P. 331 of Theorem List - Metamath Proof Explorer

Theorem List for Metamath Proof Explorer - 33001-33100   *Has distinct variable group(s)

Type	Label	Description
Statement
Theorem	bj-pr21val 33001	Value of the first projection of a couple. (Contributed by BJ, 6-Oct-2018.)
|- pr1 (| A, B|) = A
Syntax	bj-cpr2 33002	Syntax for the second class tuple projection. (Contributed by BJ, 6-Oct-2018.)
class pr2 A
Definition	df-bj-pr2 33003	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 33004, bj-pr22val 33007, bj-pr2ex 33008. (Contributed by BJ, 6-Oct-2018.) (New usage is discouraged.)
|- pr2 A = ( 1o Proj A )
Theorem	bj-pr2eq 33004	Substitution property for pr2. (Contributed by BJ, 6-Oct-2018.)
|- ( A = B -> pr2 A = pr2 B )
Theorem	bj-pr2un 33005	The second projection preserves unions. (Contributed by BJ, 6-Apr-2019.)
|- pr2 ( A u. B ) = (pr2 A u. pr2 B )
Theorem	bj-pr2val 33006	Value of the second projection. (Contributed by BJ, 6-Apr-2019.)
|- pr2 ( { A } X. tag B ) = if ( A = 1o , B , (/) )
Theorem	bj-pr22val 33007	Value of the second projection of a couple. (Contributed by BJ, 6-Oct-2018.)
|- pr2 (| A, B|) = B
Theorem	bj-pr2ex 33008	Sethood of the second projection. (Contributed by BJ, 6-Oct-2018.)
|- ( A e. V -> pr2 A e. _V )
Theorem	bj-2uplth 33009	The characteristic property of couples. Note that this holds without sethood hypotheses (compare opth 4945). (Contributed by BJ, 6-Oct-2018.)
|- ((| A, B|) = (| C, D|) <-> ( A = C /\ B = D ) )
Theorem	bj-2uplex 33010	A couple is a set if and only if its coordinates are sets. (Contributed by BJ, 6-Oct-2018.)
|- ((| A, B|) e. _V <-> ( A e. _V /\ B e. _V ) )
Theorem	bj-2upln0 33011	A couple is nonempty. (Contributed by BJ, 21-Apr-2019.)
|- (| A, B|) =/= (/)
Theorem	bj-2upln1upl 33012	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 32997 and bj-2upln0 33011 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|)
20.14.5.14  Set theory: miscellaneous Miscellaneous theorems of set theory.
Theorem	bj-disj2r 33013	Relative version of ssdifin0 4050, allowing a biconditional, and of disj2 4024. This proof does not rely, even indirectly, on ssdifin0 4050 nor disj2 4024. (Contributed by BJ, 11-Nov-2021.)
|- ( ( A i^i V ) C_ ( V \ B ) <-> ( ( A i^i B ) i^i V ) = (/) )
Theorem	bj-sscon 33014	Contraposition law for relative subsets. Relative and generalized version of ssconb 3743, which it can shorten, as well as conss2 38647. (Contributed by BJ, 11-Nov-2021.)
|- ( ( A i^i V ) C_ ( V \ B ) <-> ( B i^i V ) C_ ( V \ A ) )
Theorem	bj-vjust2 33015	Justification theorem for bj-df-v 33016. See also vjust 3201 and bj-vjust 32786. (Contributed by BJ, 30-Nov-2019.) (Proof modification is discouraged.)
|- { x | T. } = { y | T. }
Theorem	bj-df-v 33016	Alternate definition of the universal class. Actually, the current definition df-v 3202 should be proved from this one, and vex 3203 should be proved from this proposed definition together with bj-vexwv 32857, which would remove from vex 3203 dependency on ax-13 2246 (see also comment of bj-vexw 32855). (Contributed by BJ, 30-Nov-2019.) (Proof modification is discouraged.)
