P. 50 of Theorem List - Intuitionistic Logic Explorer

Theorem List for Intuitionistic Logic Explorer - 4901-5000   *Has distinct variable group(s)

Type	Label	Description
Statement
Theorem	relint 4901*	The intersection of a class is a relation if at least one member is a relation. (Contributed by NM, 8-Mar-2014.)
|- ( E. x e. A Rel x -> Rel |^| A )
Theorem	rel0 4902	The empty set is a relation. (Contributed by NM, 26-Apr-1998.)
|- Rel (/)
Theorem	relopabiv 4903*	A class of ordered pairs is a relation. For a version without a disjoint variable condition, see relopabi 4905. (Contributed by BJ, 22-Jul-2023.)
|- A = { <. x , y >. | ph }   =>    |- Rel A
Theorem	relopabv 4904*	A class of ordered pairs is a relation. For a version without a disjoint variable condition, see relopab 4906. (Contributed by SN, 8-Sep-2024.)
|- Rel { <. x , y >. | ph }
Theorem	relopabi 4905	A class of ordered pairs is a relation. (Contributed by Mario Carneiro, 21-Dec-2013.)
|- A = { <. x , y >. | ph }   =>    |- Rel A
Theorem	relopab 4906	A class of ordered pairs is a relation. (Contributed by NM, 8-Mar-1995.) (Unnecessary distinct variable restrictions were removed by Alan Sare, 9-Jul-2013.) (Proof shortened by Mario Carneiro, 21-Dec-2013.)
|- Rel { <. x , y >. | ph }
Theorem	brabv 4907	If two classes are in a relationship given by an ordered-pair class abstraction, the classes are sets. (Contributed by Alexander van der Vekens, 5-Nov-2017.)
|- ( X { <. x , y >. | ph } Y -> ( X e. _V /\ Y e. _V ) )
Theorem	mptrel 4908	The maps-to notation always describes a relationship. (Contributed by Scott Fenton, 16-Apr-2012.)
|- Rel ( x e. A |-> B )
Theorem	reli 4909	The identity relation is a relation. Part of Exercise 4.12(p) of [Mendelson] p. 235. (Contributed by NM, 26-Apr-1998.) (Revised by Mario Carneiro, 21-Dec-2013.)
|- Rel _I
Theorem	rele 4910	The membership relation is a relation. (Contributed by NM, 26-Apr-1998.) (Revised by Mario Carneiro, 21-Dec-2013.)
|- Rel _E
Theorem	opabid2 4911*	A relation expressed as an ordered pair abstraction. (Contributed by NM, 11-Dec-2006.)
|- ( Rel A -> { <. x , y >. | <. x , y >. e. A } = A )
Theorem	inopab 4912*	Intersection of two ordered pair class abstractions. (Contributed by NM, 30-Sep-2002.)
|- ( { <. x , y >. | ph } i^i { <. x , y >. | ps } ) = { <. x , y >. | ( ph /\ ps ) }
Theorem	difopab 4913*	The difference of two ordered-pair abstractions. (Contributed by Stefan O'Rear, 17-Jan-2015.)
|- ( { <. x , y >. | ph } \ { <. x , y >. | ps } ) = { <. x , y >. | ( ph /\ -. ps ) }
Theorem	inxp 4914	The intersection of two cross products. Exercise 9 of [TakeutiZaring] p. 25. (Contributed by NM, 3-Aug-1994.) (Proof shortened by Andrew Salmon, 27-Aug-2011.)
|- ( ( A X. B ) i^i ( C X. D ) ) = ( ( A i^i C ) X. ( B i^i D ) )
Theorem	xpindi 4915	Distributive law for cross product over intersection. Theorem 102 of [Suppes] p. 52. (Contributed by NM, 26-Sep-2004.)
|- ( A X. ( B i^i C ) ) = ( ( A X. B ) i^i ( A X. C ) )
Theorem	xpindir 4916	Distributive law for cross product over intersection. Similar to Theorem 102 of [Suppes] p. 52. (Contributed by NM, 26-Sep-2004.)
|- ( ( A i^i B ) X. C ) = ( ( A X. C ) i^i ( B X. C ) )
Theorem	xpiindim 4917*	Distributive law for cross product over indexed intersection. (Contributed by Jim Kingdon, 7-Dec-2018.)
