P. 53 of Theorem List - Intuitionistic Logic Explorer

Theorem List for Intuitionistic Logic Explorer - 5201-5300   *Has distinct variable group(s)

Type	Label	Description
Statement
Theorem	relcnvtr 5201	A relation is transitive iff its converse is transitive. (Contributed by FL, 19-Sep-2011.)
|- ( Rel R -> ( ( R o. R ) C_ R <-> ( `' R o. `' R ) C_ `' R ) )
Theorem	relssdmrn 5202	A relation is included in the cross product of its domain and range. Exercise 4.12(t) of [Mendelson] p. 235. (Contributed by NM, 3-Aug-1994.)
|- ( Rel A -> A C_ ( dom A X. ran A ) )
Theorem	cnvssrndm 5203	The converse is a subset of the cartesian product of range and domain. (Contributed by Mario Carneiro, 2-Jan-2017.)
|- `' A C_ ( ran A X. dom A )
Theorem	cossxp 5204	Composition as a subset of the cross product of factors. (Contributed by Mario Carneiro, 12-Jan-2017.)
|- ( A o. B ) C_ ( dom B X. ran A )
Theorem	cossxp2 5205	The composition of two relations is a relation, with bounds on its domain and codomain. (Contributed by BJ, 10-Jul-2022.)
|- ( ph -> R C_ ( A X. B ) )   &    |- ( ph -> S C_ ( B X. C ) )   =>    |- ( ph -> ( S o. R ) C_ ( A X. C ) )
Theorem	cocnvres 5206	Restricting a relation and a converse relation when they are composed together. (Contributed by BJ, 10-Jul-2022.)
|- ( S o. `' R ) = ( ( S |` dom R ) o. `' ( R |` dom S ) )
Theorem	cocnvss 5207	Upper bound for the composed of a relation and an inverse relation. (Contributed by BJ, 10-Jul-2022.)
|- ( S o. `' R ) C_ ( ran ( R |` dom S ) X. ran ( S |` dom R ) )
Theorem	relrelss 5208	Two ways to describe the structure of a two-place operation. (Contributed by NM, 17-Dec-2008.)
|- ( ( Rel A /\ Rel dom A ) <-> A C_ ( ( _V X. _V ) X. _V ) )
Theorem	unielrel 5209	The membership relation for a relation is inherited by class union. (Contributed by NM, 17-Sep-2006.)
|- ( ( Rel R /\ A e. R ) -> U. A e. U. R )
Theorem	relfld 5210	The double union of a relation is its field. (Contributed by NM, 17-Sep-2006.)
|- ( Rel R -> U. U. R = ( dom R u. ran R ) )
Theorem	relresfld 5211	Restriction of a relation to its field. (Contributed by FL, 15-Apr-2012.)
|- ( Rel R -> ( R |` U. U. R ) = R )
Theorem	relcoi2 5212	Composition with the identity relation restricted to a relation's field. (Contributed by FL, 2-May-2011.)
|- ( Rel R -> ( ( _I |` U. U. R ) o. R ) = R )
Theorem	relcoi1 5213	Composition with the identity relation restricted to a relation's field. (Contributed by FL, 8-May-2011.)
|- ( Rel R -> ( R o. ( _I |` U. U. R ) ) = R )
Theorem	unidmrn 5214	The double union of the converse of a class is its field. (Contributed by NM, 4-Jun-2008.)
|- U. U. `' A = ( dom A u. ran A )
Theorem	relcnvfld 5215	if R is a relation, its double union equals the double union of its converse. (Contributed by FL, 5-Jan-2009.)
|- ( Rel R -> U. U. R = U. U. `' R )
Theorem	dfdm2 5216	Alternate definition of domain df-dm 4684 that doesn't require dummy variables. (Contributed by NM, 2-Aug-2010.)
|- dom A = U. U. ( `' A o. A )
Theorem	unixpm 5217*	The double class union of an inhabited cross product is the union of its members. (Contributed by Jim Kingdon, 18-Dec-2018.)
|- ( E. x x e. ( A X. B ) -> U. U. ( A X. B ) = ( A u. B ) )
Theorem	unixp0im 5218	The union of an empty cross product is empty. (Contributed by Jim Kingdon, 18-Dec-2018.)
|- ( ( A X. B ) = (/) -> U. ( A X. B ) = (/) )
Theorem	cnvexg 5219	The converse of a set is a set. Corollary 6.8(1) of [TakeutiZaring] p. 26. (Contributed by NM, 17-Mar-1998.)
|- ( A e. V -> `' A e. _V )
Theorem	cnvex 5220	The converse of a set is a set. Corollary 6.8(1) of [TakeutiZaring] p. 26. (Contributed by NM, 19-Dec-2003.)