|- _V = { x | T. }
Theorem	bj-df-nul 33017	Alternate definition of the empty class/set. (Contributed by BJ, 30-Nov-2019.) (Proof modification is discouraged.)
|- (/) = { x | F. }
Theorem	bj-nul 33018*	Two formulations of the axiom of the empty set ax-nul 4789. Proposal: place it right before ax-nul 4789. (Contributed by BJ, 30-Nov-2019.) (Proof modification is discouraged.)
|- ( (/) e. _V <-> E. x A. y -. y e. x )
Theorem	bj-nuliota 33019*	Definition of the empty set using the definite description binder. See also bj-nuliotaALT 33020. (Contributed by BJ, 30-Nov-2019.) (Proof modification is discouraged.)
|- (/) = ( iota x A. y -. y e. x )
Theorem	bj-nuliotaALT 33020*	Alternate proof of bj-nuliota 33019. Note that this alternate proof uses the fact that iota x ph evaluates to (/) when there is no x satisfying ph (iotanul 5866). 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.)
|- (/) = ( iota x A. y -. y e. x )
Theorem	bj-vtoclgfALT 33021	Alternate proof of vtoclgf 3264. Proof from vtoclgft 3254. (This may have been the original proof before shortening.) (Contributed by BJ, 30-Sep-2019.) (Proof modification is discouraged.) (New usage is discouraged.)
|- F/_ x A   &    |- F/ x ps   &    |- ( x = A -> ( ph <-> ps ) )   &    |- ph   =>    |- ( A e. V -> ps )
Theorem	bj-pwcfsdom 33022	Remove hypothesis from pwcfsdom 9405. Illustration of how to remove a "proof-facilitating hypothesis". (Can use it to shorten theorems using pwcfsdom 9405.) (Contributed by BJ, 14-Sep-2019.)
|- ( aleph ` A ) ~< ( ( aleph ` A ) ^m ( cf ` ( aleph ` A ) ) )
Theorem	bj-grur1 33023	Remove hypothesis from grur1 9642. 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 e. Univ /\ U e. U. ( R1 " On ) ) -> U = ( R1 ` ( U i^i On ) ) )
20.14.5.15  Evaluation
Theorem	bj-evaleq 33024	Equality theorem for the Slot construction. This is currently a duplicate of sloteq 15862 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 )
Theorem	bj-evalfun 33025	The evaluation at a class is a function. (Contributed by BJ, 27-Dec-2021.)
|- Fun Slot A
Theorem	bj-evalfn 33026	The evaluation at a class is a function on the universal class. (General form of slotfn 15875). (Contributed by Mario Carneiro, 22-Sep-2015.) (Revised by BJ, 27-Dec-2021.)
|- Slot A Fn _V
Theorem	bj-evalval 33027	Value of the evaluation at a class. (Closed form of strfvnd 15876 and strfvn 15879). (Contributed by NM, 9-Sep-2011.) (Revised by Mario Carneiro, 15-Nov-2014.) (Revised by BJ, 27-Dec-2021.)
|- ( F e. V -> (Slot A ` F ) = ( F ` A ) )
Theorem	bj-evalid 33028	The evaluation at a set of the identity function is that set. (General form of ndxarg 15882.) 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.)
|- ( ( V e. W /\ A e. V ) -> (Slot A ` ( _I |` V ) ) = A )
Theorem	bj-ndxarg 33029	Proof of ndxarg 15882 from bj-evalid 33028. (Contributed by BJ, 27-Dec-2021.) (Proof modification is discouraged.)
|- E = Slot N   &    |- N e. NN   =>    |- ( E ` ndx ) = N
Theorem	bj-ndxid 33030	Proof of ndxid 15883 from ndxarg 15882. (Contributed by BJ, 27-Dec-2021.) (Proof modification is discouraged.)
