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Theorem List for Metamath Proof Explorer - 34301-34400   *Has distinct variable group(s)
TypeLabelDescription
Statement
 
20.15.5.22  Maps-to notation for functions with three arguments
 
Theorembj-0nelmpt 34301 The empty set is not an element of a function (given in maps-to notation). (Contributed by BJ, 30-Dec-2020.)
¬ ∅ ∈ (𝑥𝐴𝐵)
 
Theorembj-mptval 34302 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 34303* An equivalent definition of df-mpo 7150. (Contributed by BJ, 30-Dec-2020.)
(𝑥𝐴, 𝑦𝐵𝐶) = {⟨𝑠, 𝑡⟩ ∣ ∃𝑥𝐴𝑦𝐵 (𝑠 = ⟨𝑥, 𝑦⟩ ∧ 𝑡 = 𝐶)}
 
Theorembj-mpomptALT 34304* Alternate proof of mpompt 7255. (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 34305 Syntax for maps-to notation for functions with three arguments.
class (𝑥𝐴, 𝑦𝐵, 𝑧𝐶𝐷)
 
Definitiondf-bj-mpt3 34306* Define maps-to notation for functions with three arguments. See df-mpt 5139 and df-mpo 7150 for functions with one and two arguments respectively. This definition is analogous to bj-dfmpoa 34303. (Contributed by BJ, 11-Apr-2020.)
(𝑥𝐴, 𝑦𝐵, 𝑧𝐶𝐷) = {⟨𝑠, 𝑡⟩ ∣ ∃𝑥𝐴𝑦𝐵𝑧𝐶 (𝑠 = ⟨𝑥, 𝑦, 𝑧⟩ ∧ 𝑡 = 𝐷)}
 
20.15.5.23  Currying

Currying and uncurrying. See also df-cur 7924 and df-unc 7925. Contrary to these, the definitions in this section are parameterized.

 
Syntaxcsethom 34307 Syntax for the set of set morphisms.
class Set
 
Definitiondf-bj-sethom 34308* Define the set of functions (morphisms of sets) between two sets. Same as df-map 8398 with arguments swapped. TODO: prove the same staple lemmas as for m.

Remark: one may define Set⟶ = (𝑥 ∈ dom Struct , 𝑦 ∈ dom Struct ↦ {𝑓𝑓:(Base‘𝑥)⟶(Base‘𝑦)}) so that for morphisms between other structures, one could write ... = {𝑓 ∈ (𝑥 Set𝑦) ∣ ...}.

(Contributed by BJ, 11-Apr-2020.)

Set⟶ = (𝑥 ∈ V, 𝑦 ∈ V ↦ {𝑓𝑓:𝑥𝑦})
 
Syntaxctophom 34309 Syntax for the set of topological morphisms.
class Top
 
Definitiondf-bj-tophom 34310* Define the set of continuous functions (morphisms of topological spaces) between two topological spaces. Similar to df-cn 21765 (which is in terms of topologies instead of topological spaces). (Contributed by BJ, 10-Feb-2022.)
Top⟶ = (𝑥 ∈ TopSp, 𝑦 ∈ TopSp ↦ {𝑓 ∈ ((Base‘𝑥) Set⟶ (Base‘𝑦)) ∣ ∀𝑢 ∈ (TopOpen‘𝑦)(𝑓𝑢) ∈ (TopOpen‘𝑥)})
 
Syntaxcmgmhom 34311 Syntax for the set of magma morphisms.
class Mgm
 
Definitiondf-bj-mgmhom 34312* Define the set of magma morphisms between two magmas. If domain and codomain are semigroups, monoids, or groups, then one obtains the set of morphisms of these structures. (Contributed by BJ, 10-Feb-2022.)
Mgm⟶ = (𝑥 ∈ Mgm, 𝑦 ∈ Mgm ↦ {𝑓 ∈ ((Base‘𝑥) Set⟶ (Base‘𝑦)) ∣ ∀𝑢 ∈ (Base‘𝑥)∀𝑣 ∈ (Base‘𝑥)(𝑓‘(𝑢(+g𝑥)𝑣)) = ((𝑓𝑢)(+g𝑦)(𝑓𝑣))})
 
Syntaxctopmgmhom 34313 Syntax for the set of topological magma morphisms.
class TopMgm
 
Definitiondf-bj-topmgmhom 34314* Define the set of topological magma morphisms (continuous magma morphisms) between two topological magmas. If domain and codomain are topological semigroups, monoids, or groups, then one obtains the set of morphisms of these structures. This definition is currently stated with topological monoid domain and codomain, since topological magmas are currently not defined in set.mm. (Contributed by BJ, 10-Feb-2022.)
TopMgm⟶ = (𝑥 ∈ TopMnd, 𝑦 ∈ TopMnd ↦ ((𝑥 Top𝑦) ∩ (𝑥 Mgm𝑦)))
 
Syntaxccur- 34315 Syntax for the parameterized currying function.
class curry_
 
Definitiondf-bj-cur 34316* Define currying. See also df-cur 7924. (Contributed by BJ, 11-Apr-2020.)
curry_ = (𝑥 ∈ V, 𝑦 ∈ V, 𝑧 ∈ V ↦ (𝑓 ∈ ((𝑥 × 𝑦) Set𝑧) ↦ (𝑎𝑥 ↦ (𝑏𝑦 ↦ (𝑓‘⟨𝑎, 𝑏⟩)))))
 
