard axioms of unordered pair ax-pr 5420 and of union ax-un 7708 (see snex 5424 and unex 7716). Conversely, the axiom of unordered pair ax-pr 5420 is implied by the axioms of singleton and of binary union, as proved in bj-prexg 35724 and bj-prex 35725.

The axioms of union ax-un 7708 and of powerset ax-pow 5356 are independent of these axioms: consider respectively the class of pseudo-hereditarily sets of cardinality less than a given singular strong limit cardinal, see

Greg Oman, On the axiom of union, Arch. Math. Logic (2010) 49:283--289

(that model does have finite unions), and the class of well-founded hereditarily countable sets (or hereditarily less than a given uncountable regular cardinal). See also https://mathoverflow.net/questions/81815 5356 and https://mathoverflow.net/questions/48365 5356.

A proof by finite induction shows that the existence of finite unions is equivalent to the existence of binary unions and of nullary unions (the latter being the axiom of the empty set ax-nul 5299).

The axiom of binary union is useful in theories without the axioms of union ax-un 7708 and of powerset ax-pow 5356. For instance, the class of well-founded sets hereditarily of cardinality at most 𝑛 ∈ ℕ0 with ordinary membership relation is a model of { ax-ext 2702, ax-rep 5278, ax-sep 5292, ax-nul 5299, ax-reg 9569 } and the axioms of existence of unordered 𝑚-tuples for all 𝑚𝑛, and in most cases one would like to rule out such models, hence the need for extra axioms, typically variants of powersets or unions.

The axiom of adjunction ax-bj-adj 35727 is more widely used, and is an axiom of General Set Theory. We prove how to retrieve it from binary union and singleton in bj-adjfrombun 35731 and conversely how to prove from adjunction singleton (bj-snfromadj 35729) and unordered pair (bj-prfromadj 35730).

 
Theorembj-abex 35715* Two ways of stating that the extension of a formula is a set. (Contributed by BJ, 18-Jan-2025.) (Proof modification is discouraged.)
({𝑥𝜑} ∈ V ↔ ∃𝑦𝑥(𝑥𝑦𝜑))
 
Theorembj-clex 35716* Two ways of stating that a class is a set. (Contributed by BJ, 18-Jan-2025.) (Proof modification is discouraged.)
(𝑥𝐴𝜑)       (𝐴 ∈ V ↔ ∃𝑦𝑥(𝑥𝑦𝜑))
 
Theorembj-axsn 35717* Two ways of stating the axiom of singleton (which is the universal closure of either side, see ax-bj-sn 35718). (Contributed by BJ, 12-Jan-2025.) (Proof modification is discouraged.)
({𝑥} ∈ V ↔ ∃𝑦𝑧(𝑧𝑦𝑧 = 𝑥))
 
Axiomax-bj-sn 35718* Axiom of singleton. (Contributed by BJ, 12-Jan-2025.)
𝑥𝑦𝑧(𝑧𝑦𝑧 = 𝑥)
 
Theorembj-snexg 35719 A singleton built on a set is a set. Contrary to bj-snex 35720, this proof is intuitionistically valid and does not require ax-nul 5299. (Contributed by NM, 7-Aug-1994.) Extract it from snex 5424 and prove it from ax-bj-sn 35718. (Revised by BJ, 12-Jan-2025.) (Proof modification is discouraged.)
(𝐴𝑉 → {𝐴} ∈ V)
 
Theorembj-snex 35720 A singleton is a set. See also snex 5424, snexALT 5374. (Contributed by NM, 7-Aug-1994.) Prove it from ax-bj-sn 35718. (Revised by BJ, 12-Jan-2025.) (Proof modification is discouraged.)
{𝐴} ∈ V
 
Theorembj-axbun 35721* Two ways of stating the axiom of binary union (which is the universal closure of either side, see ax-bj-bun 35722). (Contributed by BJ, 12-Jan-2025.) (Proof modification is discouraged.)
((𝑥𝑦) ∈ V ↔ ∃𝑧𝑡(𝑡𝑧 ↔ (𝑡𝑥𝑡𝑦)))
 
Axiomax-bj-bun 35722* Axiom of binary union. (Contributed by BJ, 12-Jan-2025.)
𝑥𝑦𝑧𝑡(𝑡𝑧 ↔ (𝑡𝑥𝑡𝑦))
 
Theorembj-unexg 35723 Existence of binary unions of sets, proved from ax-bj-bun 35722. (Contributed by BJ, 12-Jan-2025.) (Proof modification is discouraged.)
((𝐴𝑉𝐵𝑊) → (𝐴𝐵) ∈ V)
 
