P. 39 of Theorem List - Intuitionistic Logic Explorer

Theorem List for Intuitionistic Logic Explorer - 3801-3900   *Has distinct variable group(s)

Type	Label	Description
Statement
Theorem	snssg 3801	The singleton formed on a set is included in a class if and only if the set is an element of that class. Theorem 7.4 of [Quine] p. 49. (Contributed by NM, 22-Jul-2001.) (Proof shortened by BJ, 1-Jan-2025.)
|- ( A e. V -> ( A e. B <-> { A } C_ B ) )
Theorem	snss 3802	The singleton of an element of a class is a subset of the class (inference form of snssg 3801). Theorem 7.4 of [Quine] p. 49. (Contributed by NM, 21-Jun-1993.) (Proof shortened by BJ, 1-Jan-2025.)
|- A e. _V   =>    |- ( A e. B <-> { A } C_ B )
Theorem	snssgOLD 3803	Obsolete version of snssgOLD 3803 as of 1-Jan-2025. (Contributed by NM, 22-Jul-2001.) (Proof modification is discouraged.) (New usage is discouraged.)
|- ( A e. V -> ( A e. B <-> { A } C_ B ) )
Theorem	difsn 3804	An element not in a set can be removed without affecting the set. (Contributed by NM, 16-Mar-2006.) (Proof shortened by Andrew Salmon, 29-Jun-2011.)
|- ( -. A e. B -> ( B \ { A } ) = B )
Theorem	difprsnss 3805	Removal of a singleton from an unordered pair. (Contributed by NM, 16-Mar-2006.) (Proof shortened by Andrew Salmon, 29-Jun-2011.)
|- ( { A , B } \ { A } ) C_ { B }
Theorem	difprsn1 3806	Removal of a singleton from an unordered pair. (Contributed by Thierry Arnoux, 4-Feb-2017.)
|- ( A =/= B -> ( { A , B } \ { A } ) = { B } )
Theorem	difprsn2 3807	Removal of a singleton from an unordered pair. (Contributed by Alexander van der Vekens, 5-Oct-2017.)
|- ( A =/= B -> ( { A , B } \ { B } ) = { A } )
Theorem	diftpsn3 3808	Removal of a singleton from an unordered triple. (Contributed by Alexander van der Vekens, 5-Oct-2017.)
|- ( ( A =/= C /\ B =/= C ) -> ( { A , B , C } \ { C } ) = { A , B } )
Theorem	difpr 3809	Removing two elements as pair of elements corresponds to removing each of the two elements as singletons. (Contributed by Alexander van der Vekens, 13-Jul-2018.)
|- ( A \ { B , C } ) = ( ( A \ { B } ) \ { C } )
Theorem	difsnb 3810	( B \ { A } ) equals B if and only if A is not a member of B. Generalization of difsn 3804. (Contributed by David Moews, 1-May-2017.)
|- ( -. A e. B <-> ( B \ { A } ) = B )
Theorem	snssi 3811	The singleton of an element of a class is a subset of the class. (Contributed by NM, 6-Jun-1994.)
|- ( A e. B -> { A } C_ B )
Theorem	snssd 3812	The singleton of an element of a class is a subset of the class (deduction form). (Contributed by Jonathan Ben-Naim, 3-Jun-2011.)
|- ( ph -> A e. B )   =>    |- ( ph -> { A } C_ B )
Theorem	difsnss 3813	If we remove a single element from a class then put it back in, we end up with a subset of the original class. If equality is decidable, we can replace subset with equality as seen in nndifsnid 6651. (Contributed by Jim Kingdon, 10-Aug-2018.)
|- ( B e. A -> ( ( A \ { B } ) u. { B } ) C_ A )
Theorem	pw0 3814	Compute the power set of the empty set. Theorem 89 of [Suppes] p. 47. (Contributed by NM, 5-Aug-1993.) (Proof shortened by Andrew Salmon, 29-Jun-2011.)