|- ( E. y y e. A -> ( C X. |^|_ x e. A B ) = |^|_ x e. A ( C X. B ) )
Theorem	xpriindim 4918*	Distributive law for cross product over relativized indexed intersection. (Contributed by Jim Kingdon, 7-Dec-2018.)
|- ( E. y y e. A -> ( C X. ( D i^i |^|_ x e. A B ) ) = ( ( C X. D ) i^i |^|_ x e. A ( C X. B ) ) )
Theorem	eliunxp 4919*	Membership in a union of cross products. Analogue of elxp 4791 for nonconstant B ( x ). (Contributed by Mario Carneiro, 29-Dec-2014.)
|- ( C e. U_ x e. A ( { x } X. B ) <-> E. x E. y ( C = <. x , y >. /\ ( x e. A /\ y e. B ) ) )
Theorem	opeliunxp2 4920*	Membership in a union of cross products. (Contributed by Mario Carneiro, 14-Feb-2015.)
|- ( x = C -> B = E )   =>    |- ( <. C , D >. e. U_ x e. A ( { x } X. B ) <-> ( C e. A /\ D e. E ) )
Theorem	raliunxp 4921*	Write a double restricted quantification as one universal quantifier. In this version of ralxp 4923, B ( y ) is not assumed to be constant. (Contributed by Mario Carneiro, 29-Dec-2014.)
|- ( x = <. y , z >. -> ( ph <-> ps ) )   =>    |- ( A. x e. U_ y e. A ( { y } X. B ) ph <-> A. y e. A A. z e. B ps )
Theorem	rexiunxp 4922*	Write a double restricted quantification as one universal quantifier. In this version of rexxp 4924, B ( y ) is not assumed to be constant. (Contributed by Mario Carneiro, 14-Feb-2015.)
|- ( x = <. y , z >. -> ( ph <-> ps ) )   =>    |- ( E. x e. U_ y e. A ( { y } X. B ) ph <-> E. y e. A E. z e. B ps )
Theorem	ralxp 4923*	Universal quantification restricted to a cross product is equivalent to a double restricted quantification. The hypothesis specifies an implicit substitution. (Contributed by NM, 7-Feb-2004.) (Revised by Mario Carneiro, 29-Dec-2014.)
|- ( x = <. y , z >. -> ( ph <-> ps ) )   =>    |- ( A. x e. ( A X. B ) ph <-> A. y e. A A. z e. B ps )
Theorem	rexxp 4924*	Existential quantification restricted to a cross product is equivalent to a double restricted quantification. (Contributed by NM, 11-Nov-1995.) (Revised by Mario Carneiro, 14-Feb-2015.)
|- ( x = <. y , z >. -> ( ph <-> ps ) )   =>    |- ( E. x e. ( A X. B ) ph <-> E. y e. A E. z e. B ps )
Theorem	djussxp 4925*	Disjoint union is a subset of a cross product. (Contributed by Stefan O'Rear, 21-Nov-2014.)
|- U_ x e. A ( { x } X. B ) C_ ( A X. _V )
Theorem	ralxpf 4926*	Version of ralxp 4923 with bound-variable hypotheses. (Contributed by NM, 18-Aug-2006.) (Revised by Mario Carneiro, 15-Oct-2016.)
|- F/ y ph   &    |- F/ z ph   &    |- F/ x ps   &    |- ( x = <. y , z >. -> ( ph <-> ps ) )   =>    |- ( A. x e. ( A X. B ) ph <-> A. y e. A A. z e. B ps )
Theorem	rexxpf 4927*	Version of rexxp 4924 with bound-variable hypotheses. (Contributed by NM, 19-Dec-2008.) (Revised by Mario Carneiro, 15-Oct-2016.)
|- F/ y ph   &    |- F/ z ph   &    |- F/ x ps   &    |- ( x = <. y , z >. -> ( ph <-> ps ) )   =>    |- ( E. x e. ( A X. B ) ph <-> E. y e. A E. z e. B ps )
Theorem	iunxpf 4928*	Indexed union on a cross product is equals a double indexed union. The hypothesis specifies an implicit substitution. (Contributed by NM, 19-Dec-2008.)
|- F/_ y C   &    |- F/_ z C   &    |- F/_ x D   &    |- ( x = <. y , z >. -> C = D )   =>    |- U_ x e. ( A X. B ) C = U_ y e. A U_ z e. B D
Theorem	opabbi2dv 4929*	Deduce equality of a relation and an ordered-pair class builder. Compare abbi2dv 2359. (Contributed by NM, 24-Feb-2014.)
