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Theorem List for Metamath Proof Explorer - 35201-35300   *Has distinct variable group(s)
TypeLabelDescription
Statement
 
Theorembj-pr1eq 35201 Substitution property for pr1. (Contributed by BJ, 6-Apr-2019.)
(𝐴 = 𝐵 → pr1 𝐴 = pr1 𝐵)
 
Theorembj-pr1un 35202 The first projection preserves unions. (Contributed by BJ, 6-Apr-2019.)
pr1 (𝐴𝐵) = (pr1 𝐴 ∪ pr1 𝐵)
 
Theorembj-pr1val 35203 Value of the first projection. (Contributed by BJ, 6-Apr-2019.)
pr1 ({𝐴} × tag 𝐵) = if(𝐴 = ∅, 𝐵, ∅)
 
Theorembj-pr11val 35204 Value of the first projection of a monuple. (Contributed by BJ, 6-Apr-2019.)
pr1𝐴⦆ = 𝐴
 
Theorembj-pr1ex 35205 Sethood of the first projection. (Contributed by BJ, 6-Oct-2018.)
(𝐴𝑉 → pr1 𝐴 ∈ V)
 
Theorembj-1uplth 35206 The characteristic property of monuples. Note that this holds without sethood hypotheses. (Contributed by BJ, 6-Apr-2019.)
(⦅𝐴⦆ = ⦅𝐵⦆ ↔ 𝐴 = 𝐵)
 
Theorembj-1uplex 35207 A monuple is a set if and only if its coordinates are sets. (Contributed by BJ, 6-Apr-2019.)
(⦅𝐴⦆ ∈ V ↔ 𝐴 ∈ V)
 
Theorembj-1upln0 35208 A monuple is nonempty. (Contributed by BJ, 6-Apr-2019.)
𝐴⦆ ≠ ∅
 
Syntaxbj-c2uple 35209 Syntax for Morse couple. (Contributed by BJ, 6-Oct-2018.)
class 𝐴, 𝐵
 
Definitiondf-bj-2upl 35210 Definition of the Morse couple. See df-bj-1upl 35197. New usage is discouraged because the precise definition is generally unimportant compared to the characteristic properties bj-2upleq 35211, bj-2uplth 35220, bj-2uplex 35221, and the properties of the projections (see df-bj-pr1 35200 and df-bj-pr2 35214). (Contributed by BJ, 6-Oct-2018.) (New usage is discouraged.)
𝐴, 𝐵⦆ = (⦅𝐴⦆ ∪ ({1o} × tag 𝐵))
 
Theorembj-2upleq 35211 Substitution property for ⦅ − , − ⦆. (Contributed by BJ, 6-Oct-2018.)
(𝐴 = 𝐵 → (𝐶 = 𝐷 → ⦅𝐴, 𝐶⦆ = ⦅𝐵, 𝐷⦆))
 
Theorembj-pr21val 35212 Value of the first projection of a couple. (Contributed by BJ, 6-Oct-2018.)
pr1𝐴, 𝐵⦆ = 𝐴
 
Syntaxbj-cpr2 35213 Syntax for the second class tuple projection. (Contributed by BJ, 6-Oct-2018.)
class pr2 𝐴
 
Definitiondf-bj-pr2 35214 Definition of the second projection of a class tuple. New usage is discouraged because the precise definition is generally unimportant compared to the characteristic properties bj-pr2eq 35215, bj-pr22val 35218, bj-pr2ex 35219. (Contributed by BJ, 6-Oct-2018.) (New usage is discouraged.)
pr2 𝐴 = (1o Proj 𝐴)
 
Theorembj-pr2eq 35215 Substitution property for pr2. (Contributed by BJ, 6-Oct-2018.)
(𝐴 = 𝐵 → pr2 𝐴 = pr2 𝐵)
 
Theorembj-pr2un 35216 The second projection preserves unions. (Contributed by BJ, 6-Apr-2019.)
pr2 (𝐴𝐵) = (pr2 𝐴 ∪ pr2 𝐵)
 