Syntaxcunc- 34317 Notation for the parameterized uncurrying function.
class uncurry_
 
Definitiondf-bj-unc 34318* Define uncurrying. See also df-unc 7925. (Contributed by BJ, 11-Apr-2020.)
uncurry_ = (𝑥 ∈ V, 𝑦 ∈ V, 𝑧 ∈ V ↦ (𝑓 ∈ (𝑥 Set⟶ (𝑦 Set𝑧)) ↦ (𝑎𝑥, 𝑏𝑦 ↦ ((𝑓𝑎)‘𝑏))))
 
20.15.5.24  Setting components of extensible structures

Groundwork for changing the definition, syntax and token for component-setting in extensible structures. See https://github.com/metamath/set.mm/issues/2401

 
Syntaxcstrset 34319 Syntax for component-setting in extensible structures.
class [𝐵 / 𝐴]struct𝑆
 
Definitiondf-strset 34320 Component-setting in extensible structures. Define the extensible structure [𝐵 / 𝐴]struct𝑆, which is like the extensible structure 𝑆 except that the value 𝐵 has been put in the slot 𝐴 (replacing the current value if there was already one). In such expressions, 𝐴 is generally substituted for slot mnemonics like Base or +g or dist. (Contributed by BJ, 13-Feb-2022.)
[𝐵 / 𝐴]struct𝑆 = ((𝑆 ↾ (V ∖ {(𝐴‘ndx)})) ∪ {⟨(𝐴‘ndx), 𝐵⟩})
 
Theoremsetsstrset 34321 Relation between df-sets 16480 and df-strset 34320. Temporary theorem kept during the transition from the former to the latter. (Contributed by BJ, 13-Feb-2022.)
((𝑆𝑉𝐵𝑊) → [𝐵 / 𝐴]struct𝑆 = (𝑆 sSet ⟨(𝐴‘ndx), 𝐵⟩))
 
20.15.6  Extended real and complex numbers, real and complex projective lines

In this section, we indroduce several supersets of the set of real numbers and the set of complex numbers.

Once they are given their usual topologies, which are locally compact, both topological spaces have a one-point compactification. They are denoted by ℝ̂ and ℂ̂ respectively, defined in df-bj-cchat 34408 and df-bj-rrhat 34410, and the point at infinity is denoted by , defined in df-bj-infty 34406.

Both and also have "directional compactifications", denoted respectively by ℝ̅, defined in df-bj-rrbar 34404 (already defined as *, see df-xr 10668) and ℂ̅, defined in df-bj-ccbar 34391.

Since ℂ̅ does not seem to be standard, we describe it in some detail. It is obtained by adding to a "point at infinity at the end of each ray with origin at 0". Although ℂ̅ is not an important object in itself, the motivation for introducing it is to provide a common superset to both ℝ̅ and and to define algebraic operations (addition, opposite, multiplication, inverse) as widely as reasonably possible.

Mathematically, ℂ̅ is the quotient of ((ℂ × ℝ≥0) ∖ {⟨0, 0⟩}) by the diagonal multiplicative action of >0 (think of the closed "northern hemisphere" in ^3 identified with (ℂ × ℝ), that each open ray from 0 included in the closed northern half-space intersects exactly once).

Since in set.mm, we want to have a genuine inclusion ℂ ⊆ ℂ̅, we instead define ℂ̅ as the (disjoint) union of with a circle at infinity denoted by . To have a genuine inclusion ℝ̅ ⊆ ℂ̅, we define +∞ and -∞ as certain points in .

Thanks to this framework, one has the genuine inclusions ℝ ⊆ ℝ̅ and ℝ ⊆ ℝ̂ and similarly ℂ ⊆ ℂ̅ and ℂ ⊆ ℂ̂. Furthermore, one has ℝ ⊆ ℂ as well as ℝ̅ ⊆ ℂ̅ and ℝ̂ ⊆ ℂ̂.

Furthermore, we define the main algebraic operations on (ℂ̅ ∪ ℂ̂), which is not very mathematical, but "overloads" the operations, so that one can use the same notation in all cases.

 
20.15.6.1  Complements on class abstractions of ordered pairs and binary relations
 
Theorembj-nfald 34322 Variant of nfald 2339. (Contributed by BJ, 25-Dec-2023.)
(𝜑 → ∀𝑦𝜑)    &   (𝜑 → Ⅎ𝑥𝜓)       (𝜑 → Ⅎ𝑥𝑦𝜓)
 
Theorembj-nfexd 34323 Variant of nfexd 2340. (Contributed by BJ, 25-Dec-2023.)
(𝜑 → ∀𝑦𝜑)    &   (𝜑 → Ⅎ𝑥𝜓)       (𝜑 → Ⅎ𝑥𝑦𝜓)
 