|- Rel A   &    |- ( ph -> ( <. x , y >. e. A <-> ps ) )   =>    |- ( ph -> A = { <. x , y >. | ps } )
Theorem	relop 4930*	A necessary and sufficient condition for a Kuratowski ordered pair to be a relation. (Contributed by NM, 3-Jun-2008.) (Avoid depending on this detail.)
|- A e. _V   &    |- B e. _V   =>    |- ( Rel <. A , B >. <-> E. x E. y ( A = { x } /\ B = { x , y } ) )
Theorem	ideqg 4931	For sets, the identity relation is the same as equality. (Contributed by NM, 30-Apr-2004.) (Proof shortened by Andrew Salmon, 27-Aug-2011.)
|- ( B e. V -> ( A _I B <-> A = B ) )
Theorem	ideq 4932	For sets, the identity relation is the same as equality. (Contributed by NM, 13-Aug-1995.)
|- B e. _V   =>    |- ( A _I B <-> A = B )
Theorem	ididg 4933	A set is identical to itself. (Contributed by NM, 28-May-2008.) (Proof shortened by Andrew Salmon, 27-Aug-2011.)
|- ( A e. V -> A _I A )
Theorem	issetid 4934	Two ways of expressing set existence. (Contributed by NM, 16-Feb-2008.) (Proof shortened by Andrew Salmon, 27-Aug-2011.) (Revised by Mario Carneiro, 26-Apr-2015.)
|- ( A e. _V <-> A _I A )
Theorem	coss1 4935	Subclass theorem for composition. (Contributed by FL, 30-Dec-2010.)
|- ( A C_ B -> ( A o. C ) C_ ( B o. C ) )
Theorem	coss2 4936	Subclass theorem for composition. (Contributed by NM, 5-Apr-2013.)
|- ( A C_ B -> ( C o. A ) C_ ( C o. B ) )
Theorem	coeq1 4937	Equality theorem for composition of two classes. (Contributed by NM, 3-Jan-1997.)
|- ( A = B -> ( A o. C ) = ( B o. C ) )
Theorem	coeq2 4938	Equality theorem for composition of two classes. (Contributed by NM, 3-Jan-1997.)
|- ( A = B -> ( C o. A ) = ( C o. B ) )
Theorem	coeq1i 4939	Equality inference for composition of two classes. (Contributed by NM, 16-Nov-2000.)
|- A = B   =>    |- ( A o. C ) = ( B o. C )
Theorem	coeq2i 4940	Equality inference for composition of two classes. (Contributed by NM, 16-Nov-2000.)
|- A = B   =>    |- ( C o. A ) = ( C o. B )
Theorem	coeq1d 4941	Equality deduction for composition of two classes. (Contributed by NM, 16-Nov-2000.)
|- ( ph -> A = B )   =>    |- ( ph -> ( A o. C ) = ( B o. C ) )
Theorem	coeq2d 4942	Equality deduction for composition of two classes. (Contributed by NM, 16-Nov-2000.)
|- ( ph -> A = B )   =>    |- ( ph -> ( C o. A ) = ( C o. B ) )
Theorem	coeq12i 4943	Equality inference for composition of two classes. (Contributed by FL, 7-Jun-2012.)
|- A = B   &    |- C = D   =>    |- ( A o. C ) = ( B o. D )
Theorem	coeq12d 4944	Equality deduction for composition of two classes. (Contributed by FL, 7-Jun-2012.)
|- ( ph -> A = B )   &    |- ( ph -> C = D )   =>    |- ( ph -> ( A o. C ) = ( B o. D ) )
Theorem	nfco 4945	Bound-variable hypothesis builder for function value. (Contributed by NM, 1-Sep-1999.)
|- F/_ x A   &    |- F/_ x B   =>    |- F/_ x ( A o. B )
Theorem	elco 4946*	Elements of a composed relation. (Contributed by BJ, 10-Jul-2022.)
|- ( A e. ( R o. S ) <-> E. x E. y E. z ( A = <. x , z >. /\ ( x S y /\ y R z ) ) )
Theorem	brcog 4947*	Ordered pair membership in a composition. (Contributed by NM, 24-Feb-2015.)