Theorembj-pr2val 35217 Value of the second projection. (Contributed by BJ, 6-Apr-2019.)
pr2 ({𝐴} × tag 𝐵) = if(𝐴 = 1o, 𝐵, ∅)
 
Theorembj-pr22val 35218 Value of the second projection of a couple. (Contributed by BJ, 6-Oct-2018.)
pr2𝐴, 𝐵⦆ = 𝐵
 
Theorembj-pr2ex 35219 Sethood of the second projection. (Contributed by BJ, 6-Oct-2018.)
(𝐴𝑉 → pr2 𝐴 ∈ V)
 
Theorembj-2uplth 35220 The characteristic property of couples. Note that this holds without sethood hypotheses (compare opth 5392). (Contributed by BJ, 6-Oct-2018.)
(⦅𝐴, 𝐵⦆ = ⦅𝐶, 𝐷⦆ ↔ (𝐴 = 𝐶𝐵 = 𝐷))
 
Theorembj-2uplex 35221 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 35222 A couple is nonempty. (Contributed by BJ, 21-Apr-2019.)
𝐴, 𝐵⦆ ≠ ∅
 
Theorembj-2upln1upl 35223 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 35208 and bj-2upln0 35222 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
 
20.15.5.16  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 35224 Relative version of cleqf 2939. (Contributed by BJ, 27-Dec-2023.)
𝑥𝐴    &   𝑥𝐵    &   𝑥𝑉       ((𝑉𝐴) = (𝑉𝐵) ↔ ∀𝑥𝑉 (𝑥𝐴𝑥𝐵))
 
Theorembj-rcleq 35225* Relative version of dfcleq 2732. (Contributed by BJ, 27-Dec-2023.)
((𝑉𝐴) = (𝑉𝐵) ↔ ∀𝑥𝑉 (𝑥𝐴𝑥𝐵))
 
Theorembj-reabeq 35226* Relative form of abeq2 2873. (Contributed by BJ, 27-Dec-2023.)
((𝑉𝐴) = {𝑥𝑉𝜑} ↔ ∀𝑥𝑉 (𝑥𝐴𝜑))
 
Theorembj-disj2r 35227 Relative version of ssdifin0 4417, allowing a biconditional, and of disj2 4392. (Contributed by BJ, 11-Nov-2021.) This proof does not rely, even indirectly, on ssdifin0 4417 nor disj2 4392. (Proof modification is discouraged.)
((𝐴𝑉) ⊆ (𝑉𝐵) ↔ ((𝐴𝐵) ∩ 𝑉) = ∅)
 
Theorembj-sscon 35228 Contraposition law for relative subclasses. Relative and generalized version of ssconb 4073, which it can shorten, as well as conss2 42068. (Contributed by BJ, 11-Nov-2021.) This proof does not rely, even indirectly, on ssconb 4073 nor conss2 42068. (Proof modification is discouraged.)
((𝐴𝑉) ⊆ (𝑉𝐵) ↔ (𝐵𝑉) ⊆ (𝑉𝐴))
 
20.15.5.17  Set theory: miscellaneous

Miscellaneous theorems of set theory.

 
Theoremeleq2w2ALT 35229 Alternate proof of eleq2w2 2735 and special instance of eleq2 2828. (Contributed by BJ, 22-Sep-2024.) (Proof modification is discouraged.) (New usage is discouraged.)
(𝐴 = 𝐵 → (𝑥𝐴𝑥𝐵))
 
Theorembj-clel3gALT 35230* Alternate proof of clel3g 3592. (Contributed by BJ, 1-Sep-2024.) (Proof modification is discouraged.) (New usage is discouraged.)
(𝐵𝑉 → (𝐴𝐵 ↔ ∃𝑥(𝑥 = 𝐵𝐴𝑥)))
 