|- E = Slot N   &    |- N e. NN   =>    |- E = Slot ( E ` ndx )
Theorem	bj-evalidval 33031	Closed general form of strndxid 15885. Both sides are equal to ( F ` A ) by bj-evalid 33028 and bj-evalval 33027 respectively, but bj-evalidval 33031 adds something to bj-evalid 33028 and bj-evalval 33027 in that Slot A appears on both sides. (Contributed by BJ, 27-Dec-2021.)
|- ( ( V e. W /\ A e. V /\ F e. U ) -> ( F ` (Slot A ` ( _I |` V ) ) ) = (Slot A ` F ) )
20.14.5.16  Elementwise operations
Syntax	celwise 33032	Syntax for elementwise operations.
class elwise
Definition	df-elwise 33033*	Define the elementwise operation associated with a given operation. For instance, + is the addition of complex numbers (axaddf 9966), 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 x e. A and y e. B. The set of odd natural numbers is ( ( { 2 } (elwise ` x. ) NN0 ) (elwise ` + ) { 1 } ), or less formally 2 NN0 + 1. (Contributed by BJ, 22-Dec-2021.)
|- elwise = ( o e. _V |-> ( x e. _V , y e. _V |-> { z | E. u e. x E. v e. y z = ( u o v ) } ) )
20.14.5.17  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 16246), topologies (df-top 20699), pi-systems, rings of sets, delta-rings, lambda-systems/Dynkin systems, algebras/fields of sets, sigma-algebras/sigma-fields/tribes (df-siga 30171), 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 ( X ↾t A ) the elementwise intersection on the family X by the class A. REMARK: many theorems are already in set.mm ; MM>search *rest* /J
Theorem	bj-rest00 33034	An elementwise intersection on the empty family is the empty set. TODO: this is 0rest 16090. (Contributed by BJ, 27-Apr-2021.)
|- ( (/) ↾t A ) = (/)
Theorem	bj-restsn 33035	An elementwise intersection on the singleton on a set is the singleton on the intersection by that set. Generalization of bj-restsn0 33038 and bj-restsnid 33040. (Contributed by BJ, 27-Apr-2021.)
|- ( ( Y e. V /\ A e. W ) -> ( { Y } ↾t A ) = { ( Y i^i A ) } )
Theorem	bj-restsnss 33036	Special case of bj-restsn 33035. (Contributed by BJ, 27-Apr-2021.)
|- ( ( Y e. V /\ A C_ Y ) -> ( { Y } ↾t A ) = { A } )
Theorem	bj-restsnss2 33037	Special case of bj-restsn 33035. (Contributed by BJ, 27-Apr-2021.)
|- ( ( A e. V /\ Y C_ A ) -> ( { Y } ↾t A ) = { Y } )
Theorem	bj-restsn0 33038	An elementwise intersection on the singleton on the empty set is the singleton on the empty set. Special case of bj-restsn 33035 and bj-restsnss2 33037. TODO: this is restsn 20974. (Contributed by BJ, 27-Apr-2021.)
|- ( A e. V -> ( { (/) } ↾t A ) = { (/) } )
Theorem	bj-restsn10 33039	Special case of bj-restsn 33035, bj-restsnss 33036, and bj-rest10 33041. (Contributed by BJ, 27-Apr-2021.)
|- ( X e. V -> ( { X } ↾t (/) ) = { (/) } )
Theorem	bj-restsnid 33040	The elementwise intersection on the singleton on a class by that class is the singleton on that class. Special case of bj-restsn 33035 and bj-restsnss 33036. (Contributed by BJ, 27-Apr-2021.)
|- ( { A } ↾t A ) = { A }
Theorem	bj-rest10 33041	An elementwise intersection on a nonempty family by the empty set is the singleton on the empty set. TODO: this generalizes rest0 20973 and could replace it. (Contributed by BJ, 27-Apr-2021.)
|- ( X e. V -> ( X =/= (/) -> ( X ↾t (/) ) = { (/) } ) )
Theorem	bj-rest10b 33042	Alternate version of bj-rest10 33041. (Contributed by BJ, 27-Apr-2021.)