Theoremcopsex2d 34324* Implicit substitution deduction for ordered pairs. (Contributed by BJ, 25-Dec-2023.)
(𝜑 → ∀𝑥𝜑)    &   (𝜑 → ∀𝑦𝜑)    &   (𝜑 → Ⅎ𝑥𝜒)    &   (𝜑 → Ⅎ𝑦𝜒)    &   (𝜑𝐴𝑈)    &   (𝜑𝐵𝑉)    &   ((𝜑 ∧ (𝑥 = 𝐴𝑦 = 𝐵)) → (𝜓𝜒))       (𝜑 → (∃𝑥𝑦(⟨𝐴, 𝐵⟩ = ⟨𝑥, 𝑦⟩ ∧ 𝜓) ↔ 𝜒))
 
Theoremcopsex2b 34325* Biconditional form of copsex2d 34324. TODO: prove a relative version, that is, with 𝑥𝑉𝑦𝑊...(𝐴𝑉𝐵𝑊). (Contributed by BJ, 27-Dec-2023.)
(𝜑 → ∀𝑥𝜑)    &   (𝜑 → ∀𝑦𝜑)    &   (𝜑 → Ⅎ𝑥𝜒)    &   (𝜑 → Ⅎ𝑦𝜒)    &   ((𝜑 ∧ (𝑥 = 𝐴𝑦 = 𝐵)) → (𝜓𝜒))       (𝜑 → (∃𝑥𝑦(⟨𝐴, 𝐵⟩ = ⟨𝑥, 𝑦⟩ ∧ 𝜓) ↔ ((𝐴 ∈ V ∧ 𝐵 ∈ V) ∧ 𝜒)))
 
Theoremopelopabd 34326* Membership of an ordere pair in a class abstraction of ordered pairs. (Contributed by BJ, 17-Dec-2023.)
(𝜑 → ∀𝑥𝜑)    &   (𝜑 → ∀𝑦𝜑)    &   (𝜑 → Ⅎ𝑥𝜒)    &   (𝜑 → Ⅎ𝑦𝜒)    &   (𝜑𝐴𝑈)    &   (𝜑𝐵𝑉)    &   ((𝜑 ∧ (𝑥 = 𝐴𝑦 = 𝐵)) → (𝜓𝜒))       (𝜑 → (⟨𝐴, 𝐵⟩ ∈ {⟨𝑥, 𝑦⟩ ∣ 𝜓} ↔ 𝜒))
 
Theoremopelopabb 34327* Membership of an ordered pair in a class abstraction of ordered pairs, biconditional form. (Contributed by BJ, 17-Dec-2023.)
(𝜑 → ∀𝑥𝜑)    &   (𝜑 → ∀𝑦𝜑)    &   (𝜑 → Ⅎ𝑥𝜒)    &   (𝜑 → Ⅎ𝑦𝜒)    &   ((𝜑 ∧ (𝑥 = 𝐴𝑦 = 𝐵)) → (𝜓𝜒))       (𝜑 → (⟨𝐴, 𝐵⟩ ∈ {⟨𝑥, 𝑦⟩ ∣ 𝜓} ↔ ((𝐴 ∈ V ∧ 𝐵 ∈ V) ∧ 𝜒)))
 
Theoremopelopabbv 34328* Membership of an ordered pair in a class abstraction of ordered pairs, biconditional form. (Contributed by BJ, 17-Dec-2023.)
(𝜑𝑅 = {⟨𝑥, 𝑦⟩ ∣ 𝜓})    &   ((𝜑 ∧ (𝑥 = 𝐴𝑦 = 𝐵)) → (𝜓𝜒))       (𝜑 → (⟨𝐴, 𝐵⟩ ∈ 𝑅 ↔ ((𝐴 ∈ V ∧ 𝐵 ∈ V) ∧ 𝜒)))
 
Theorembj-opelrelex 34329 The coordinates of an ordered pair that belongs to a relation are sets. TODO: Slightly shorter than brrelex12 5598, which could be proved from it. (Contributed by BJ, 27-Dec-2023.)
((Rel 𝑅 ∧ ⟨𝐴, 𝐵⟩ ∈ 𝑅) → (𝐴 ∈ V ∧ 𝐵 ∈ V))
 
Theorembj-opelresdm 34330 If an ordered pair is in a restricted binary relation, then its first component is an element of the restricting class. See also opelres 5853. (Contributed by BJ, 25-Dec-2023.)
(⟨𝐴, 𝐵⟩ ∈ (𝑅𝑋) → 𝐴𝑋)
 
Theorembj-brresdm 34331 If two classes are related by a restricted binary relation, then the first class is an element of the restricting class. See also brres 5854 and brrelex1 5599.

Remark: there are many pairs like bj-opelresdm 34330 / bj-brresdm 34331, where one uses membership of ordered pairs and the other, related classes (for instance, bj-opelresdm 34330 / brrelex12 5598 or the opelopabg 5417 / brabg 5418 family). They are straightforwardly equivalent by df-br 5059. The latter is indeed a very direct definition, introducing a "shorthand", and barely necessary, were it not for the frequency of the expression 𝐴𝑅𝐵. Therefore, in the spirit of "definitions are here to be used", most theorems, apart from the most elementary ones, should only have the "br" version, not the "opel" one. (Contributed by BJ, 25-Dec-2023.)