Theorembj-pw0ALT 35231 Alternate proof of pw0 4746. The proofs have a similar structure: pw0 4746 uses the definitions of powerclass and singleton as class abstractions, whereas bj-pw0ALT 35231 uses characterizations of their elements. Both proofs then use transitivity of a congruence relation (equality for pw0 4746 and biconditional for bj-pw0ALT 35231) to translate the property ss0b 4332 into the wanted result. To translate a biconditional into a class equality, pw0 4746 uses abbii 2809 (which yields an equality of class abstractions), while bj-pw0ALT 35231 uses eqriv 2736 (which requires a biconditional of membership of a given setvar variable). Note that abbii 2809, through its closed form abbi1 2807, is proved from eqrdv 2737, which is the deduction form of eqriv 2736. In the other direction, velpw 4539 and velsn 4578 are proved from the definitions of powerclass and singleton using elabg 3608, which is a version of abbii 2809 suited for membership characterizations. (Contributed by BJ, 14-Apr-2024.) (Proof modification is discouraged.) (New usage is discouraged.)
𝒫 ∅ = {∅}
 
Theorembj-sselpwuni 35232 Quantitative version of ssexg 5248: 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 35233 Quantitative version of uniexr 7622: 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 35234 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 4537 and elpw2g 5269 (the latter of which could be proved from it). (Contributed by BJ, 31-Dec-2023.)
((𝐴𝐵) ∈ 𝑉 → (𝐴 ∈ 𝒫 𝐵𝐴𝐵))
 
Theorembj-vjust 35235 Justification theorem for dfv2 3436 if it were the definition. See also vjust 3434. (Contributed by BJ, 30-Nov-2019.) (Proof modification is discouraged.)
{𝑥 ∣ ⊤} = {𝑦 ∣ ⊤}
 
Theorembj-nul 35236* Two formulations of the axiom of the empty set ax-nul 5231. Proposal: place it right before ax-nul 5231. (Contributed by BJ, 30-Nov-2019.) (Proof modification is discouraged.)
(∅ ∈ V ↔ ∃𝑥𝑦 ¬ 𝑦𝑥)
 
Theorembj-nuliota 35237* Definition of the empty set using the definite description binder. See also bj-nuliotaALT 35238. (Contributed by BJ, 30-Nov-2019.) (Proof modification is discouraged.)
∅ = (℩𝑥𝑦 ¬ 𝑦𝑥)
 
Theorembj-nuliotaALT 35238* Alternate proof of bj-nuliota 35237. Note that this alternate proof uses the fact that 𝑥𝜑 evaluates to when there is no 𝑥 satisfying 𝜑 (iotanul 6415). 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 35239 Alternate proof of vtoclgf 3504. Proof from vtoclgft 3493. (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 35240 Join of elsng 4576 and elsn2g 4600. (Contributed by BJ, 18-Nov-2023.)
((𝐴𝑉𝐵𝑊) → (𝐴 ∈ {𝐵} ↔ 𝐴 = 𝐵))
 
Theorembj-elsnb 35241 Biconditional version of elsng 4576. (Contributed by BJ, 18-Nov-2023.)
(𝐴 ∈ {𝐵} ↔ (𝐴 ∈ V ∧ 𝐴 = 𝐵))
 
Theorembj-pwcfsdom 35242 Remove hypothesis from pwcfsdom 10348. Illustration of how to remove a "proof-facilitating hypothesis". (Can use it to shorten theorems using pwcfsdom 10348.) (Contributed by BJ, 14-Sep-2019.)
(ℵ‘𝐴) ≺ ((ℵ‘𝐴) ↑m (cf‘(ℵ‘𝐴)))
 
Theorembj-grur1 35243 Remove hypothesis from grur1 10585. 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 35244* 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 5224. 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 5227. Relabel ("sepbi"?).

(∃𝑥𝑧(𝜑𝑧𝑥) ↔ ∃𝑦𝑧(𝑧𝑦𝜑))
 
Theorembj-dfid2ALT 35245 Alternate version of dfid2 5492. (Contributed by BJ, 9-Nov-2024.) (Proof modification is discouraged.) Use df-id 5490 instead to make the semantics of the construction df-opab 5138 clearer. (New usage is discouraged.)
I = {⟨𝑥, 𝑥⟩ ∣ ⊤}
 
Theorembj-0nelopab 35246 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 5736 (this is a very limited reordering).