|- ( X e. ( V \ { (/) } ) -> ( X ↾t (/) ) = { (/) } )
Theorem	bj-restn0 33043	An elementwise intersection on a nonempty family is nonempty. (Contributed by BJ, 27-Apr-2021.)
|- ( ( X e. V /\ A e. W ) -> ( X =/= (/) -> ( X ↾t A ) =/= (/) ) )
Theorem	bj-restn0b 33044	Alternate version of bj-restn0 33043. (Contributed by BJ, 27-Apr-2021.)
|- ( ( X e. ( V \ { (/) } ) /\ A e. W ) -> ( X ↾t A ) =/= (/) )
Theorem	bj-restpw 33045	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 20982 (which uses distop 20799 and restopn2 20981). (Contributed by BJ, 27-Apr-2021.)
|- ( ( Y e. V /\ A e. W ) -> ( ~P Y ↾t A ) = ~P ( Y i^i A ) )
Theorem	bj-rest0 33046	An elementwise intersection on a family containing the empty set contains the empty set. (Contributed by BJ, 27-Apr-2021.)
|- ( ( X e. V /\ A e. W ) -> ( (/) e. X -> (/) e. ( X ↾t A ) ) )
Theorem	bj-restb 33047	An elementwise intersection by a set on a family containing a superset of that set contains that set. (Contributed by BJ, 27-Apr-2021.)
|- ( X e. V -> ( ( A C_ B /\ B e. X ) -> A e. ( X ↾t A ) ) )
Theorem	bj-restv 33048	An elementwise intersection by a subset on a family containing the whole set contains the whole subset. (Contributed by BJ, 27-Apr-2021.)
|- ( ( A C_ U. X /\ U. X e. X ) -> A e. ( X ↾t A ) )
Theorem	bj-resta 33049	An elementwise intersection by a set on a family containing that set contains that set. (Contributed by BJ, 27-Apr-2021.)
|- ( X e. V -> ( A e. X -> A e. ( X ↾t A ) ) )
Theorem	bj-restuni 33050	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 20966 and restuni2 20971. (Contributed by BJ, 27-Apr-2021.)
|- ( ( X e. V /\ A e. W ) -> U. ( X ↾t A ) = ( U. X i^i A ) )
Theorem	bj-restuni2 33051	The union of an elementwise intersection on a family of sets by a subset is equal to that subset. See also restuni 20966 and restuni2 20971. (Contributed by BJ, 27-Apr-2021.)
|- ( ( X e. V /\ A C_ U. X ) -> U. ( X ↾t A ) = A )
Theorem	bj-restreg 33052	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.)
|- ( ( A e. V /\ A =/= (/) ) -> (/) e. ( A ↾t A ) )
20.14.5.18  Moore collections (complements)
Theorem	bj-intss 33053	A nonempty intersection of a family of subsets of a class is included in that class. (Contributed by BJ, 7-Dec-2021.)
|- ( A C_ ~P X -> ( A =/= (/) -> |^| A C_ X ) )
Theorem	bj-raldifsn 33054*	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 -> ( ph <-> ps ) )   =>    |- ( B e. A -> ( A. x e. A ph <-> ( A. x e. ( A \ { B } ) ph /\ ps ) ) )
Theorem	bj-0int 33055*	If A is a collection of subsets of X, like a topology, two equivalent ways to say that arbitrary intersections of elements of A relative to X belong to some class B (in typical applications, A itself). (Contributed by BJ, 7-Dec-2021.)