|- ( ( A e. V /\ B e. W ) -> ( A ( C o. D ) B <-> E. x ( A D x /\ x C B ) ) )
Theorem	opelco2g 4948*	Ordered pair membership in a composition. (Contributed by NM, 27-Jan-1997.) (Revised by Mario Carneiro, 24-Feb-2015.)
|- ( ( A e. V /\ B e. W ) -> ( <. A , B >. e. ( C o. D ) <-> E. x ( <. A , x >. e. D /\ <. x , B >. e. C ) ) )
Theorem	brcogw 4949	Ordered pair membership in a composition. (Contributed by Thierry Arnoux, 14-Jan-2018.)
|- ( ( ( A e. V /\ B e. W /\ X e. Z ) /\ ( A D X /\ X C B ) ) -> A ( C o. D ) B )
Theorem	eqbrrdva 4950*	Deduction from extensionality principle for relations, given an equivalence only on the relation's domain and range. (Contributed by Thierry Arnoux, 28-Nov-2017.)
|- ( ph -> A C_ ( C X. D ) )   &    |- ( ph -> B C_ ( C X. D ) )   &    |- ( ( ph /\ x e. C /\ y e. D ) -> ( x A y <-> x B y ) )   =>    |- ( ph -> A = B )
Theorem	brco 4951*	Binary relation on a composition. (Contributed by NM, 21-Sep-2004.) (Revised by Mario Carneiro, 24-Feb-2015.)
|- A e. _V   &    |- B e. _V   =>    |- ( A ( C o. D ) B <-> E. x ( A D x /\ x C B ) )
Theorem	opelco 4952*	Ordered pair membership in a composition. (Contributed by NM, 27-Dec-1996.) (Revised by Mario Carneiro, 24-Feb-2015.)
|- A e. _V   &    |- B e. _V   =>    |- ( <. A , B >. e. ( C o. D ) <-> E. x ( A D x /\ x C B ) )
Theorem	cnvss 4953	Subset theorem for converse. (Contributed by NM, 22-Mar-1998.)
|- ( A C_ B -> `' A C_ `' B )
Theorem	cnveq 4954	Equality theorem for converse. (Contributed by NM, 13-Aug-1995.)
|- ( A = B -> `' A = `' B )
Theorem	cnveqi 4955	Equality inference for converse. (Contributed by NM, 23-Dec-2008.)
|- A = B   =>    |- `' A = `' B
Theorem	cnveqd 4956	Equality deduction for converse. (Contributed by NM, 6-Dec-2013.)
|- ( ph -> A = B )   =>    |- ( ph -> `' A = `' B )
Theorem	elcnv 4957*	Membership in a converse. Equation 5 of [Suppes] p. 62. (Contributed by NM, 24-Mar-1998.)
|- ( A e. `' R <-> E. x E. y ( A = <. x , y >. /\ y R x ) )
Theorem	elcnv2 4958*	Membership in a converse. Equation 5 of [Suppes] p. 62. (Contributed by NM, 11-Aug-2004.)
|- ( A e. `' R <-> E. x E. y ( A = <. x , y >. /\ <. y , x >. e. R ) )
Theorem	nfcnv 4959	Bound-variable hypothesis builder for converse. (Contributed by NM, 31-Jan-2004.) (Revised by Mario Carneiro, 15-Oct-2016.)
|- F/_ x A   =>    |- F/_ x `' A
Theorem	opelcnvg 4960	Ordered-pair membership in converse. (Contributed by NM, 13-May-1999.) (Proof shortened by Andrew Salmon, 27-Aug-2011.)
|- ( ( A e. C /\ B e. D ) -> ( <. A , B >. e. `' R <-> <. B , A >. e. R ) )
Theorem	brcnvg 4961	The converse of a binary relation swaps arguments. Theorem 11 of [Suppes] p. 61. (Contributed by NM, 10-Oct-2005.)
|- ( ( A e. C /\ B e. D ) -> ( A `' R B <-> B R A ) )
Theorem	opelcnv 4962	Ordered-pair membership in converse. (Contributed by NM, 13-Aug-1995.)
|- A e. _V   &    |- B e. _V   =>    |- ( <. A , B >. e. `' R <-> <. B , A >. e. R )
Theorem	brcnv 4963	The converse of a binary relation swaps arguments. Theorem 11 of [Suppes] p. 61. (Contributed by NM, 13-Aug-1995.)