¬ ∅ ∈ {⟨𝑥, 𝑦⟩ ∣ 𝜑}
 
Theorembj-brrelex12ALT 35247 Two classes related by a binary relation are both sets. Alternate proof of brrelex12 5640. (Contributed by BJ, 14-Jul-2023.) (Proof modification is discouraged.) (New usage is discouraged.)
((Rel 𝑅𝐴𝑅𝐵) → (𝐴 ∈ V ∧ 𝐵 ∈ V))
 
Theorembj-epelg 35248 The membership relation and the membership predicate agree when the "containing" class is a set. General version of epel 5499 and closed form of epeli 5498. (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 5644 available. (Proof shortened by BJ, 14-Jul-2023.) (Proof modification is discouraged.)
(𝐵𝑉 → (𝐴 E 𝐵𝐴𝐵))
 
Theorembj-epelb 35249 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 5645 available. Check if it is shorter to prove bj-epelg 35248 first or bj-epelb 35249 first. (Contributed by BJ, 14-Jul-2023.)
(𝐴 E 𝐵 ↔ (𝐴𝐵𝐵 ∈ V))
 
Theorembj-nsnid 35250 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 4654): ¬ ({𝐴} ∈ 𝐴 ↔ (∅ ∈ 𝐴𝐴 ∈ V)). (Contributed by BJ, 4-Feb-2023.)
(𝐴𝑉 → ¬ {𝐴} ∈ 𝐴)
 
Theorembj-rdg0gALT 35251 Alternate proof of rdg0g 8267. More direct since it bypasses tz7.44-1 8246 and rdg0 8261 (and vtoclg 3506, vtoclga 3514). (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)
 
20.15.5.18  Evaluation at a class

This section treats the existing predicate Slot (df-slot 16892) as "evaluation at a class" and for the moment does not introduce new syntax for it.

 
Theorembj-evaleq 35252 Equality theorem for the Slot construction. This is currently a duplicate of sloteq 16893 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 35253 The evaluation at a class is a function. (Contributed by BJ, 27-Dec-2021.)
Fun Slot 𝐴
 
Theorembj-evalfn 35254 The evaluation at a class is a function on the universal class. (General form of slotfn 16894). (Contributed by Mario Carneiro, 22-Sep-2015.) (Revised by BJ, 27-Dec-2021.)
Slot 𝐴 Fn V
 
Theorembj-evalval 35255 Value of the evaluation at a class. (Closed form of strfvnd 16895 and strfvn 16896). (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 35256 The evaluation at a set of the identity function is that set. (General form of ndxarg 16906.) 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 35257 Proof of ndxarg 16906 from bj-evalid 35256. (Contributed by BJ, 27-Dec-2021.) (Proof modification is discouraged.)
𝐸 = Slot 𝑁    &   𝑁 ∈ ℕ       (𝐸‘ndx) = 𝑁
 
Theorembj-evalidval 35258 Closed general form of strndxid 16908. Both sides are equal to (𝐹𝐴) by bj-evalid 35256 and bj-evalval 35255 respectively, but bj-evalidval 35258 adds something to bj-evalid 35256 and bj-evalval 35255 in that Slot 𝐴 appears on both sides. (Contributed by BJ, 27-Dec-2021.)
((𝑉𝑊𝐴𝑉𝐹𝑈) → (𝐹‘(Slot 𝐴‘( I ↾ 𝑉))) = (Slot 𝐴𝐹))
 
20.15.5.19  Elementwise operations
 
Syntaxcelwise 35259 Syntax for elementwise operations.
class elwise
 
Definitiondf-elwise 35260* Define the elementwise operation associated with a given operation. For instance, + is the addition of complex numbers (axaddf 10910), 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)}))
 
20.15.5.20  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 17304), topologies (df-top 22052), pi-systems, rings of sets, delta-rings, lambda-systems/Dynkin systems, algebras/fields of sets, sigma-algebras/sigma-fields/tribes (df-siga 32086), 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 35261 An elementwise intersection on the empty family is the empty set. TODO: this is 0rest 17149. (Contributed by BJ, 27-Apr-2021.)
(∅ ↾t 𝐴) = ∅
 