|- ~P (/) = { (/) }
Theorem	snsspr1 3815	A singleton is a subset of an unordered pair containing its member. (Contributed by NM, 27-Aug-2004.)
|- { A } C_ { A , B }
Theorem	snsspr2 3816	A singleton is a subset of an unordered pair containing its member. (Contributed by NM, 2-May-2009.)
|- { B } C_ { A , B }
Theorem	snsstp1 3817	A singleton is a subset of an unordered triple containing its member. (Contributed by NM, 9-Oct-2013.)
|- { A } C_ { A , B , C }
Theorem	snsstp2 3818	A singleton is a subset of an unordered triple containing its member. (Contributed by NM, 9-Oct-2013.)
|- { B } C_ { A , B , C }
Theorem	snsstp3 3819	A singleton is a subset of an unordered triple containing its member. (Contributed by NM, 9-Oct-2013.)
|- { C } C_ { A , B , C }
Theorem	prsstp12 3820	A pair is a subset of an unordered triple containing its members. (Contributed by Jim Kingdon, 11-Aug-2018.)
|- { A , B } C_ { A , B , C }
Theorem	prsstp13 3821	A pair is a subset of an unordered triple containing its members. (Contributed by Jim Kingdon, 11-Aug-2018.)
|- { A , C } C_ { A , B , C }
Theorem	prsstp23 3822	A pair is a subset of an unordered triple containing its members. (Contributed by Jim Kingdon, 11-Aug-2018.)
|- { B , C } C_ { A , B , C }
Theorem	prss 3823	A pair of elements of a class is a subset of the class. Theorem 7.5 of [Quine] p. 49. (Contributed by NM, 30-May-1994.) (Proof shortened by Andrew Salmon, 29-Jun-2011.)
|- A e. _V   &    |- B e. _V   =>    |- ( ( A e. C /\ B e. C ) <-> { A , B } C_ C )
Theorem	prssg 3824	A pair of elements of a class is a subset of the class. Theorem 7.5 of [Quine] p. 49. (Contributed by NM, 22-Mar-2006.) (Proof shortened by Andrew Salmon, 29-Jun-2011.)
|- ( ( A e. V /\ B e. W ) -> ( ( A e. C /\ B e. C ) <-> { A , B } C_ C ) )
Theorem	prssi 3825	A pair of elements of a class is a subset of the class. (Contributed by NM, 16-Jan-2015.)
|- ( ( A e. C /\ B e. C ) -> { A , B } C_ C )
Theorem	prssd 3826	Deduction version of prssi 3825: A pair of elements of a class is a subset of the class. (Contributed by Glauco Siliprandi, 17-Aug-2020.)
|- ( ph -> A e. C )   &    |- ( ph -> B e. C )   =>    |- ( ph -> { A , B } C_ C )
Theorem	prsspwg 3827	An unordered pair belongs to the power class of a class iff each member belongs to the class. (Contributed by Thierry Arnoux, 3-Oct-2016.) (Revised by NM, 18-Jan-2018.)
|- ( ( A e. V /\ B e. W ) -> ( { A , B } C_ ~P C <-> ( A C_ C /\ B C_ C ) ) )
Theorem	ssprss 3828	A pair as subset of a pair. (Contributed by AV, 26-Oct-2020.)
|- ( ( A e. V /\ B e. W ) -> ( { A , B } C_ { C , D } <-> ( ( A = C \/ A = D ) /\ ( B = C \/ B = D ) ) ) )
Theorem	ssprsseq 3829	A proper pair is a subset of a pair iff it is equal to the superset. (Contributed by AV, 26-Oct-2020.)
|- ( ( A e. V /\ B e. W /\ A =/= B ) -> ( { A , B } C_ { C , D } <-> { A , B } = { C , D } ) )
Theorem	sssnr 3830	Empty set and the singleton itself are subsets of a singleton. Concerning the converse, see exmidsssn 4285. (Contributed by Jim Kingdon, 10-Aug-2018.)