|- A e. _V   &    |- B e. _V   =>    |- ( A `' R B <-> B R A )
Theorem	csbcnvg 4964	Move class substitution in and out of the converse of a function. (Contributed by Thierry Arnoux, 8-Feb-2017.)
|- ( A e. V -> `' [_ A / x ]_ F = [_ A / x ]_ `' F )
Theorem	cnvco 4965	Distributive law of converse over class composition. Theorem 26 of [Suppes] p. 64. (Contributed by NM, 19-Mar-1998.) (Proof shortened by Andrew Salmon, 27-Aug-2011.)
|- `' ( A o. B ) = ( `' B o. `' A )
Theorem	cnvuni 4966*	The converse of a class union is the (indexed) union of the converses of its members. (Contributed by NM, 11-Aug-2004.)
|- `' U. A = U_ x e. A `' x
Theorem	dfdm3 4967*	Alternate definition of domain. Definition 6.5(1) of [TakeutiZaring] p. 24. (Contributed by NM, 28-Dec-1996.)
|- dom A = { x | E. y <. x , y >. e. A }
Theorem	dfrn2 4968*	Alternate definition of range. Definition 4 of [Suppes] p. 60. (Contributed by NM, 27-Dec-1996.)
|- ran A = { y | E. x x A y }
Theorem	dfrn3 4969*	Alternate definition of range. Definition 6.5(2) of [TakeutiZaring] p. 24. (Contributed by NM, 28-Dec-1996.)
|- ran A = { y | E. x <. x , y >. e. A }
Theorem	elrn2g 4970*	Membership in a range. (Contributed by Scott Fenton, 2-Feb-2011.)
|- ( A e. V -> ( A e. ran B <-> E. x <. x , A >. e. B ) )
Theorem	elrng 4971*	Membership in a range. (Contributed by Scott Fenton, 2-Feb-2011.)
|- ( A e. V -> ( A e. ran B <-> E. x x B A ) )
Theorem	ssrelrn 4972*	If a relation is a subset of a cartesian product, then for each element of the range of the relation there is an element of the first set of the cartesian product which is related to the element of the range by the relation. (Contributed by AV, 24-Oct-2020.)
|- ( ( R C_ ( A X. B ) /\ Y e. ran R ) -> E. a e. A a R Y )
Theorem	dfdm4 4973	Alternate definition of domain. (Contributed by NM, 28-Dec-1996.)
|- dom A = ran `' A
Theorem	dfdmf 4974*	Definition of domain, using bound-variable hypotheses instead of distinct variable conditions. (Contributed by NM, 8-Mar-1995.) (Revised by Mario Carneiro, 15-Oct-2016.)
|- F/_ x A   &    |- F/_ y A   =>    |- dom A = { x | E. y x A y }
Theorem	csbdmg 4975	Distribute proper substitution through the domain of a class. (Contributed by Jim Kingdon, 8-Dec-2018.)
|- ( A e. V -> [_ A / x ]_ dom B = dom [_ A / x ]_ B )
Theorem	eldmg 4976*	Domain membership. Theorem 4 of [Suppes] p. 59. (Contributed by Mario Carneiro, 9-Jul-2014.)
|- ( A e. V -> ( A e. dom B <-> E. y A B y ) )
Theorem	eldm2g 4977*	Domain membership. Theorem 4 of [Suppes] p. 59. (Contributed by NM, 27-Jan-1997.) (Revised by Mario Carneiro, 9-Jul-2014.)
|- ( A e. V -> ( A e. dom B <-> E. y <. A , y >. e. B ) )
Theorem	eldm 4978*	Membership in a domain. Theorem 4 of [Suppes] p. 59. (Contributed by NM, 2-Apr-2004.)
|- A e. _V   =>    |- ( A e. dom B <-> E. y A B y )
Theorem	eldm2 4979*	Membership in a domain. Theorem 4 of [Suppes] p. 59. (Contributed by NM, 1-Aug-1994.)
|- A e. _V   =>    |- ( A e. dom B <-> E. y <. A , y >. e. B )
Theorem	dmss 4980	Subset theorem for domain. (Contributed by NM, 11-Aug-1994.)
|- ( A C_ B -> dom A C_ dom B )
Theorem	dmeq 4981	Equality theorem for domain. (Contributed by NM, 11-Aug-1994.)