Theorembj-restsn 35262 An elementwise intersection on the singleton on a set is the singleton on the intersection by that set. Generalization of bj-restsn0 35265 and bj-restsnid 35267. (Contributed by BJ, 27-Apr-2021.)
((𝑌𝑉𝐴𝑊) → ({𝑌} ↾t 𝐴) = {(𝑌𝐴)})
 
Theorembj-restsnss 35263 Special case of bj-restsn 35262. (Contributed by BJ, 27-Apr-2021.)
((𝑌𝑉𝐴𝑌) → ({𝑌} ↾t 𝐴) = {𝐴})
 
Theorembj-restsnss2 35264 Special case of bj-restsn 35262. (Contributed by BJ, 27-Apr-2021.)
((𝐴𝑉𝑌𝐴) → ({𝑌} ↾t 𝐴) = {𝑌})
 
Theorembj-restsn0 35265 An elementwise intersection on the singleton on the empty set is the singleton on the empty set. Special case of bj-restsn 35262 and bj-restsnss2 35264. TODO: this is restsn 22330. (Contributed by BJ, 27-Apr-2021.)
(𝐴𝑉 → ({∅} ↾t 𝐴) = {∅})
 
Theorembj-restsn10 35266 Special case of bj-restsn 35262, bj-restsnss 35263, and bj-rest10 35268. (Contributed by BJ, 27-Apr-2021.)
(𝑋𝑉 → ({𝑋} ↾t ∅) = {∅})
 
Theorembj-restsnid 35267 The elementwise intersection on the singleton on a class by that class is the singleton on that class. Special case of bj-restsn 35262 and bj-restsnss 35263. (Contributed by BJ, 27-Apr-2021.)
({𝐴} ↾t 𝐴) = {𝐴}
 
Theorembj-rest10 35268 An elementwise intersection on a nonempty family by the empty set is the singleton on the empty set. TODO: this generalizes rest0 22329 and could replace it. (Contributed by BJ, 27-Apr-2021.)
(𝑋𝑉 → (𝑋 ≠ ∅ → (𝑋t ∅) = {∅}))
 
Theorembj-rest10b 35269 Alternate version of bj-rest10 35268. (Contributed by BJ, 27-Apr-2021.)
(𝑋 ∈ (𝑉 ∖ {∅}) → (𝑋t ∅) = {∅})
 
Theorembj-restn0 35270 An elementwise intersection on a nonempty family is nonempty. (Contributed by BJ, 27-Apr-2021.)
((𝑋𝑉𝐴𝑊) → (𝑋 ≠ ∅ → (𝑋t 𝐴) ≠ ∅))
 
Theorembj-restn0b 35271 Alternate version of bj-restn0 35270. (Contributed by BJ, 27-Apr-2021.)
((𝑋 ∈ (𝑉 ∖ {∅}) ∧ 𝐴𝑊) → (𝑋t 𝐴) ≠ ∅)
 
Theorembj-restpw 35272 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 22338 (which uses distop 22154 and restopn2 22337). (Contributed by BJ, 27-Apr-2021.)
((𝑌𝑉𝐴𝑊) → (𝒫 𝑌t 𝐴) = 𝒫 (𝑌𝐴))
 
Theorembj-rest0 35273 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 35274 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 35275 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 35276 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 35277 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 22322 and restuni2 22327. (Contributed by BJ, 27-Apr-2021.)
((𝑋𝑉𝐴𝑊) → (𝑋t 𝐴) = ( 𝑋𝐴))
 
Theorembj-restuni2 35278 The union of an elementwise intersection on a family of sets by a subset is equal to that subset. See also restuni 22322 and restuni2 22327. (Contributed by BJ, 27-Apr-2021.)
((𝑋𝑉𝐴 𝑋) → (𝑋t 𝐴) = 𝐴)
 
Theorembj-restreg 35279 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))
 
20.15.5.21  Moore collections (complements)
 
Theorembj-raldifsn 35280* 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 35281* 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 35282* A Moore collection is a set. Therefore, the class Moore of all Moore sets defined in df-bj-moore 35284 is actually the class of all Moore collections. This is also illustrated by the lack of sethood condition in bj-ismoore 35285.

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 7622).

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 5367) , 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 35291. (Contributed by BJ, 8-Dec-2021.)