(𝐴(𝑅𝑋)𝐵𝐴𝑋)
 
Theorembrabd0 34332* Expressing that two sets are related by a binary relation which is expressed as a class abstraction of ordered pairs. (Contributed by BJ, 17-Dec-2023.)
(𝜑 → ∀𝑥𝜑)    &   (𝜑 → ∀𝑦𝜑)    &   (𝜑 → Ⅎ𝑥𝜒)    &   (𝜑 → Ⅎ𝑦𝜒)    &   (𝜑𝐴𝑈)    &   (𝜑𝐵𝑉)    &   (𝜑𝑅 = {⟨𝑥, 𝑦⟩ ∣ 𝜓})    &   ((𝜑 ∧ (𝑥 = 𝐴𝑦 = 𝐵)) → (𝜓𝜒))       (𝜑 → (𝐴𝑅𝐵𝜒))
 
Theorembrabd 34333* Expressing that two sets are related by a binary relation which is expressed as a class abstraction of ordered pairs. (Contributed by BJ, 17-Dec-2023.)
(𝜑𝐴𝑈)    &   (𝜑𝐵𝑉)    &   (𝜑𝑅 = {⟨𝑥, 𝑦⟩ ∣ 𝜓})    &   ((𝜑 ∧ (𝑥 = 𝐴𝑦 = 𝐵)) → (𝜓𝜒))       (𝜑 → (𝐴𝑅𝐵𝜒))
 
Theorembj-brab2a1 34334* "Unbounded" version of brab2a 5638. (Contributed by BJ, 25-Dec-2023.)
((𝑥 = 𝐴𝑦 = 𝐵) → (𝜑𝜓))    &   𝑅 = {⟨𝑥, 𝑦⟩ ∣ 𝜑}       (𝐴𝑅𝐵 ↔ ((𝐴 ∈ V ∧ 𝐵 ∈ V) ∧ 𝜓))
 
20.15.6.2  Identity relation (complements)

Complements on the identity relation.

 
Theorembj-opabssvv 34335* A variant of relopabiv 5687 (which could be proved from it, similarly to relxp 5567 from xpss 5565). (Contributed by BJ, 28-Dec-2023.)
{⟨𝑥, 𝑦⟩ ∣ 𝜑} ⊆ (V × V)
 
Theorembj-funidres 34336 The restricted identity relation is a function. (Contributed by BJ, 27-Dec-2023.)

TODO: relabel funi 6381 to funid.

Fun ( I ↾ 𝑉)
 
Theorembj-opelidb 34337 Characterization of the ordered pair elements of the identity relation.

Remark: in deduction-style proofs, one could save a few syntactic steps by using another antecedent than which already appears in the proof. Here for instance this could be the definition I = {⟨𝑥, 𝑦⟩ ∣ 𝑥 = 𝑦} but this would make the proof less easy to read. (Contributed by BJ, 27-Dec-2023.)

(⟨𝐴, 𝐵⟩ ∈ I ↔ ((𝐴 ∈ V ∧ 𝐵 ∈ V) ∧ 𝐴 = 𝐵))
 
Theorembj-opelidb1 34338 Characterization of the ordered pair elements of the identity relation. Variant of bj-opelidb 34337 where only the sethood of the first component is expressed. (Contributed by BJ, 27-Dec-2023.)
(⟨𝐴, 𝐵⟩ ∈ I ↔ (𝐴 ∈ V ∧ 𝐴 = 𝐵))
 
Theorembj-inexeqex 34339 Lemma for bj-opelid 34341 (but not specific to the identity relation): if the intersection of two classes is a set and the two classes are equal, then both are sets (all three classes are equal, so they all belong to 𝑉, but it is more convenient to have V in the consequent for theorems using it). (Contributed by BJ, 27-Dec-2023.)
(((𝐴𝐵) ∈ 𝑉𝐴 = 𝐵) → (𝐴 ∈ V ∧ 𝐵 ∈ V))
 
Theorembj-elsn0 34340 If the intersection of two classes is a set, then these classes are equal if and only if one is an element of the singleton formed on the other. Stronger form of elsng 4573 and elsn2g 4595 (which could be proved from it). (Contributed by BJ, 20-Jan-2024.)
((𝐴𝐵) ∈ 𝑉 → (𝐴 ∈ {𝐵} ↔ 𝐴 = 𝐵))
 
Theorembj-opelid 34341 Characterization of the ordered pair elements of the identity relation when the intersection of their components are sets. Note that the antecedent is more general than either component being a set. (Contributed by BJ, 29-Mar-2020.)
((𝐴𝐵) ∈ 𝑉 → (⟨𝐴, 𝐵⟩ ∈ I ↔ 𝐴 = 𝐵))
 
Theorembj-ideqg 34342 Characterization of the classes related by the identity relation when their intersection is a set. Note that the antecedent is more general than either class being a set. (Contributed by NM, 30-Apr-2004.) Weaken the antecedent to sethood of the intersection. (Revised by BJ, 24-Dec-2023.)

TODO: replace ideqg 5716, or at least prove ideqg 5716 from it.