|- ( A C_ ~P X -> ( ( X e. B /\ A. x e. ( ~P A \ { (/) } ) |^| x e. B ) <-> A. x e. ~P A ( X i^i |^| x ) e. B ) )
Theorem	bj-mooreset 33056*	A Moore collection is a set. That is, if we define a "Moore predicate" by (Moore A <-> A. x e. ~P A ( U. A i^i |^| x ) e. A ), then any class satisfying that predicate is actually a set. Therefore, the definition df-bj-moore 33058 is sufficient. Note that the closed sets of a topology form a Moore collection, so this remark also applies to topologies and many other families of sets (namely, as soon as the whole set is required to be a closed set, as can be seen from the proof, which relies crucially on uniexr 6972). Note: if, in the above predicate, we substitute ~P X for A, then the last e. ~P X could be weakened to C_ ~P X, and then the predicate would be obviously satisfied since U. ~P X = X, making ~P X a Moore collection in this weaker sense, even if X is a proper class, but the addition of this single case does not add anything interesting. (Contributed by BJ, 8-Dec-2021.)
|- ( A. x e. ~P A ( U. A i^i |^| x ) e. A -> A e. _V )
Syntax	cmoore 33057	Syntax for the class of Moore collections.
class Moore_
Definition	df-bj-moore 33058*	Define the class of Moore collections. This is to df-mre 16246 what df-top 20699 is to df-topon 20716. For the sake of consistency, the function defined at df-mre 16246 should be denoted by "MooreOn". Note: df-mre 16246 singles out the empty intersection. This is not necessary. It could be written instead Moore = ( x e. _V |-> { y e. ~P ~P x | A. z e. ~P y ( x i^i |^| z ) e. y } ) and the equivalence of both definitions is proved by bj-0int 33055. 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 33056). (Contributed by BJ, 27-Apr-2021.)
|- Moore_ = { x | A. y e. ~P x ( U. x i^i |^| y ) e. x }
Theorem	bj-ismoore 33059*	Characterization of Moore collections among sets. (Contributed by BJ, 9-Dec-2021.)
|- ( A e. V -> ( A e. Moore_ <-> A. x e. ~P A ( U. A i^i |^| x ) e. A ) )
Theorem	bj-ismoorec 33060*	Characterization of Moore collections. (Contributed by BJ, 9-Dec-2021.)
|- ( A e. Moore_ <-> ( A e. _V /\ A. x e. ~P A ( U. A i^i |^| x ) e. A ) )
Theorem	bj-ismoored0 33061	Necessary condition to be a Moore collection. (Contributed by BJ, 9-Dec-2021.)
|- ( A e. Moore_ -> U. A e. A )
Theorem	bj-ismoored 33062	Necessary condition to be a Moore collection. (Contributed by BJ, 9-Dec-2021.)
|- ( ph -> A e. Moore_ )   &    |- ( ph -> B C_ A )   =>    |- ( ph -> ( U. A i^i |^| B ) e. A )
Theorem	bj-ismoored2 33063	Necessary condition to be a Moore collection. (Contributed by BJ, 9-Dec-2021.)
|- ( ph -> A e. Moore_ )   &    |- ( ph -> B C_ A )   &    |- ( ph -> B =/= (/) )   =>    |- ( ph -> |^| B e. A )
Theorem	bj-ismooredr 33064*	Sufficient condition to be a Moore collection. (Contributed by BJ, 9-Dec-2021.)
|- ( ph -> A e. V )   &    |- ( ( ph /\ x C_ A ) -> ( U. A i^i |^| x ) e. A )   =>    |- ( ph -> A e. Moore_ )
Theorem	bj-ismooredr2 33065*	Sufficient condition to be a Moore collection. (Contributed by BJ, 9-Dec-2021.)
|- ( ph -> A e. V )   &    |- ( ph -> U. A e. A )   &    |- ( ( ( ph /\ x C_ A ) /\ x =/= (/) ) -> |^| x e. A )   =>    |- ( ph -> A e. Moore_ )
Theorem	bj-discrmoore 33066	The discrete Moore collection on a set. (Contributed by BJ, 9-Dec-2021.)
|- ( A e. _V <-> ~P A e. Moore_ )
Theorem	bj-0nmoore 33067	The empty set is not a Moore collection. (Contributed by BJ, 9-Dec-2021.)