Theorembj-prexg 35724 Existence of unordered pairs formed on sets, proved from ax-bj-sn 35718 and ax-bj-bun 35722. Contrary to bj-prex 35725, this proof is intuitionistically valid and does not require ax-nul 5299. (Contributed by BJ, 12-Jan-2025.) (Proof modification is discouraged.)
((𝐴𝑉𝐵𝑊) → {𝐴, 𝐵} ∈ V)
 
Theorembj-prex 35725 Existence of unordered pairs proved from ax-bj-sn 35718 and ax-bj-bun 35722. (Contributed by BJ, 12-Jan-2025.) (Proof modification is discouraged.)
{𝐴, 𝐵} ∈ V
 
Theorembj-axadj 35726* Two ways of stating the axiom of adjunction (which is the universal closure of either side, see ax-bj-adj 35727). (Contributed by BJ, 12-Jan-2025.) (Proof modification is discouraged.)
((𝑥 ∪ {𝑦}) ∈ V ↔ ∃𝑧𝑡(𝑡𝑧 ↔ (𝑡𝑥𝑡 = 𝑦)))
 
Axiomax-bj-adj 35727* Axiom of adjunction. (Contributed by BJ, 19-Jan-2025.)
𝑥𝑦𝑧𝑡(𝑡𝑧 ↔ (𝑡𝑥𝑡 = 𝑦))
 
Theorembj-adjg1 35728 Existence of the result of the adjunction (generalized only in the first term since this suffices for current applications). (Contributed by BJ, 19-Jan-2025.) (Proof modification is discouraged.)
(𝐴𝑉 → (𝐴 ∪ {𝑥}) ∈ V)
 
Theorembj-snfromadj 35729 Singleton from adjunction and empty set. (Contributed by BJ, 19-Jan-2025.) (Proof modification is discouraged.)
{𝑥} ∈ V
 
Theorembj-prfromadj 35730 Unordered pair from adjunction. (Contributed by BJ, 19-Jan-2025.) (Proof modification is discouraged.)
{𝑥, 𝑦} ∈ V
 
Theorembj-adjfrombun 35731 Adjunction from singleton and binary union. (Contributed by BJ, 19-Jan-2025.) (Proof modification is discouraged.)
(𝑥 ∪ {𝑦}) ∈ V
 
21.15.5.18  Set theory: miscellaneous

Miscellaneous theorems of set theory.

 
Theoremeleq2w2ALT 35732 Alternate proof of eleq2w2 2727 and special instance of eleq2 2821. (Contributed by BJ, 22-Sep-2024.) (Proof modification is discouraged.) (New usage is discouraged.)
(𝐴 = 𝐵 → (𝑥𝐴𝑥𝐵))
 
Theorembj-clel3gALT 35733* Alternate proof of clel3g 3646. (Contributed by BJ, 1-Sep-2024.) (Proof modification is discouraged.) (New usage is discouraged.)
(𝐵𝑉 → (𝐴𝐵 ↔ ∃𝑥(𝑥 = 𝐵𝐴𝑥)))
 
Theorembj-pw0ALT 35734 Alternate proof of pw0 4808. The proofs have a similar structure: pw0 4808 uses the definitions of powerclass and singleton as class abstractions, whereas bj-pw0ALT 35734 uses characterizations of their elements. Both proofs then use transitivity of a congruence relation (equality for pw0 4808 and biconditional for bj-pw0ALT 35734) to translate the property ss0b 4393 into the wanted result. To translate a biconditional into a class equality, pw0 4808 uses abbii 2801 (which yields an equality of class abstractions), while bj-pw0ALT 35734 uses eqriv 2728 (which requires a biconditional of membership of a given setvar variable). Note that abbii 2801, through its closed form abbi 2799, is proved from eqrdv 2729, which is the deduction form of eqriv 2728. In the other direction, velpw 4601 and velsn 4638 are proved from the definitions of powerclass and singleton using elabg 3662, which is a version of abbii 2801 suited for membership characterizations. (Contributed by BJ, 14-Apr-2024.) (Proof modification is discouraged.) (New usage is discouraged.)
𝒫 ∅ = {∅}
 
Theorembj-sselpwuni 35735 Quantitative version of ssexg 5316: a subset of an element of a class is an element of the powerclass of the union of that class. (Contributed by BJ, 6-Apr-2024.)
((𝐴𝐵𝐵𝑉) → 𝐴 ∈ 𝒫 𝑉)
 
Theorembj-unirel 35736 Quantitative version of uniexr 7733: if the union of a class is an element of a class, then that class is an element of the double powerclass of the union of this class. (Contributed by BJ, 6-Apr-2024.)
( 𝐴𝑉𝐴 ∈ 𝒫 𝒫 𝑉)
 