((𝐴𝐵) ∈ 𝑉 → (𝐴 I 𝐵𝐴 = 𝐵))
 
Theorembj-ideqgALT 34343 Alternate proof of bj-ideqg 34342 from brabga 5413 instead of bj-opelid 34341 itself proved from bj-opelidb 34337. (Contributed by BJ, 27-Dec-2023.) (Proof modification is discouraged.) (New usage is discouraged.)
((𝐴𝐵) ∈ 𝑉 → (𝐴 I 𝐵𝐴 = 𝐵))
 
Theorembj-ideqb 34344 Characterization of classes related by the identity relation. (Contributed by BJ, 24-Dec-2023.)
(𝐴 I 𝐵 ↔ (𝐴 ∈ V ∧ 𝐴 = 𝐵))
 
Theorembj-idres 34345 Alternate expression for the restricted identity relation. The advantage of that expression is to expose it as a "bounded" class, being included in the Cartesian square of the restricting class. (Contributed by BJ, 27-Dec-2023.)

This is an alternate of idinxpresid 5909 (see idinxpres 5908). See also elrid 5907 and elidinxp 5905. (Proof modification is discouraged.)

( I ↾ 𝐴) = ( I ∩ (𝐴 × 𝐴))
 
Theorembj-opelidres 34346 Characterization of the ordered pairs in the restricted identity relation when the intersection of their component belongs to the restricting class. TODO: prove bj-idreseq 34347 from it. (Contributed by BJ, 29-Mar-2020.)
(𝐴𝑉 → (⟨𝐴, 𝐵⟩ ∈ ( I ↾ 𝑉) ↔ 𝐴 = 𝐵))
 
Theorembj-idreseq 34347 Sufficient condition for the restricted identity relation to agree with equality. Note that the instance of bj-ideqg 34342 with V substituted for 𝑉 is a direct consequence of bj-idreseq 34347. This is a strengthening of resieq 5858 which should be proved from it (note that currently, resieq 5858 relies on ideq 5717). Note that the intersection in the antecedent is not very meaningful, but is a device to prove versions with either class assumed to be a set. It could be enough to prove the version with a disjunctive antecedent: ((𝐴𝐶𝐵𝐶) → .... (Contributed by BJ, 25-Dec-2023.)
((𝐴𝐵) ∈ 𝐶 → (𝐴( I ↾ 𝐶)𝐵𝐴 = 𝐵))
 
Theorembj-idreseqb 34348 Characterization for two classes to be related under the restricted identity relation. (Contributed by BJ, 24-Dec-2023.)
(𝐴( I ↾ 𝐶)𝐵 ↔ (𝐴𝐶𝐴 = 𝐵))
 
Theorembj-ideqg1 34349 For sets, the identity relation is the same thing as equality. (Contributed by NM, 30-Apr-2004.) (Proof shortened by Andrew Salmon, 27-Aug-2011.) Generalize to a disjunctive antecedent. (Revised by BJ, 24-Dec-2023.)

TODO: delete once bj-ideqg 34342 is in the main section.

((𝐴𝑉𝐵𝑊) → (𝐴 I 𝐵𝐴 = 𝐵))
 
Theorembj-ideqg1ALT 34350 Alternate proof of bj-ideqg1 using brabga 5413 instead of the "unbounded" version bj-brab2a1 34334 or brab2a 5638. (Contributed by BJ, 25-Dec-2023.) (Proof modification is discouraged.) (New usage is discouraged.)

TODO: delete once bj-ideqg 34342 is in the main section.

((𝐴𝑉𝐵𝑊) → (𝐴 I 𝐵𝐴 = 𝐵))
 
Theorembj-opelidb1ALT 34351 Characterization of the couples in I. (Contributed by BJ, 29-Mar-2020.) (Proof modification is discouraged.) (New usage is discouraged.)
(⟨𝐴, 𝐵⟩ ∈ I ↔ (𝐴 ∈ V ∧ 𝐴 = 𝐵))
 
Theorembj-elid3 34352 Characterization of the couples in I whose first component is a setvar. (Contributed by BJ, 29-Mar-2020.)
(⟨𝑥, 𝐴⟩ ∈ I ↔ 𝑥 = 𝐴)
 
Theorembj-elid4 34353 Characterization of the elements of I. (Contributed by BJ, 22-Jun-2019.)
(𝐴 ∈ (𝑉 × 𝑊) → (𝐴 ∈ I ↔ (1st𝐴) = (2nd𝐴)))
 
Theorembj-elid5 34354 Characterization of the elements of I. (Contributed by BJ, 22-Jun-2019.)
(𝐴 ∈ I ↔ (𝐴 ∈ (V × V) ∧ (1st𝐴) = (2nd𝐴)))
 
Theorembj-elid6 34355 Characterization of the elements of the diagonal of a Cartesian square. (Contributed by BJ, 22-Jun-2019.)
(𝐵 ∈ ( I ↾ 𝐴) ↔ (𝐵 ∈ (𝐴 × 𝐴) ∧ (1st𝐵) = (2nd𝐵)))
 
Theorembj-elid7 34356 Characterization of the elements of the diagonal of a Cartesian square. (Contributed by BJ, 22-Jun-2019.)
(⟨𝐵, 𝐶⟩ ∈ ( I ↾ 𝐴) ↔ (𝐵𝐴𝐵 = 𝐶))
 
20.15.6.3  Functionalized identity (diagonal in a Cartesian square)

This subsection defines a functionalized version of the identity relation, that can also be seen as the diagonal in a Cartesian square).