|- ( A = B -> dom A = dom B )
Theorem	dmeqi 4982	Equality inference for domain. (Contributed by NM, 4-Mar-2004.)
|- A = B   =>    |- dom A = dom B
Theorem	dmeqd 4983	Equality deduction for domain. (Contributed by NM, 4-Mar-2004.)
|- ( ph -> A = B )   =>    |- ( ph -> dom A = dom B )
Theorem	opeldm 4984	Membership of first of an ordered pair in a domain. (Contributed by NM, 30-Jul-1995.)
|- A e. _V   &    |- B e. _V   =>    |- ( <. A , B >. e. C -> A e. dom C )
Theorem	breldm 4985	Membership of first of a binary relation in a domain. (Contributed by NM, 30-Jul-1995.)
|- A e. _V   &    |- B e. _V   =>    |- ( A R B -> A e. dom R )
Theorem	opeldmg 4986	Membership of first of an ordered pair in a domain. (Contributed by Jim Kingdon, 9-Jul-2019.)
|- ( ( A e. V /\ B e. W ) -> ( <. A , B >. e. C -> A e. dom C ) )
Theorem	breldmg 4987	Membership of first of a binary relation in a domain. (Contributed by NM, 21-Mar-2007.)
|- ( ( A e. C /\ B e. D /\ A R B ) -> A e. dom R )
Theorem	dmun 4988	The domain of a union is the union of domains. Exercise 56(a) of [Enderton] p. 65. (Contributed by NM, 12-Aug-1994.) (Proof shortened by Andrew Salmon, 27-Aug-2011.)
|- dom ( A u. B ) = ( dom A u. dom B )
Theorem	dmin 4989	The domain of an intersection belong to the intersection of domains. Theorem 6 of [Suppes] p. 60. (Contributed by NM, 15-Sep-2004.)
|- dom ( A i^i B ) C_ ( dom A i^i dom B )
Theorem	dmiun 4990	The domain of an indexed union. (Contributed by Mario Carneiro, 26-Apr-2016.)
|- dom U_ x e. A B = U_ x e. A dom B
Theorem	dmuni 4991*	The domain of a union. Part of Exercise 8 of [Enderton] p. 41. (Contributed by NM, 3-Feb-2004.)
|- dom U. A = U_ x e. A dom x
Theorem	dmopab 4992*	The domain of a class of ordered pairs. (Contributed by NM, 16-May-1995.) (Revised by Mario Carneiro, 4-Dec-2016.)
|- dom { <. x , y >. | ph } = { x | E. y ph }
Theorem	dmopabss 4993*	Upper bound for the domain of a restricted class of ordered pairs. (Contributed by NM, 31-Jan-2004.)
|- dom { <. x , y >. | ( x e. A /\ ph ) } C_ A
Theorem	dmopab3 4994*	The domain of a restricted class of ordered pairs. (Contributed by NM, 31-Jan-2004.)
|- ( A. x e. A E. y ph <-> dom { <. x , y >. | ( x e. A /\ ph ) } = A )
Theorem	dm0 4995	The domain of the empty set is empty. Part of Theorem 3.8(v) of [Monk1] p. 36. (Contributed by NM, 4-Jul-1994.) (Proof shortened by Andrew Salmon, 27-Aug-2011.)
|- dom (/) = (/)
Theorem	dmi 4996	The domain of the identity relation is the universe. (Contributed by NM, 30-Apr-1998.) (Proof shortened by Andrew Salmon, 27-Aug-2011.)
|- dom _I = _V
Theorem	dmv 4997	The domain of the universe is the universe. (Contributed by NM, 8-Aug-2003.)
|- dom _V = _V
Theorem	dm0rn0 4998	An empty domain implies an empty range. For a similar theorem for whether the domain and range are inhabited, see dmmrnm 5001. (Contributed by NM, 21-May-1998.)
|- ( dom A = (/) <-> ran A = (/) )
Theorem	reldm0 4999	A relation is empty iff its domain is empty. For a similar theorem for whether the relation and domain are inhabited, see reldmm 5000. (Contributed by NM, 15-Sep-2004.)
|- ( Rel A -> ( A = (/) <-> dom A = (/) ) )
Theorem	reldmm 5000*	A relation is inhabited iff its domain is inhabited. (Contributed by Jim Kingdon, 30-Jan-2026.)
|- ( Rel A -> ( E. x x e. A <-> E. y y e. dom A ) )