Theorembj-elpwg 35737 If the intersection of two classes is a set, then inclusion among these classes is equivalent to membership in the powerclass. Common generalization of elpwg 4599 and elpw2g 5337 (the latter of which could be proved from it). (Contributed by BJ, 31-Dec-2023.)
((𝐴𝐵) ∈ 𝑉 → (𝐴 ∈ 𝒫 𝐵𝐴𝐵))
 
Theorembj-velpwALT 35738* This theorem bj-velpwALT 35738 and the next theorem bj-elpwgALT 35739 are alternate proofs of velpw 4601 and elpwg 4599 respectively, where one proves first the setvar case and then generalizes using vtoclbg 3556 instead of proving first the general case using elab2g 3666 and then specifying. Here, this results in needing an extra DV condition, a longer combined proof and use of ax-12 2171. In other cases, that order is better (e.g., vsnex 5422 proved before snexg 5423). (Contributed by BJ, 17-Jan-2025.) (Proof modification is discouraged.) (New usage is discouraged.)
(𝑥 ∈ 𝒫 𝐴𝑥𝐴)
 
Theorembj-elpwgALT 35739 Alternate proof of elpwg 4599. See comment for bj-velpwALT 35738. (Contributed by BJ, 17-Jan-2025.) (Proof modification is discouraged.) (New usage is discouraged.)
(𝐴𝑉 → (𝐴 ∈ 𝒫 𝐵𝐴𝐵))
 
Theorembj-vjust 35740 Justification theorem for dfv2 3476 if it were the definition. See also vjust 3474. (Contributed by BJ, 30-Nov-2019.) (Proof modification is discouraged.)
{𝑥 ∣ ⊤} = {𝑦 ∣ ⊤}
 
Theorembj-nul 35741* Two formulations of the axiom of the empty set ax-nul 5299. Proposal: place it right before ax-nul 5299. (Contributed by BJ, 30-Nov-2019.) (Proof modification is discouraged.)
(∅ ∈ V ↔ ∃𝑥𝑦 ¬ 𝑦𝑥)
 
Theorembj-nuliota 35742* Definition of the empty set using the definite description binder. See also bj-nuliotaALT 35743. (Contributed by BJ, 30-Nov-2019.) (Proof modification is discouraged.)
∅ = (℩𝑥𝑦 ¬ 𝑦𝑥)
 
Theorembj-nuliotaALT 35743* Alternate proof of bj-nuliota 35742. Note that this alternate proof uses the fact that 𝑥𝜑 evaluates to when there is no 𝑥 satisfying 𝜑 (iotanul 6510). This is an implementation detail of the encoding currently used in set.mm and should be avoided. (Contributed by BJ, 30-Nov-2019.) (Proof modification is discouraged.) (New usage is discouraged.)
∅ = (℩𝑥𝑦 ¬ 𝑦𝑥)
 
Theorembj-vtoclgfALT 35744 Alternate proof of vtoclgf 3551. Proof from vtoclgft 3537. (This may have been the original proof before shortening.) (Contributed by BJ, 30-Sep-2019.) (Proof modification is discouraged.) (New usage is discouraged.)
𝑥𝐴    &   𝑥𝜓    &   (𝑥 = 𝐴 → (𝜑𝜓))    &   𝜑       (𝐴𝑉𝜓)
 
Theorembj-elsn12g 35745 Join of elsng 4636 and elsn2g 4660. (Contributed by BJ, 18-Nov-2023.)
((𝐴𝑉𝐵𝑊) → (𝐴 ∈ {𝐵} ↔ 𝐴 = 𝐵))
 
Theorembj-elsnb 35746 Biconditional version of elsng 4636. (Contributed by BJ, 18-Nov-2023.)
(𝐴 ∈ {𝐵} ↔ (𝐴 ∈ V ∧ 𝐴 = 𝐵))
 
Theorembj-pwcfsdom 35747 Remove hypothesis from pwcfsdom 10560. Illustration of how to remove a "proof-facilitating hypothesis". (Can use it to shorten theorems using pwcfsdom 10560.) (Contributed by BJ, 14-Sep-2019.)
(ℵ‘𝐴) ≺ ((ℵ‘𝐴) ↑m (cf‘(ℵ‘𝐴)))
 
Theorembj-grur1 35748 Remove hypothesis from grur1 10797. Illustration of how to remove a "definitional hypothesis". This makes its uses longer, but the theorem feels more self-contained. It looks preferable when the defined term appears only once in the conclusion. (Contributed by BJ, 14-Sep-2019.)
((𝑈 ∈ Univ ∧ 𝑈 (𝑅1 “ On)) → 𝑈 = (𝑅1‘(𝑈 ∩ On)))
 