As explained in df-bj-diag 34358, it will probably be deleted.

 
Syntaxcdiag2 34357 Syntax for the diagonal of the Cartesian square of a set.
class Id
 
Definitiondf-bj-diag 34358 Define the functionalized identity, which can also be seen as the diagonal function. Its value is given in bj-diagval 34359 when it is viewed as the functionalized identity, and in bj-diagval2 34360 when it is viewed as the diagonal function.

Indeed, Definition df-br 5059 identifies a binary relation with the class of couples that are related by that binary relation (see eqrel2 35440 for the extensionality property of binary relations). As a consequence, the identity relation, or identity function (see funi 6381), on any class, can alternatively be seen as the diagonal of the cartesian square of that class.

The identity relation on the universal class, I, is an "identity relation generator", since its restriction to any class is the identity relation on that class. It may be useful to consider a functionalized version of that fact, and that is the purpose of df-bj-diag 34358.

Note: most proofs will only use its values (Id‘𝐴), in which case it may be enough to use ( I ↾ 𝐴) everywhere and dispense with this definition. (Contributed by BJ, 22-Jun-2019.)

Id = (𝑥 ∈ V ↦ ( I ↾ 𝑥))
 
Theorembj-diagval 34359 Value of the funtionalized identity, or equivalently of the diagonal function. This expression views it as the functionalized identity, whereas bj-diagval2 34360 views it as the diagonal function. See df-bj-diag 34358 for the terminology. (Contributed by BJ, 22-Jun-2019.)
(𝐴𝑉 → (Id‘𝐴) = ( I ↾ 𝐴))
 
Theorembj-diagval2 34360 Value of the funtionalized identity, or equivalently of the diagonal function. This expression views it as the diagonal function, whereas bj-diagval 34359 views it as the functionalized identity. See df-bj-diag 34358 for the terminology. (Contributed by BJ, 22-Jun-2019.)
(𝐴𝑉 → (Id‘𝐴) = ( I ∩ (𝐴 × 𝐴)))
 
Theorembj-eldiag 34361 Characterization of the elements of the diagonal of a Cartesian square. Subsumed by bj-elid6 34355. (Contributed by BJ, 22-Jun-2019.)
(𝐴𝑉 → (𝐵 ∈ (Id‘𝐴) ↔ (𝐵 ∈ (𝐴 × 𝐴) ∧ (1st𝐵) = (2nd𝐵))))
 
Theorembj-eldiag2 34362 Characterization of the elements of the diagonal of a Cartesian square. Subsumed by bj-elid7 34356. (Contributed by BJ, 22-Jun-2019.)
(𝐴𝑉 → (⟨𝐵, 𝐶⟩ ∈ (Id‘𝐴) ↔ (𝐵𝐴𝐵 = 𝐶)))
 
20.15.6.4  Direct image and inverse image

Definitions of the functionalized direct image and inverse image.

The functionalized direct (resp. inverse) image is the morphism component of the covariant (resp. contravariant) powerset endofunctor of the category of sets and relations (and, up to restriction, of its subcategory of sets and functions). Its object component is the powerset operation 𝒫 defined in df-pw 4539.

 
Syntaxcimdir 34363 Syntax for the functionalized direct image.
class 𝒫*
 
Definitiondf-imdir 34364* Definition of the functionalized direct image, which maps a binary relation between two given sets to its associated direct image relation. (Contributed by BJ, 16-Dec-2023.)
𝒫* = (𝑎 ∈ V, 𝑏 ∈ V ↦ (𝑟 ∈ 𝒫 (𝑎 × 𝑏) ↦ {⟨𝑥, 𝑦⟩ ∣ ((𝑥𝑎𝑦𝑏) ∧ (𝑟𝑥) = 𝑦)}))
 
Theorembj-imdirval 34365* Value of the functionalized direct image. (Contributed by BJ, 16-Dec-2023.)
(𝜑𝐴𝑈)    &   (𝜑𝐵𝑉)       (𝜑 → (𝐴𝒫*𝐵) = (𝑟 ∈ 𝒫 (𝐴 × 𝐵) ↦ {⟨𝑥, 𝑦⟩ ∣ ((𝑥𝐴𝑦𝐵) ∧ (𝑟𝑥) = 𝑦)}))
 
Theorembj-imdirval2 34366* Value of the functionalized direct image. (Contributed by BJ, 16-Dec-2023.)
(𝜑𝐴𝑈)    &   (𝜑𝐵𝑉)    &   (𝜑𝑅 ⊆ (𝐴 × 𝐵))       (𝜑 → ((𝐴𝒫*𝐵)‘𝑅) = {⟨𝑥, 𝑦⟩ ∣ ((𝑥𝐴𝑦𝐵) ∧ (𝑅𝑥) = 𝑦)})
 