|- ( ( A = (/) \/ A = { B } ) -> A C_ { B } )
Theorem	sssnm 3831*	The inhabited subset of a singleton. (Contributed by Jim Kingdon, 10-Aug-2018.)
|- ( E. x x e. A -> ( A C_ { B } <-> A = { B } ) )
Theorem	eqsnm 3832*	Two ways to express that an inhabited set equals a singleton. (Contributed by Jim Kingdon, 11-Aug-2018.)
|- ( E. x x e. A -> ( A = { B } <-> A. x e. A x = B ) )
Theorem	ssprr 3833	The subsets of a pair. (Contributed by Jim Kingdon, 11-Aug-2018.)
|- ( ( ( A = (/) \/ A = { B } ) \/ ( A = { C } \/ A = { B , C } ) ) -> A C_ { B , C } )
Theorem	sstpr 3834	The subsets of a triple. (Contributed by Jim Kingdon, 11-Aug-2018.)
|- ( ( ( ( A = (/) \/ A = { B } ) \/ ( A = { C } \/ A = { B , C } ) ) \/ ( ( A = { D } \/ A = { B , D } ) \/ ( A = { C , D } \/ A = { B , C , D } ) ) ) -> A C_ { B , C , D } )
Theorem	tpss 3835	A triplet of elements of a class is a subset of the class. (Contributed by NM, 9-Apr-1994.) (Proof shortened by Andrew Salmon, 29-Jun-2011.)
|- A e. _V   &    |- B e. _V   &    |- C e. _V   =>    |- ( ( A e. D /\ B e. D /\ C e. D ) <-> { A , B , C } C_ D )
Theorem	tpssi 3836	A triple of elements of a class is a subset of the class. (Contributed by Alexander van der Vekens, 1-Feb-2018.)
|- ( ( A e. D /\ B e. D /\ C e. D ) -> { A , B , C } C_ D )
Theorem	sneqr 3837	If the singletons of two sets are equal, the two sets are equal. Part of Exercise 4 of [TakeutiZaring] p. 15. (Contributed by NM, 27-Aug-1993.)
|- A e. _V   =>    |- ( { A } = { B } -> A = B )
Theorem	snsssn 3838	If a singleton is a subset of another, their members are equal. (Contributed by NM, 28-May-2006.)
|- A e. _V   =>    |- ( { A } C_ { B } -> A = B )
Theorem	sneqrg 3839	Closed form of sneqr 3837. (Contributed by Scott Fenton, 1-Apr-2011.)
|- ( A e. V -> ( { A } = { B } -> A = B ) )
Theorem	sneqbg 3840	Two singletons of sets are equal iff their elements are equal. (Contributed by Scott Fenton, 16-Apr-2012.)
|- ( A e. V -> ( { A } = { B } <-> A = B ) )
Theorem	snsspw 3841	The singleton of a class is a subset of its power class. (Contributed by NM, 5-Aug-1993.)
|- { A } C_ ~P A
Theorem	prsspw 3842	An unordered pair belongs to the power class of a class iff each member belongs to the class. (Contributed by NM, 10-Dec-2003.) (Proof shortened by Andrew Salmon, 26-Jun-2011.)
|- A e. _V   &    |- B e. _V   =>    |- ( { A , B } C_ ~P C <-> ( A C_ C /\ B C_ C ) )
Theorem	preqr1g 3843	Reverse equality lemma for unordered pairs. If two unordered pairs have the same second element, the first elements are equal. Closed form of preqr1 3845. (Contributed by Jim Kingdon, 21-Sep-2018.)
|- ( ( A e. _V /\ B e. _V ) -> ( { A , C } = { B , C } -> A = B ) )
Theorem	preqr2g 3844	Reverse equality lemma for unordered pairs. If two unordered pairs have the same second element, the second elements are equal. Closed form of preqr2 3846. (Contributed by Jim Kingdon, 21-Sep-2018.)