Theorembj-bm1.3ii 35749* The extension of a predicate (𝜑(𝑧)) is included in a set (𝑥) if and only if it is a set (𝑦). Sufficiency is obvious, and necessity is the content of the axiom of separation ax-sep 5292. Similar to Theorem 1.3(ii) of [BellMachover] p. 463. (Contributed by NM, 21-Jun-1993.) Generalized to a closed form biconditional with existential quantifications using two different setvars 𝑥, 𝑦 (which need not be disjoint). (Revised by BJ, 8-Aug-2022.)

TODO: move in place of bm1.3ii 5295. Relabel ("sepbi"?).

(∃𝑥𝑧(𝜑𝑧𝑥) ↔ ∃𝑦𝑧(𝑧𝑦𝜑))
 
Theorembj-dfid2ALT 35750 Alternate version of dfid2 5569. (Contributed by BJ, 9-Nov-2024.) (Proof modification is discouraged.) Use df-id 5567 instead to make the semantics of the construction df-opab 5204 clearer. (New usage is discouraged.)
I = {⟨𝑥, 𝑥⟩ ∣ ⊤}
 
Theorembj-0nelopab 35751 The empty set is never an element in an ordered-pair class abstraction. (Contributed by Alexander van der Vekens, 5-Nov-2017.) (Proof shortened by BJ, 22-Jul-2023.)

TODO: move to the main section when one can reorder sections so that we can use relopab 5816 (this is a very limited reordering).

¬ ∅ ∈ {⟨𝑥, 𝑦⟩ ∣ 𝜑}
 
Theorembj-brrelex12ALT 35752 Two classes related by a binary relation are both sets. Alternate proof of brrelex12 5720. (Contributed by BJ, 14-Jul-2023.) (Proof modification is discouraged.) (New usage is discouraged.)
((Rel 𝑅𝐴𝑅𝐵) → (𝐴 ∈ V ∧ 𝐵 ∈ V))
 
Theorembj-epelg 35753 The membership relation and the membership predicate agree when the "containing" class is a set. General version of epel 5576 and closed form of epeli 5575. (Contributed by Scott Fenton, 27-Mar-2011.) (Revised by Mario Carneiro, 28-Apr-2015.) TODO: move it to the main section after reordering to have brrelex1i 5724 available. (Proof shortened by BJ, 14-Jul-2023.) (Proof modification is discouraged.)
(𝐵𝑉 → (𝐴 E 𝐵𝐴𝐵))
 
Theorembj-epelb 35754 Two classes are related by the membership relation if and only if they are related by the membership relation (i.e., the first is an element of the second) and the second is a set (hence so is the first). TODO: move to Main after reordering to have brrelex2i 5725 available. Check if it is shorter to prove bj-epelg 35753 first or bj-epelb 35754 first. (Contributed by BJ, 14-Jul-2023.)
(𝐴 E 𝐵 ↔ (𝐴𝐵𝐵 ∈ V))
 
Theorembj-nsnid 35755 A set does not contain the singleton formed on it. More precisely, one can prove that a class contains the singleton formed on it if and only if it is proper and contains the empty set (since it is "the singleton formed on" any proper class, see snprc 4714): ¬ ({𝐴} ∈ 𝐴 ↔ (∅ ∈ 𝐴𝐴 ∈ V)). (Contributed by BJ, 4-Feb-2023.)
(𝐴𝑉 → ¬ {𝐴} ∈ 𝐴)
 
Theorembj-rdg0gALT 35756 Alternate proof of rdg0g 8409. More direct since it bypasses tz7.44-1 8388 and rdg0 8403 (and vtoclg 3553, vtoclga 3562). (Contributed by NM, 25-Apr-1995.) More direct proof. (Revised by BJ, 17-Nov-2024.) (Proof modification is discouraged.) (New usage is discouraged.)
(𝐴𝑉 → (rec(𝐹, 𝐴)‘∅) = 𝐴)
 
21.15.5.19  Evaluation at a class

This section treats the existing predicate Slot (df-slot 17097) as "evaluation at a class" and for the moment does not introduce new syntax for it.