Theorembj-imdirval3 34367 Value of the functionalized direct image. (Contributed by BJ, 16-Dec-2023.)
(𝜑𝐴𝑈)    &   (𝜑𝐵𝑉)    &   (𝜑𝑅 ⊆ (𝐴 × 𝐵))       (𝜑 → (𝑋((𝐴𝒫*𝐵)‘𝑅)𝑌 ↔ ((𝑋𝐴𝑌𝐵) ∧ (𝑅𝑋) = 𝑌)))
 
Theorembj-imdirid 34368 Functorial property of the direct image: the direct image by the identity on a set is the identity on the powerset. (Contributed by BJ, 24-Dec-2023.)
(𝜑𝐴𝑈)       (𝜑 → ((𝐴𝒫*𝐴)‘( I ↾ 𝐴)) = ( I ↾ 𝒫 𝐴))
 
Syntaxcinvdir 34369 Syntax for the functionalized inverse image.
class 𝒫*
 
Definitiondf-invdir 34370* Definition of the functionalized inverse image, which maps a binary relation between two given sets to its associated inverse image relation. (Contributed by BJ, 23-Dec-2023.)
𝒫* = (𝑎 ∈ V, 𝑏 ∈ V ↦ (𝑟 ∈ 𝒫 (𝑎 × 𝑏) ↦ {⟨𝑥, 𝑦⟩ ∣ ((𝑥𝑎𝑦𝑏) ∧ 𝑥 = (𝑟𝑦))}))
 
20.15.6.5  Extended numbers and projective lines as sets

We parameterize the set of infinite extended complex numbers (df-bj-ccinfty 34387) using the real numbers (df-r 10536) via the function +∞e. Since at that point, we have only defined the set of real numbers but no operations on it, we define a temporary "fractional part" function, which is more convenient to define on the temporary reals R (df-nr 10467) since we can use operations on the latter. We also define the temporary real "one-half" in order to define minus infinity (df-bj-minfty 34399) and then we can define the sets of extended real numbers and of extended complex numbers, and the projective real and complex lines, as well as addition and negation on these, and also the order relation on the extended reals (which bypasses the intermediate definition of a temporary order on the real numbers and then a superseding one on the extended real numbers).

 
Syntaxcfractemp 34371 Syntax for the fractional part of a tempopary real.
class {R
 
Definitiondf-bj-fractemp 34372* Temporary definition: fractional part of a temporary real.

To understand this definition, recall the canonical injection ω⟶R, 𝑛 ↦ [{𝑥Q𝑥 <Q ⟨suc 𝑛, 1o⟩}, 1P] ~R where we successively take the successor of 𝑛 to land in positive integers, then take the couple with 1o as second component to land in positive rationals, then take the Dedekind cut that positive rational forms, and finally take the equivalence class of the couple with 1P as second component. Adding one at the beginning and subtracting it at the end is necessary since the constructions used in set.mm use the positive integers, positive rationals, and positive reals as intermediate number systems. (Contributed by BJ, 22-Jan-2023.) The precise definition is irrelevant and should generally not be used. One could even inline it. The definitive fractional part of an extended or projective complex number will be defined later. (New usage is discouraged.)

{R = (𝑥R ↦ (𝑦R ((𝑦 = 0R ∨ (0R <R 𝑦𝑦 <R 1R)) ∧ ∃𝑛 ∈ ω ([⟨{𝑧Q𝑧 <Q ⟨suc 𝑛, 1o⟩}, 1P⟩] ~R +R 𝑦) = 𝑥)))
 
Syntaxcinftyexpitau 34373 Syntax for the function +∞e parameterizing .
class +∞e
 
Definitiondf-bj-inftyexpitau 34374 Definition of the auxiliary function +∞e parameterizing the circle at infinity in ℂ̅. We use coupling with {R} to simplify the proof of bj-inftyexpitaudisj 34380. (Contributed by BJ, 22-Jan-2023.) The precise definition is irrelevant and should generally not be used. TODO: prove only the necessary lemmas to prove (𝐴 ∈ ℝ ∧ 𝐵 ∈ ℝ) → ((+∞e𝐴) = (+∞e𝐵) ↔ (𝐴𝐵) ∈ ℤ)). (New usage is discouraged.)
+∞e = (𝑥 ∈ ℝ ↦ ⟨({R‘(1st𝑥)), {R}⟩)
 
SyntaxcccinftyN 34375 Syntax for the circle at infinity ∞N.
class ∞N
 
Definitiondf-bj-ccinftyN 34376 Definition of the circle at infinity ∞N. (Contributed by BJ, 22-Jun-2019.) The precise definition is irrelevant and should generally not be used. (New usage is discouraged.)
∞N = ran +∞e
 
Theorembj-inftyexpitaufo 34377 The function +∞e written as a surjection with domain and range. (Contributed by BJ, 4-Feb-2023.)
+∞e:ℝ–onto→ℂ∞N
 
Syntaxchalf 34378 Syntax for the temporary one-half.
class 1/2
 
Definitiondf-bj-onehalf 34379 Define the temporary real "one-half". Once the machinery is developed, the real number "one-half" is commonly denoted by (1 / 2). (Contributed by BJ, 4-Feb-2023.) (New usage is discouraged.)