(∀𝑥 ∈ 𝒫 𝐴( 𝐴 𝑥) ∈ 𝐴𝐴 ∈ V)
 
Syntaxcmoore 35283 Syntax for the class of Moore collections.
class Moore
 
Definitiondf-bj-moore 35284* 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 35282, and as illustrated by the lack of sethood condition in bj-ismoore 35285.

This is to df-mre 17304 (defining Moore) what df-top 22052 (defining Top) is to df-topon 22069 (defining TopOn).

For the sake of consistency, the function defined at df-mre 17304 should be denoted by "MooreOn".

Note: df-mre 17304 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 35281.

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 35282).

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 17304 and df-topon 22069) or the class of all families of that kind, independent of a base set (like df-bj-moore 35284 or df-top 22052). 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 35285* 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 35282 for the RHS). (Contributed by BJ, 9-Dec-2021.)
(𝐴Moore ↔ ∀𝑥 ∈ 𝒫 𝐴( 𝐴 𝑥) ∈ 𝐴)
 
Theorembj-ismoored0 35286 Necessary condition to be a Moore collection. (Contributed by BJ, 9-Dec-2021.)
(𝐴Moore 𝐴𝐴)
 
Theorembj-ismoored 35287 Necessary condition to be a Moore collection. (Contributed by BJ, 9-Dec-2021.)
(𝜑𝐴Moore)    &   (𝜑𝐵𝐴)       (𝜑 → ( 𝐴 𝐵) ∈ 𝐴)
 
Theorembj-ismoored2 35288 Necessary condition to be a Moore collection. (Contributed by BJ, 9-Dec-2021.)
(𝜑𝐴Moore)    &   (𝜑𝐵𝐴)    &   (𝜑𝐵 ≠ ∅)       (𝜑 𝐵𝐴)
 
Theorembj-ismooredr 35289* 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 35290* Sufficient condition to be a Moore collection (variant of bj-ismooredr 35289 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 35291 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 35292 The empty set is not a Moore collection. (Contributed by BJ, 9-Dec-2021.)
¬ ∅ ∈ Moore
 
Theorembj-snmoore 35293 A singleton is a Moore collection. See bj-snmooreb 35294 for a biconditional version. (Contributed by BJ, 10-Apr-2024.)
(𝐴𝑉 → {𝐴} ∈ Moore)
 
Theorembj-snmooreb 35294 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 35295 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 35293). 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)
 
20.15.5.22  Maps-to notation for functions with three arguments
 
Theorembj-0nelmpt 35296 The empty set is not an element of a function (given in maps-to notation). (Contributed by BJ, 30-Dec-2020.)
¬ ∅ ∈ (𝑥𝐴𝐵)
 
Theorembj-mptval 35297 Value of a function given in maps-to notation. (Contributed by BJ, 30-Dec-2020.)
𝑥𝐴       (∀𝑥𝐴 𝐵𝑉 → (𝑋𝐴 → (((𝑥𝐴𝐵)‘𝑋) = 𝑌𝑋(𝑥𝐴𝐵)𝑌)))
 
Theorembj-dfmpoa 35298* An equivalent definition of df-mpo 7289. (Contributed by BJ, 30-Dec-2020.)
(𝑥𝐴, 𝑦𝐵𝐶) = {⟨𝑠, 𝑡⟩ ∣ ∃𝑥𝐴𝑦𝐵 (𝑠 = ⟨𝑥, 𝑦⟩ ∧ 𝑡 = 𝐶)}
 
Theorembj-mpomptALT 35299* Alternate proof of mpompt 7397. (Contributed by BJ, 30-Dec-2020.) (Proof modification is discouraged.) (New usage is discouraged.)
(𝑧 = ⟨𝑥, 𝑦⟩ → 𝐶 = 𝐷)       (𝑧 ∈ (𝐴 × 𝐵) ↦ 𝐶) = (𝑥𝐴, 𝑦𝐵𝐷)
 
Syntaxcmpt3 35300 Syntax for maps-to notation for functions with three arguments.
class (𝑥𝐴, 𝑦𝐵, 𝑧𝐶𝐷)
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