 
Theorembj-evaleq 35757 Equality theorem for the Slot construction. This is currently a duplicate of sloteq 17098 but may diverge from it if/when a token Eval is introduced for evaluation in order to separate it from Slot and any of its possible modifications. (Contributed by BJ, 27-Dec-2021.) (Proof modification is discouraged.)
(𝐴 = 𝐵 → Slot 𝐴 = Slot 𝐵)
 
Theorembj-evalfun 35758 The evaluation at a class is a function. (Contributed by BJ, 27-Dec-2021.)
Fun Slot 𝐴
 
Theorembj-evalfn 35759 The evaluation at a class is a function on the universal class. (General form of slotfn 17099). (Contributed by Mario Carneiro, 22-Sep-2015.) (Revised by BJ, 27-Dec-2021.)
Slot 𝐴 Fn V
 
Theorembj-evalval 35760 Value of the evaluation at a class. (Closed form of strfvnd 17100 and strfvn 17101). (Contributed by NM, 9-Sep-2011.) (Revised by Mario Carneiro, 15-Nov-2014.) (Revised by BJ, 27-Dec-2021.)
(𝐹𝑉 → (Slot 𝐴𝐹) = (𝐹𝐴))
 
Theorembj-evalid 35761 The evaluation at a set of the identity function is that set. (General form of ndxarg 17111.) The restriction to a set 𝑉 is necessary since the argument of the function Slot 𝐴 (like that of any function) has to be a set for the evaluation to be meaningful. (Contributed by BJ, 27-Dec-2021.)
((𝑉𝑊𝐴𝑉) → (Slot 𝐴‘( I ↾ 𝑉)) = 𝐴)
 
Theorembj-ndxarg 35762 Proof of ndxarg 17111 from bj-evalid 35761. (Contributed by BJ, 27-Dec-2021.) (Proof modification is discouraged.)
𝐸 = Slot 𝑁    &   𝑁 ∈ ℕ       (𝐸‘ndx) = 𝑁
 
Theorembj-evalidval 35763 Closed general form of strndxid 17113. Both sides are equal to (𝐹𝐴) by bj-evalid 35761 and bj-evalval 35760 respectively, but bj-evalidval 35763 adds something to bj-evalid 35761 and bj-evalval 35760 in that Slot 𝐴 appears on both sides. (Contributed by BJ, 27-Dec-2021.)
((𝑉𝑊𝐴𝑉𝐹𝑈) → (𝐹‘(Slot 𝐴‘( I ↾ 𝑉))) = (Slot 𝐴𝐹))
 
21.15.5.20  Elementwise operations
 
Syntaxcelwise 35764 Syntax for elementwise operations.
class elwise
 
Definitiondf-elwise 35765* Define the elementwise operation associated with a given operation. For instance, + is the addition of complex numbers (axaddf 11122), so if 𝐴 and 𝐵 are sets of complex numbers, then (𝐴(elwise‘ + )𝐵) is the set of numbers of the form (𝑥 + 𝑦) with 𝑥𝐴 and 𝑦𝐵. The set of odd natural numbers is (({2}(elwise‘ · )ℕ0)(elwise‘ + ){1}), or less formally 2ℕ0 + 1. (Contributed by BJ, 22-Dec-2021.)
elwise = (𝑜 ∈ V ↦ (𝑥 ∈ V, 𝑦 ∈ V ↦ {𝑧 ∣ ∃𝑢𝑥𝑣𝑦 𝑧 = (𝑢𝑜𝑣)}))
 
21.15.5.21  Elementwise intersection (families of sets induced on a subset)

Many kinds of structures are given by families of subsets of a given set: Moore collections (df-mre 17512), topologies (df-top 22325), pi-systems, rings of sets, delta-rings, lambda-systems/Dynkin systems, algebras/fields of sets, sigma-algebras/sigma-fields/tribes (df-siga 32938), sigma rings, monotone classes, matroids/independent sets, bornologies, filters.

There is a natural notion of structure induced on a subset. It is often given by an elementwise intersection, namely, the family of intersections of sets in the original family with the given subset. In this subsection, we define this notion and prove its main properties. Classical conditions on families of subsets include being nonempty, containing the whole set, containing the empty set, being stable under unions, intersections, subsets, supersets, (relative) complements. Therefore, we prove related properties for the elementwise intersection.

We will call (𝑋t 𝐴) the elementwise intersection on the family 𝑋 by the class 𝐴.

REMARK: many theorems are already in set.mm: "MM> SEARCH *rest* / JOIN".