|- ( ( A e. _V /\ B e. _V ) -> ( { C , A } = { C , B } -> A = B ) )
Theorem	preqr1 3845	Reverse equality lemma for unordered pairs. If two unordered pairs have the same second element, the first elements are equal. (Contributed by NM, 18-Oct-1995.)
|- A e. _V   &    |- B e. _V   =>    |- ( { A , C } = { B , C } -> A = B )
Theorem	preqr2 3846	Reverse equality lemma for unordered pairs. If two unordered pairs have the same first element, the second elements are equal. (Contributed by NM, 5-Aug-1993.)
|- A e. _V   &    |- B e. _V   =>    |- ( { C , A } = { C , B } -> A = B )
Theorem	preq12b 3847	Equality relationship for two unordered pairs. (Contributed by NM, 17-Oct-1996.)
|- A e. _V   &    |- B e. _V   &    |- C e. _V   &    |- D e. _V   =>    |- ( { A , B } = { C , D } <-> ( ( A = C /\ B = D ) \/ ( A = D /\ B = C ) ) )
Theorem	prel12 3848	Equality of two unordered pairs. (Contributed by NM, 17-Oct-1996.)
|- A e. _V   &    |- B e. _V   &    |- C e. _V   &    |- D e. _V   =>    |- ( -. A = B -> ( { A , B } = { C , D } <-> ( A e. { C , D } /\ B e. { C , D } ) ) )
Theorem	opthpr 3849	A way to represent ordered pairs using unordered pairs with distinct members. (Contributed by NM, 27-Mar-2007.)
|- A e. _V   &    |- B e. _V   &    |- C e. _V   &    |- D e. _V   =>    |- ( A =/= D -> ( { A , B } = { C , D } <-> ( A = C /\ B = D ) ) )
Theorem	preq12bg 3850	Closed form of preq12b 3847. (Contributed by Scott Fenton, 28-Mar-2014.)
|- ( ( ( A e. V /\ B e. W ) /\ ( C e. X /\ D e. Y ) ) -> ( { A , B } = { C , D } <-> ( ( A = C /\ B = D ) \/ ( A = D /\ B = C ) ) ) )
Theorem	prneimg 3851	Two pairs are not equal if at least one element of the first pair is not contained in the second pair. (Contributed by Alexander van der Vekens, 13-Aug-2017.)
|- ( ( ( A e. U /\ B e. V ) /\ ( C e. X /\ D e. Y ) ) -> ( ( ( A =/= C /\ A =/= D ) \/ ( B =/= C /\ B =/= D ) ) -> { A , B } =/= { C , D } ) )
Theorem	preqsn 3852	Equivalence for a pair equal to a singleton. (Contributed by NM, 3-Jun-2008.)
|- A e. _V   &    |- B e. _V   &    |- C e. _V   =>    |- ( { A , B } = { C } <-> ( A = B /\ B = C ) )
Theorem	elpr2elpr 3853*	For an element A of an unordered pair which is a subset of a given set V, there is another (maybe the same) element b of the given set V being an element of the unordered pair. (Contributed by AV, 5-Dec-2020.)
|- ( ( X e. V /\ Y e. V /\ A e. { X , Y } ) -> E. b e. V { X , Y } = { A , b } )
Theorem	dfopg 3854	Value of the ordered pair when the arguments are sets. (Contributed by Mario Carneiro, 26-Apr-2015.)
|- ( ( A e. V /\ B e. W ) -> <. A , B >. = { { A } , { A , B } } )
Theorem	dfop 3855	Value of an ordered pair when the arguments are sets, with the conclusion corresponding to Kuratowski's original definition. (Contributed by NM, 25-Jun-1998.)