|- -. (/) e. Moore_
Theorem	bj-snmoore 33068	A singleton is a Moore collection. (Contributed by BJ, 9-Dec-2021.)
|- ( A e. _V <-> { A } e. Moore_ )
20.14.5.19  Maps-to notation for functions with three arguments
Theorem	bj-0nelmpt 33069	The empty set is not an element of a function (given in maps-to notation). (Contributed by BJ, 30-Dec-2020.)
|- -. (/) e. ( x e. A |-> B )
Theorem	bj-mptval 33070	Value of a function given in maps-to notation. (Contributed by BJ, 30-Dec-2020.)
|- F/_ x A   =>    |- ( A. x e. A B e. V -> ( X e. A -> ( ( ( x e. A |-> B ) ` X ) = Y <-> X ( x e. A |-> B ) Y ) ) )
Theorem	bj-dfmpt2a 33071*	An equivalent definition of df-mpt2 6655. (Contributed by BJ, 30-Dec-2020.)
|- ( x e. A , y e. B |-> C ) = { <. s , t >. | E. x e. A E. y e. B ( s = <. x , y >. /\ t = C ) }
Theorem	bj-mpt2mptALT 33072*	Alternate proof of mpt2mpt 6752. (Contributed by BJ, 30-Dec-2020.) (Proof modification is discouraged.) (New usage is discouraged.)
|- ( z = <. x , y >. -> C = D )   =>    |- ( z e. ( A X. B ) |-> C ) = ( x e. A , y e. B |-> D )
Syntax	cmpt3 33073	Extend the definition of a class to include maps-to notation for functions with three arguments.
class ( x e. A , y e. B , z e. C |-> D )
Definition	df-bj-mpt3 33074*	Define maps-to notation for functions with three arguments. See df-mpt 4730 and df-mpt2 6655 for functions with one and two arguments respectively. This definition is analogous to bj-dfmpt2a 33071. (Contributed by BJ, 11-Apr-2020.)
|- ( x e. A , y e. B , z e. C |-> D ) = { <. s , t >. | E. x e. A E. y e. B E. z e. C ( s = <. x , y , z >. /\ t = D ) }
20.14.5.20  Currying Currying and uncurrying. See also df-cur and df-unc 7394. Contrary to these, the definitions in this section are parameterized.
Syntax	csethom 33075	Notation for the set of set morphisms.
class -Set->
Syntax	ctophom 33076	Notation for the set of topological morphisms.
class -Top->
Syntax	cmagmahom 33077	Notation for the set of magma morphisms.
class -Magma->
Definition	df-bj-sethom 33078*	Define the set of functions (morphisms of sets) between two sets. Same as df-map 7859 with arguments swapped. TODO: prove the same staple lemmas as for ^m. Remark: one may define -Set-> = ( x e. ( 1st Struct ) , y e. ( 1st Struct ) |-> { f | f : ( Base x ) --> ( Base y ) } ) so that for morphisms between other structures, one could write ... { f e. ( x -Set-> y ) |... (Contributed by BJ, 11-Apr-2020.)
|- -Set-> = ( x e. _V , y e. _V |-> { f | f : x --> y } )
Definition	df-bj-tophom 33079*	Define the set of continuous functions (morphisms of topological spaces) between two topological spaces. Similar to df-cn 21031 (which is in terms of topologies instead of topological spaces). (Contributed by BJ, 10-Feb-2022.)
|- -Top-> = ( x e. TopSp , y e. TopSp |-> { f e. ( ( Base ` x ) -Set-> ( Base ` y ) ) | A. u e. ( TopOpen ` y ) ( `' f " u ) e. ( TopOpen ` x ) } )
Definition	df-bj-magmahom 33080*	Define the set of magma morphisms between two magmas. (Contributed by BJ, 10-Feb-2022.)