 
Theorembj-rest00 35766 An elementwise intersection on the empty family is the empty set. TODO: this is 0rest 17357. (Contributed by BJ, 27-Apr-2021.)
(∅ ↾t 𝐴) = ∅
 
Theorembj-restsn 35767 An elementwise intersection on the singleton on a set is the singleton on the intersection by that set. Generalization of bj-restsn0 35770 and bj-restsnid 35772. (Contributed by BJ, 27-Apr-2021.)
((𝑌𝑉𝐴𝑊) → ({𝑌} ↾t 𝐴) = {(𝑌𝐴)})
 
Theorembj-restsnss 35768 Special case of bj-restsn 35767. (Contributed by BJ, 27-Apr-2021.)
((𝑌𝑉𝐴𝑌) → ({𝑌} ↾t 𝐴) = {𝐴})
 
Theorembj-restsnss2 35769 Special case of bj-restsn 35767. (Contributed by BJ, 27-Apr-2021.)
((𝐴𝑉𝑌𝐴) → ({𝑌} ↾t 𝐴) = {𝑌})
 
Theorembj-restsn0 35770 An elementwise intersection on the singleton on the empty set is the singleton on the empty set. Special case of bj-restsn 35767 and bj-restsnss2 35769. TODO: this is restsn 22603. (Contributed by BJ, 27-Apr-2021.)
(𝐴𝑉 → ({∅} ↾t 𝐴) = {∅})
 
Theorembj-restsn10 35771 Special case of bj-restsn 35767, bj-restsnss 35768, and bj-rest10 35773. (Contributed by BJ, 27-Apr-2021.)
(𝑋𝑉 → ({𝑋} ↾t ∅) = {∅})
 
Theorembj-restsnid 35772 The elementwise intersection on the singleton on a class by that class is the singleton on that class. Special case of bj-restsn 35767 and bj-restsnss 35768. (Contributed by BJ, 27-Apr-2021.)
({𝐴} ↾t 𝐴) = {𝐴}
 
Theorembj-rest10 35773 An elementwise intersection on a nonempty family by the empty set is the singleton on the empty set. TODO: this generalizes rest0 22602 and could replace it. (Contributed by BJ, 27-Apr-2021.)
(𝑋𝑉 → (𝑋 ≠ ∅ → (𝑋t ∅) = {∅}))
 
Theorembj-rest10b 35774 Alternate version of bj-rest10 35773. (Contributed by BJ, 27-Apr-2021.)
(𝑋 ∈ (𝑉 ∖ {∅}) → (𝑋t ∅) = {∅})
 
Theorembj-restn0 35775 An elementwise intersection on a nonempty family is nonempty. (Contributed by BJ, 27-Apr-2021.)
((𝑋𝑉𝐴𝑊) → (𝑋 ≠ ∅ → (𝑋t 𝐴) ≠ ∅))
 
Theorembj-restn0b 35776 Alternate version of bj-restn0 35775. (Contributed by BJ, 27-Apr-2021.)
((𝑋 ∈ (𝑉 ∖ {∅}) ∧ 𝐴𝑊) → (𝑋t 𝐴) ≠ ∅)
 
Theorembj-restpw 35777 The elementwise intersection on a powerset is the powerset of the intersection. This allows to prove for instance that the topology induced on a subset by the discrete topology is the discrete topology on that subset. See also restdis 22611 (which uses distop 22427 and restopn2 22610). (Contributed by BJ, 27-Apr-2021.)
((𝑌𝑉𝐴𝑊) → (𝒫 𝑌t 𝐴) = 𝒫 (𝑌𝐴))
 
Theorembj-rest0 35778 An elementwise intersection on a family containing the empty set contains the empty set. (Contributed by BJ, 27-Apr-2021.)
((𝑋𝑉𝐴𝑊) → (∅ ∈ 𝑋 → ∅ ∈ (𝑋t 𝐴)))
 
Theorembj-restb 35779 An elementwise intersection by a set on a family containing a superset of that set contains that set. (Contributed by BJ, 27-Apr-2021.)
(𝑋𝑉 → ((𝐴𝐵𝐵𝑋) → 𝐴 ∈ (𝑋t 𝐴)))
 
Theorembj-restv 35780 An elementwise intersection by a subset on a family containing the whole set contains the whole subset. (Contributed by BJ, 27-Apr-2021.)
((𝐴 𝑋 𝑋𝑋) → 𝐴 ∈ (𝑋t 𝐴))
 
Theorembj-resta 35781 An elementwise intersection by a set on a family containing that set contains that set. (Contributed by BJ, 27-Apr-2021.)
(𝑋𝑉 → (𝐴𝑋𝐴 ∈ (𝑋t 𝐴)))
 
Theorembj-restuni 35782 The union of an elementwise intersection by a set is equal to the intersection with that set of the union of the family. See also restuni 22595 and restuni2 22600. (Contributed by BJ, 27-Apr-2021.)
((𝑋𝑉𝐴𝑊) → (𝑋t 𝐴) = ( 𝑋𝐴))
 