|- A e. _V   &    |- B e. _V   =>    |- <. A , B >. = { { A } , { A , B } }
Theorem	opeq1 3856	Equality theorem for ordered pairs. (Contributed by NM, 25-Jun-1998.) (Revised by Mario Carneiro, 26-Apr-2015.)
|- ( A = B -> <. A , C >. = <. B , C >. )
Theorem	opeq2 3857	Equality theorem for ordered pairs. (Contributed by NM, 25-Jun-1998.) (Revised by Mario Carneiro, 26-Apr-2015.)
|- ( A = B -> <. C , A >. = <. C , B >. )
Theorem	opeq12 3858	Equality theorem for ordered pairs. (Contributed by NM, 28-May-1995.)
|- ( ( A = C /\ B = D ) -> <. A , B >. = <. C , D >. )
Theorem	opeq1i 3859	Equality inference for ordered pairs. (Contributed by NM, 16-Dec-2006.)
|- A = B   =>    |- <. A , C >. = <. B , C >.
Theorem	opeq2i 3860	Equality inference for ordered pairs. (Contributed by NM, 16-Dec-2006.)
|- A = B   =>    |- <. C , A >. = <. C , B >.
Theorem	opeq12i 3861	Equality inference for ordered pairs. (Contributed by NM, 16-Dec-2006.) (Proof shortened by Eric Schmidt, 4-Apr-2007.)
|- A = B   &    |- C = D   =>    |- <. A , C >. = <. B , D >.
Theorem	opeq1d 3862	Equality deduction for ordered pairs. (Contributed by NM, 16-Dec-2006.)
|- ( ph -> A = B )   =>    |- ( ph -> <. A , C >. = <. B , C >. )
Theorem	opeq2d 3863	Equality deduction for ordered pairs. (Contributed by NM, 16-Dec-2006.)
|- ( ph -> A = B )   =>    |- ( ph -> <. C , A >. = <. C , B >. )
Theorem	opeq12d 3864	Equality deduction for ordered pairs. (Contributed by NM, 16-Dec-2006.) (Proof shortened by Andrew Salmon, 29-Jun-2011.)
|- ( ph -> A = B )   &    |- ( ph -> C = D )   =>    |- ( ph -> <. A , C >. = <. B , D >. )
Theorem	oteq1 3865	Equality theorem for ordered triples. (Contributed by NM, 3-Apr-2015.)
|- ( A = B -> <. A , C , D >. = <. B , C , D >. )
Theorem	oteq2 3866	Equality theorem for ordered triples. (Contributed by NM, 3-Apr-2015.)
|- ( A = B -> <. C , A , D >. = <. C , B , D >. )
Theorem	oteq3 3867	Equality theorem for ordered triples. (Contributed by NM, 3-Apr-2015.)
|- ( A = B -> <. C , D , A >. = <. C , D , B >. )
Theorem	oteq1d 3868	Equality deduction for ordered triples. (Contributed by Mario Carneiro, 11-Jan-2017.)
|- ( ph -> A = B )   =>    |- ( ph -> <. A , C , D >. = <. B , C , D >. )
Theorem	oteq2d 3869	Equality deduction for ordered triples. (Contributed by Mario Carneiro, 11-Jan-2017.)
|- ( ph -> A = B )   =>    |- ( ph -> <. C , A , D >. = <. C , B , D >. )
Theorem	oteq3d 3870	Equality deduction for ordered triples. (Contributed by Mario Carneiro, 11-Jan-2017.)
|- ( ph -> A = B )   =>    |- ( ph -> <. C , D , A >. = <. C , D , B >. )
Theorem	oteq123d 3871	Equality deduction for ordered triples. (Contributed by Mario Carneiro, 11-Jan-2017.)
|- ( ph -> A = B )   &    |- ( ph -> C = D )   &    |- ( ph -> E = F )   =>    |- ( ph -> <. A , C , E >. = <. B , D , F >. )
Theorem	nfop 3872	Bound-variable hypothesis builder for ordered pairs. (Contributed by NM, 14-Nov-1995.)
|- F/_ x A   &    |- F/_ x B   =>    |- F/_ x <. A , B >.