TODO:

$p |- 1/2 e. R. $= ? $. (riotacl 7120)

$p |- -. 0R = 1/2 $= ? $. (since -. ( 0R +R 0R ) = 1R )

$p |- 0R <R 1/2 $= ? $.

$p |- 1/2 <R 1R $= ? $.

$p |- ( {R ` 0R ) = 0R $= ? $.

$p |- ( {R ` 1/2 ) = 1/2 $= ? $.

df-minfty $a |- minfty = ( inftyexpitau ` <. 1/2 , 0R >. ) $.

1/2 = (𝑥R (𝑥 +R 𝑥) = 1R)
 
Theorembj-inftyexpitaudisj 34380 An element of the circle at infinity is not a complex number. (Contributed by BJ, 4-Feb-2023.)
¬ (+∞e𝐴) ∈ ℂ
 
Syntaxcinftyexpi 34381 Syntax for the function +∞ei parameterizing .
class +∞ei
 
Definitiondf-bj-inftyexpi 34382 Definition of the auxiliary function +∞ei parameterizing the circle at infinity in ℂ̅. We use coupling with to simplify the proof of bj-ccinftydisj 34388. It could seem more natural to define +∞ei on all of , but we want to use only basic functions in the definition of ℂ̅. TODO: transition to df-bj-inftyexpitau 34374 instead. (Contributed by BJ, 22-Jun-2019.) The precise definition is irrelevant and should generally not be used. (New usage is discouraged.)
+∞ei = (𝑥 ∈ (-π(,]π) ↦ ⟨𝑥, ℂ⟩)
 
Theorembj-inftyexpiinv 34383 Utility theorem for the inverse of +∞ei. (Contributed by BJ, 22-Jun-2019.) This utility theorem is irrelevant and should generally not be used. (New usage is discouraged.)
(𝐴 ∈ (-π(,]π) → (1st ‘(+∞ei𝐴)) = 𝐴)
 
Theorembj-inftyexpiinj 34384 Injectivity of the parameterization +∞ei. Remark: a more conceptual proof would use bj-inftyexpiinv 34383 and the fact that a function with a retraction is injective. (Contributed by BJ, 22-Jun-2019.)
((𝐴 ∈ (-π(,]π) ∧ 𝐵 ∈ (-π(,]π)) → (𝐴 = 𝐵 ↔ (+∞ei𝐴) = (+∞ei𝐵)))
 
Theorembj-inftyexpidisj 34385 An element of the circle at infinity is not a complex number. (Contributed by BJ, 22-Jun-2019.) This utility theorem is irrelevant and should generally not be used. (New usage is discouraged.)
¬ (+∞ei𝐴) ∈ ℂ
 
Syntaxcccinfty 34386 Syntax for the circle at infinity .
class
 
Definitiondf-bj-ccinfty 34387 Definition of the circle at infinity . (Contributed by BJ, 22-Jun-2019.) The precise definition is irrelevant and should generally not be used. (New usage is discouraged.)
= ran +∞ei
 
Theorembj-ccinftydisj 34388 The circle at infinity is disjoint from the set of complex numbers. (Contributed by BJ, 22-Jun-2019.)
(ℂ ∩ ℂ) = ∅
 
Theorembj-elccinfty 34389 A lemma for infinite extended complex numbers. (Contributed by BJ, 27-Jun-2019.)
(𝐴 ∈ (-π(,]π) → (+∞ei𝐴) ∈ ℂ)
 
Syntaxcccbar 34390 Syntax for the set of extended complex numbers ℂ̅.
class ℂ̅
 
Definitiondf-bj-ccbar 34391 Definition of the set of extended complex numbers ℂ̅. (Contributed by BJ, 22-Jun-2019.)
ℂ̅ = (ℂ ∪ ℂ)
 
Theorembj-ccssccbar 34392 Complex numbers are extended complex numbers. (Contributed by BJ, 27-Jun-2019.)
ℂ ⊆ ℂ̅
 
Theorembj-ccinftyssccbar 34393 Infinite extended complex numbers are extended complex numbers. (Contributed by BJ, 27-Jun-2019.)
⊆ ℂ̅
 
Syntaxcpinfty 34394 Syntax for "plus infinity".
class +∞
 
Definitiondf-bj-pinfty 34395 Definition of "plus infinity". (Contributed by BJ, 27-Jun-2019.)
+∞ = (+∞ei‘0)
 
Theorembj-pinftyccb 34396 The class +∞ is an extended complex number. (Contributed by BJ, 27-Jun-2019.)
+∞ ∈ ℂ̅
 
Theorembj-pinftynrr 34397 The extended complex number +∞ is not a complex number. (Contributed by BJ, 27-Jun-2019.)
¬ +∞ ∈ ℂ
 
Syntaxcminfty 34398 Syntax for "minus infinity".
class -∞
 
Definitiondf-bj-minfty 34399 Definition of "minus infinity". (Contributed by BJ, 27-Jun-2019.)
-∞ = (+∞ei‘π)
 
Theorembj-minftyccb 34400 The class -∞ is an extended complex number. (Contributed by BJ, 27-Jun-2019.)
-∞ ∈ ℂ̅
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