Theorembj-restuni2 35783 The union of an elementwise intersection on a family of sets by a subset is equal to that subset. See also restuni 22595 and restuni2 22600. (Contributed by BJ, 27-Apr-2021.)
((𝑋𝑉𝐴 𝑋) → (𝑋t 𝐴) = 𝐴)
 
Theorembj-restreg 35784 A reformulation of the axiom of regularity using elementwise intersection. (RK: might have to be placed later since theorems in this section are to be moved early (in the section related to the algebra of sets).) (Contributed by BJ, 27-Apr-2021.)
((𝐴𝑉𝐴 ≠ ∅) → ∅ ∈ (𝐴t 𝐴))
 
21.15.5.22  Moore collections (complements)
 
Theorembj-raldifsn 35785* All elements in a set satisfy a given property if and only if all but one satisfy that property and that one also does. Typically, this can be used for characterizations that are proved using different methods for a given element and for all others, for instance zero and nonzero numbers, or the empty set and nonempty sets. (Contributed by BJ, 7-Dec-2021.)
(𝑥 = 𝐵 → (𝜑𝜓))       (𝐵𝐴 → (∀𝑥𝐴 𝜑 ↔ (∀𝑥 ∈ (𝐴 ∖ {𝐵})𝜑𝜓)))
 
Theorembj-0int 35786* If 𝐴 is a collection of subsets of 𝑋, like a Moore collection or a topology, two equivalent ways to say that arbitrary intersections of elements of 𝐴 relative to 𝑋 belong to some class 𝐵: the LHS singles out the empty intersection (the empty intersection relative to 𝑋 is 𝑋 and the intersection of a nonempty family of subsets of 𝑋 is included in 𝑋, so there is no need to intersect it with 𝑋). In typical applications, 𝐵 is 𝐴 itself. (Contributed by BJ, 7-Dec-2021.)
(𝐴 ⊆ 𝒫 𝑋 → ((𝑋𝐵 ∧ ∀𝑥 ∈ (𝒫 𝐴 ∖ {∅}) 𝑥𝐵) ↔ ∀𝑥 ∈ 𝒫 𝐴(𝑋 𝑥) ∈ 𝐵))
 
Theorembj-mooreset 35787* A Moore collection is a set. Therefore, the class Moore of all Moore sets defined in df-bj-moore 35789 is actually the class of all Moore collections. This is also illustrated by the lack of sethood condition in bj-ismoore 35790.

Note that the closed sets of a topology form a Moore collection, so a topology is a set, and this remark also applies to many other families of sets (namely, as soon as the whole set is required to be a set of the family, then the associated kind of family has no proper classes: that this condition suffices to impose sethood can be seen in this proof, which relies crucially on uniexr 7733).

Note: if, in the above predicate, we substitute 𝒫 𝑋 for 𝐴, then the last ∈ 𝒫 𝑋 could be weakened to 𝑋, and then the predicate would be obviously satisfied since 𝒫 𝑋 = 𝑋 (unipw 5443), making 𝒫 𝑋 a Moore collection in this weaker sense, for any class 𝑋, even proper, but the addition of this single case does not add anything interesting. Instead, we have the biconditional bj-discrmoore 35796. (Contributed by BJ, 8-Dec-2021.)

(∀𝑥 ∈ 𝒫 𝐴( 𝐴 𝑥) ∈ 𝐴𝐴 ∈ V)
 
Syntaxcmoore 35788 Syntax for the class of Moore collections.
class Moore
 
Definitiondf-bj-moore 35789* Define the class of Moore collections. This is indeed the class of all Moore collections since these all are sets, as proved in bj-mooreset 35787, and as illustrated by the lack of sethood condition in bj-ismoore 35790.

This is to df-mre 17512 (defining Moore) what df-top 22325 (defining Top) is to df-topon 22342 (defining TopOn).

For the sake of consistency, the function defined at df-mre 17512 should be denoted by "MooreOn".

Note: df-mre 17512 singles out the empty intersection. This is not necessary. It could be written instead Moore = (𝑥 ∈ V ↦ {𝑦 ∈ 𝒫 𝒫 𝑥 ∣ ∀𝑧 ∈ 𝒫 𝑦(𝑥 𝑧) ∈ 𝑦}) and the equivalence of both definitions is proved by bj-0int 35786.

There is no added generality in defining a "Moore predicate" for arbitrary classes, since a Moore class satisfying such a predicate is automatically a set (see bj-mooreset 35787).