Theorem	nfopd 3873	Deduction version of bound-variable hypothesis builder nfop 3872. This shows how the deduction version of a not-free theorem such as nfop 3872 can be created from the corresponding not-free inference theorem. (Contributed by NM, 4-Feb-2008.)
|- ( ph -> F/_ x A )   &    |- ( ph -> F/_ x B )   =>    |- ( ph -> F/_ x <. A , B >. )
Theorem	opid 3874	The ordered pair <. A , A >. in Kuratowski's representation. (Contributed by FL, 28-Dec-2011.)
|- A e. _V   =>    |- <. A , A >. = { { A } }
Theorem	ralunsn 3875*	Restricted quantification over the union of a set and a singleton, using implicit substitution. (Contributed by Paul Chapman, 17-Nov-2012.) (Revised by Mario Carneiro, 23-Apr-2015.)
|- ( x = B -> ( ph <-> ps ) )   =>    |- ( B e. C -> ( A. x e. ( A u. { B } ) ph <-> ( A. x e. A ph /\ ps ) ) )
Theorem	2ralunsn 3876*	Double restricted quantification over the union of a set and a singleton, using implicit substitution. (Contributed by Paul Chapman, 17-Nov-2012.)
|- ( x = B -> ( ph <-> ch ) )   &    |- ( y = B -> ( ph <-> ps ) )   &    |- ( x = B -> ( ps <-> th ) )   =>    |- ( B e. C -> ( A. x e. ( A u. { B } ) A. y e. ( A u. { B } ) ph <-> ( ( A. x e. A A. y e. A ph /\ A. x e. A ps ) /\ ( A. y e. A ch /\ th ) ) ) )
Theorem	opprc 3877	Expansion of an ordered pair when either member is a proper class. (Contributed by Mario Carneiro, 26-Apr-2015.)
|- ( -. ( A e. _V /\ B e. _V ) -> <. A , B >. = (/) )
Theorem	opprc1 3878	Expansion of an ordered pair when the first member is a proper class. See also opprc 3877. (Contributed by NM, 10-Apr-2004.) (Revised by Mario Carneiro, 26-Apr-2015.)
|- ( -. A e. _V -> <. A , B >. = (/) )
Theorem	opprc2 3879	Expansion of an ordered pair when the second member is a proper class. See also opprc 3877. (Contributed by NM, 15-Nov-1994.) (Revised by Mario Carneiro, 26-Apr-2015.)
|- ( -. B e. _V -> <. A , B >. = (/) )
Theorem	oprcl 3880	If an ordered pair has an element, then its arguments are sets. (Contributed by Mario Carneiro, 26-Apr-2015.)
|- ( C e. <. A , B >. -> ( A e. _V /\ B e. _V ) )
Theorem	pwsnss 3881	The power set of a singleton. (Contributed by Jim Kingdon, 12-Aug-2018.)
|- { (/) , { A } } C_ ~P { A }
Theorem	pwpw0ss 3882	Compute the power set of the power set of the empty set. (See pw0 3814 for the power set of the empty set.) Theorem 90 of [Suppes] p. 48 (but with subset in place of equality). (Contributed by Jim Kingdon, 12-Aug-2018.)
|- { (/) , { (/) } } C_ ~P { (/) }
Theorem	pwprss 3883	The power set of an unordered pair. (Contributed by Jim Kingdon, 13-Aug-2018.)
|- ( { (/) , { A } } u. { { B } , { A , B } } ) C_ ~P { A , B }
Theorem	pwtpss 3884	The power set of an unordered triple. (Contributed by Jim Kingdon, 13-Aug-2018.)