|- -Magma-> = ( x e. Mgm , y e. Mgm |-> { f e. ( ( Base ` x ) -Set-> ( Base ` y ) ) | A. u e. ( Base ` x ) A. v e. ( Base ` x ) ( f ` ( u ( +g ` x ) v ) ) = ( ( f ` u ) ( +g ` y ) ( f ` v ) ) } )
Syntax	ccur- 33081	Extend class notation to include the parameterized currying function.
class curry_
Definition	df-bj-cur 33082*	Define currying. See also df-cur 7393. (Contributed by BJ, 11-Apr-2020.)
|- curry_ = ( x e. _V , y e. _V , z e. _V |-> ( f e. ( ( x X. y ) -Set-> z ) |-> ( a e. x |-> ( b e. y |-> ( f ` <. a , b >. ) ) ) ) )
Syntax	cunc- 33083	Extend class notation to include the parameterized uncurrying function.
class uncurry_
Definition	df-bj-unc 33084*	Define uncurrying. See also df-unc 7394. (Contributed by BJ, 11-Apr-2020.)
|- uncurry_ = ( x e. _V , y e. _V , z e. _V |-> ( f e. ( x -Set-> ( y -Set-> z ) ) |-> ( a e. x , b e. y |-> ( ( f ` a ) ` b ) ) ) )
20.14.6  Extended real and complex numbers, real and complex projective lines In this section, we indroduce several supersets of the set RR of real numbers and the set CC 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 RRhat and CChat respectively, defined in df-bj-cchat 33120 and df-bj-rrhat 33122, and the point at infinity is denoted by infty, defined in df-bj-infty 33118. Both RR and CC also have "directional compactifications", denoted respectively by RRbar, defined in df-bj-rrbar 33116 (already defined as RR*, see df-xr 10078) and CCbar, defined in df-bj-ccbar 33103. Since CCbar does not seem to be standard, we describe it in some detail. It is obtained by adding to CC a "point at infinity at the end of each ray with origin at 0". Although CCbar is not an important object in itself, the motivation for introducing it is to provide a common superset to both RRbar and CC and to define algebraic operations (addition, opposite, multiplication, inverse) as widely as reasonably possible. Mathematically, CCbar is the quotient of ( ( CC X. RR>=0 ) \ { <. 0 , 0 >. } ) by the diagonal multiplicative action of RR>0 (think of the closed "northern hemisphere" in RR^3 identified with ( CC X. RR ), 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 CC C_ CCbar, we instead define CCbar as the (disjoint) union of CC with a circle at infinity denoted by CCinfty. To have a genuine inclusion RRbar C_ CCbar, we define pinfty and minfty as certain points in CCinfty. Thanks to this framework, one has the genuine inclusions RR C_ RRbar and RR C_ RRhat and similarly CC C_ CCbar and CC C_ CChat. Furthermore, one has RR C_ CC as well as RRbar C_ CCbar and RRhat C_ CChat. Furthermore, we define the main algebraic operations on (CCbar u. CChat ), which is not very mathematical, but "overloads" the operations, so that one can use the same notation in all cases.
20.14.6.1  Diagonal in a Cartesian square Complements on the idendity relation and definition of the diagonal in the Cartesian square of a set.
Theorem	bj-elid 33085	Characterization of the elements of _I. (Contributed by BJ, 22-Jun-2019.)
|- ( A e. _I <-> ( A e. ( _V X. _V ) /\ ( 1st ` A ) = ( 2nd ` A ) ) )
Theorem	bj-elid2 33086	Characterization of the elements of _I. (Contributed by BJ, 22-Jun-2019.)
|- ( A e. ( V X. W ) -> ( A e. _I <-> ( 1st ` A ) = ( 2nd ` A ) ) )
Theorem	bj-elid3 33087	Characterization of the elements of _I. (Contributed by BJ, 29-Mar-2020.)
|- ( <. A , B >. e. _I <-> ( A e. _V /\ A = B ) )
Syntax	cdiag2 33088	Syntax for the diagonal of the Cartesian square of a set.