TODO: move to the main section. For many families of sets, one can define both the function associating to each set the set of families of that kind on it (like df-mre 17512 and df-topon 22342) or the class of all families of that kind, independent of a base set (like df-bj-moore 35789 or df-top 22325). In general, the former will be more useful and the extra generality of the latter is not necessary. Moore collections, however, are particular in that they are more ubiquitous and are used in a wide variety of applications (for many families of sets, the family of families of a given kind is often a Moore collection, for instance). Therefore, in the case of Moore families, having both definitions is useful.

(Contributed by BJ, 27-Apr-2021.)

Moore = {𝑥 ∣ ∀𝑦 ∈ 𝒫 𝑥( 𝑥 𝑦) ∈ 𝑥}
 
Theorembj-ismoore 35790* Characterization of Moore collections. Note that there is no sethood hypothesis on 𝐴: it is implied by either side (this is obvious for the LHS, and is the content of bj-mooreset 35787 for the RHS). (Contributed by BJ, 9-Dec-2021.)
(𝐴Moore ↔ ∀𝑥 ∈ 𝒫 𝐴( 𝐴 𝑥) ∈ 𝐴)
 
Theorembj-ismoored0 35791 Necessary condition to be a Moore collection. (Contributed by BJ, 9-Dec-2021.)
(𝐴Moore 𝐴𝐴)
 
Theorembj-ismoored 35792 Necessary condition to be a Moore collection. (Contributed by BJ, 9-Dec-2021.)
(𝜑𝐴Moore)    &   (𝜑𝐵𝐴)       (𝜑 → ( 𝐴 𝐵) ∈ 𝐴)
 
Theorembj-ismoored2 35793 Necessary condition to be a Moore collection. (Contributed by BJ, 9-Dec-2021.)
(𝜑𝐴Moore)    &   (𝜑𝐵𝐴)    &   (𝜑𝐵 ≠ ∅)       (𝜑 𝐵𝐴)
 
Theorembj-ismooredr 35794* Sufficient condition to be a Moore collection. Note that there is no sethood hypothesis on 𝐴: it is a consequence of the only hypothesis. (Contributed by BJ, 9-Dec-2021.)
((𝜑𝑥𝐴) → ( 𝐴 𝑥) ∈ 𝐴)       (𝜑𝐴Moore)
 
Theorembj-ismooredr2 35795* Sufficient condition to be a Moore collection (variant of bj-ismooredr 35794 singling out the empty intersection). Note that there is no sethood hypothesis on 𝐴: it is a consequence of the first hypothesis. (Contributed by BJ, 9-Dec-2021.)
(𝜑 𝐴𝐴)    &   ((𝜑 ∧ (𝑥𝐴𝑥 ≠ ∅)) → 𝑥𝐴)       (𝜑𝐴Moore)
 
Theorembj-discrmoore 35796 The powerclass 𝒫 𝐴 is a Moore collection if and only if 𝐴 is a set. It is then called the discrete Moore collection. (Contributed by BJ, 9-Dec-2021.)
(𝐴 ∈ V ↔ 𝒫 𝐴Moore)
 
Theorembj-0nmoore 35797 The empty set is not a Moore collection. (Contributed by BJ, 9-Dec-2021.)
¬ ∅ ∈ Moore
 
Theorembj-snmoore 35798 A singleton is a Moore collection. See bj-snmooreb 35799 for a biconditional version. (Contributed by BJ, 10-Apr-2024.)
(𝐴𝑉 → {𝐴} ∈ Moore)
 
Theorembj-snmooreb 35799 A singleton is a Moore collection, biconditional version. (Contributed by BJ, 9-Dec-2021.) (Proof shortened by BJ, 10-Apr-2024.)
(𝐴 ∈ V ↔ {𝐴} ∈ Moore)
 
Theorembj-prmoore 35800 A pair formed of two nested sets is a Moore collection. (Note that in the statement, if 𝐵 is a proper class, we are in the case of bj-snmoore 35798). A direct consequence is {∅, 𝐴} ∈ Moore.

More generally, any nonempty well-ordered chain of sets that is a set is a Moore collection.

We also have the biconditional ((𝐴𝐵) ∈ 𝑉 ({𝐴, 𝐵} ∈ Moore ↔ (𝐴𝐵𝐵𝐴))). (Contributed by BJ, 11-Apr-2024.)

((𝐴𝑉𝐴𝐵) → {𝐴, 𝐵} ∈ Moore)
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454 45301-45400 455 45401-45500 456 45501-45600 457 45601-45700 458 45701-45800 459 45801-45900 460 45901-46000 461 46001-46100 462 46101-46200 463 46201-46300 464 46301-46400 465 46401-46500 466 46501-46600 467 46601-46700 468 46701-46800 469 46801-46900 470 46901-47000 471 47001-47100 472 47101-47200 473 47201-47300 474 47301-47400 475 47401-47500
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