|- ( ( { (/) , { A } } u. { { B } , { A , B } } ) u. ( { { C } , { A , C } } u. { { B , C } , { A , B , C } } ) ) C_ ~P { A , B , C }
Theorem	pwpwpw0ss 3885	Compute the power set of the power set of the power set of the empty set. (See also pw0 3814 and pwpw0ss 3882.) (Contributed by Jim Kingdon, 13-Aug-2018.)
|- ( { (/) , { (/) } } u. { { { (/) } } , { (/) , { (/) } } } ) C_ ~P { (/) , { (/) } }
Theorem	pwv 3886	The power class of the universe is the universe. Exercise 4.12(d) of [Mendelson] p. 235. (Contributed by NM, 14-Sep-2003.)
|- ~P _V = _V
2.1.18  The union of a class
Syntax	cuni 3887	Extend class notation to include the union of a class. Read: "union (of) A".
class U. A
Definition	df-uni 3888*	Define the union of a class i.e. the collection of all members of the members of the class. Definition 5.5 of [TakeutiZaring] p. 16. For example, U. { { 1 , 3 } , { 1 , 8 } } = { 1 , 3 , 8 }. This is similar to the union of two classes df-un 3201. (Contributed by NM, 23-Aug-1993.)
|- U. A = { x | E. y ( x e. y /\ y e. A ) }
Theorem	dfuni2 3889*	Alternate definition of class union. (Contributed by NM, 28-Jun-1998.)
|- U. A = { x | E. y e. A x e. y }
Theorem	eluni 3890*	Membership in class union. (Contributed by NM, 22-May-1994.)
|- ( A e. U. B <-> E. x ( A e. x /\ x e. B ) )
Theorem	eluni2 3891*	Membership in class union. Restricted quantifier version. (Contributed by NM, 31-Aug-1999.)
|- ( A e. U. B <-> E. x e. B A e. x )
Theorem	elunii 3892	Membership in class union. (Contributed by NM, 24-Mar-1995.)
|- ( ( A e. B /\ B e. C ) -> A e. U. C )
Theorem	nfuni 3893	Bound-variable hypothesis builder for union. (Contributed by NM, 30-Dec-1996.) (Proof shortened by Andrew Salmon, 27-Aug-2011.)
|- F/_ x A   =>    |- F/_ x U. A
Theorem	nfunid 3894	Deduction version of nfuni 3893. (Contributed by NM, 18-Feb-2013.)
|- ( ph -> F/_ x A )   =>    |- ( ph -> F/_ x U. A )
Theorem	csbunig 3895	Distribute proper substitution through the union of a class. (Contributed by Alan Sare, 10-Nov-2012.)
|- ( A e. V -> [_ A / x ]_ U. B = U. [_ A / x ]_ B )
Theorem	unieq 3896	Equality theorem for class union. Exercise 15 of [TakeutiZaring] p. 18. (Contributed by NM, 10-Aug-1993.) (Proof shortened by Andrew Salmon, 29-Jun-2011.)
|- ( A = B -> U. A = U. B )
Theorem	unieqi 3897	Inference of equality of two class unions. (Contributed by NM, 30-Aug-1993.)
|- A = B   =>    |- U. A = U. B
Theorem	unieqd 3898	Deduction of equality of two class unions. (Contributed by NM, 21-Apr-1995.)
|- ( ph -> A = B )   =>    |- ( ph -> U. A = U. B )
Theorem	eluniab 3899*	Membership in union of a class abstraction. (Contributed by NM, 11-Aug-1994.) (Revised by Mario Carneiro, 14-Nov-2016.)
|- ( A e. U. { x | ph } <-> E. x ( A e. x /\ ph ) )
Theorem	elunirab 3900*	Membership in union of a class abstraction. (Contributed by NM, 4-Oct-2006.)
|- ( A e. U. { x e. B | ph } <-> E. x e. B ( A e. x /\ ph ) )