class Diag
Definition	df-bj-diag 33089	Define the diagonal of the Cartesian square of a set. (Contributed by BJ, 22-Jun-2019.)
|- Diag = ( x e. _V |-> ( _I i^i ( x X. x ) ) )
Theorem	bj-diagval 33090	Value of the diagonal. (Contributed by BJ, 22-Jun-2019.)
|- ( A e. V -> (Diag ` A ) = ( _I i^i ( A X. A ) ) )
Theorem	bj-eldiag 33091	Characterization of the elements of the diagonal of a Cartesian square. (Contributed by BJ, 22-Jun-2019.)
|- ( A e. V -> ( B e. (Diag ` A ) <-> ( B e. ( A X. A ) /\ ( 1st ` B ) = ( 2nd ` B ) ) ) )
Theorem	bj-eldiag2 33092	Characterization of the elements of the diagonal of a Cartesian square. (Contributed by BJ, 22-Jun-2019.)
|- ( A e. V -> ( <. B , C >. e. (Diag ` A ) <-> ( B e. A /\ B = C ) ) )
20.14.6.2  Extended numbers and projective lines as sets TODO(?): replace df-bj-inftyexpi 33094 with a function inftyexpi2pi defined on ( 0 [,) 1 ) since we plan to put this section as early as possible, before the definition of pi. It would be best to use df-0r 9882 and df-1r 9883 but intervals are defined for real numbers, and not these temporary reals. It looks like to define the sets, the addition and the opposite, one only needs some basic results about addition, opposite and ordering, which could use df-plr 9879, df-ltr 9881, df-0r 9882, df-1r 9883, df-ltr 9881. The idea is then to define the order relation directly on RRbar, skipping RR.
Syntax	cinftyexpi 33093	Syntax for the function inftyexpi parameterizing CCinfty.
class inftyexpi
Definition	df-bj-inftyexpi 33094	Definition of the auxiliary function inftyexpi parameterizing the circle at infinity CCinfty in CCbar. We use coupling with CC to simplify the proof of bj-ccinftydisj 33100. It could seem more natural to define inftyexpi on all of RR using prcpal but we want to use only basic functions in the definition of CCbar. (Contributed by BJ, 22-Jun-2019.) The precise definition is irrelevant and should generally not be used. (New usage is discouraged.)
|- inftyexpi = ( x e. ( -u pi (,] pi ) |-> <. x , CC >. )
Theorem	bj-inftyexpiinv 33095	Utility theorem for the inverse of inftyexpi. (Contributed by BJ, 22-Jun-2019.) This utility theorem is irrelevant and should generally not be used. (New usage is discouraged.)
|- ( A e. ( -u pi (,] pi ) -> ( 1st ` (inftyexpi ` A ) ) = A )
Theorem	bj-inftyexpiinj 33096	Injectivity of the parameterization inftyexpi. Remark: a more conceptual proof would use bj-inftyexpiinv 33095 and the fact that a function with a retraction is injective. (Contributed by BJ, 22-Jun-2019.)
|- ( ( A e. ( -u pi (,] pi ) /\ B e. ( -u pi (,] pi ) ) -> ( A = B <-> (inftyexpi ` A ) = (inftyexpi ` B ) ) )
Theorem	bj-inftyexpidisj 33097	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.)
|- -. (inftyexpi ` A ) e. CC
Syntax	cccinfty 33098	Syntax for the circle at infinity CCinfty.
class CCinfty
Definition	df-bj-ccinfty 33099	Definition of the circle at infinity CCinfty. (Contributed by BJ, 22-Jun-2019.) The precise definition is irrelevant and should generally not be used. (New usage is discouraged.)
|- CCinfty = ran inftyexpi
Theorem	bj-ccinftydisj 33100	The circle at infinity is disjoint from the set of complex numbers. (Contributed by BJ, 22-Jun-2019.)
|- ( CC i^i CCinfty ) = (/)