|- A e. _V   =>    |- `' A e. _V
Theorem	relcnvexb 5221	A relation is a set iff its converse is a set. (Contributed by FL, 3-Mar-2007.)
|- ( Rel R -> ( R e. _V <-> `' R e. _V ) )
Theorem	ressn 5222	Restriction of a class to a singleton. (Contributed by Mario Carneiro, 28-Dec-2014.)
|- ( A |` { B } ) = ( { B } X. ( A " { B } ) )
Theorem	cnviinm 5223*	The converse of an intersection is the intersection of the converse. (Contributed by Jim Kingdon, 18-Dec-2018.)
|- ( E. y y e. A -> `' |^|_ x e. A B = |^|_ x e. A `' B )
Theorem	cnvpom 5224*	The converse of a partial order relation is a partial order relation. (Contributed by NM, 15-Jun-2005.)
|- ( E. x x e. A -> ( R Po A <-> `' R Po A ) )
Theorem	cnvsom 5225*	The converse of a strict order relation is a strict order relation. (Contributed by Jim Kingdon, 19-Dec-2018.)
|- ( E. x x e. A -> ( R Or A <-> `' R Or A ) )
Theorem	coexg 5226	The composition of two sets is a set. (Contributed by NM, 19-Mar-1998.)
|- ( ( A e. V /\ B e. W ) -> ( A o. B ) e. _V )
Theorem	coex 5227	The composition of two sets is a set. (Contributed by NM, 15-Dec-2003.)
|- A e. _V   &    |- B e. _V   =>    |- ( A o. B ) e. _V
Theorem	xpcom 5228*	Composition of two cross products. (Contributed by Jim Kingdon, 20-Dec-2018.)
|- ( E. x x e. B -> ( ( B X. C ) o. ( A X. B ) ) = ( A X. C ) )
2.6.7  Definite description binder (inverted iota)
Syntax	cio 5229	Extend class notation with Russell's definition description binder (inverted iota).
class ( iota x ph )
Theorem	iotajust 5230*	Soundness justification theorem for df-iota 5231. (Contributed by Andrew Salmon, 29-Jun-2011.)
|- U. { y | { x | ph } = { y } } = U. { z | { x | ph } = { z } }
Definition	df-iota 5231*	Define Russell's definition description binder, which can be read as "the unique x such that ph," where ph ordinarily contains x as a free variable. Our definition is meaningful only when there is exactly one x such that ph is true (see iotaval 5242); otherwise, it evaluates to the empty set (see iotanul 5246). Russell used the inverted iota symbol iota to represent the binder. Sometimes proofs need to expand an iota-based definition. That is, given "X = the x for which ... x ... x ..." holds, the proof needs to get to "... X ... X ...". A general strategy to do this is to use iotacl 5255 (for unbounded iota). This can be easier than applying a version that applies an explicit substitution, because substituting an iota into its own property always has a bound variable clash which must be first renamed or else guarded with NF. (Contributed by Andrew Salmon, 30-Jun-2011.)
|- ( iota x ph ) = U. { y | { x | ph } = { y } }
Theorem	dfiota2 5232*	Alternate definition for descriptions. Definition 8.18 in [Quine] p. 56. (Contributed by Andrew Salmon, 30-Jun-2011.)
|- ( iota x ph ) = U. { y | A. x ( ph <-> x = y ) }
Theorem	nfiota1 5233	Bound-variable hypothesis builder for the iota class. (Contributed by Andrew Salmon, 11-Jul-2011.) (Revised by Mario Carneiro, 15-Oct-2016.)
|- F/_ x ( iota x ph )
Theorem	nfiotadw 5234*	Bound-variable hypothesis builder for the iota class. (Contributed by Jim Kingdon, 21-Dec-2018.)
|- F/ y ph   &    |- ( ph -> F/ x ps )   =>    |- ( ph -> F/_ x ( iota y ps ) )
Theorem	nfiotaw 5235*	Bound-variable hypothesis builder for the iota class. (Contributed by NM, 23-Aug-2011.)
|- F/ x ph   =>    |- F/_ x ( iota y ph )
Theorem	cbviota 5236	Change bound variables in a description binder. (Contributed by Andrew Salmon, 1-Aug-2011.)
|- ( x = y -> ( ph <-> ps ) )   &    |- F/ y ph   &    |- F/ x ps   =>    |- ( iota x ph ) = ( iota y ps )
Theorem	cbviotav 5237*	Change bound variables in a description binder. (Contributed by Andrew Salmon, 1-Aug-2011.)
|- ( x = y -> ( ph <-> ps ) )   =>    |- ( iota x ph ) = ( iota y ps )
Theorem	sb8iota 5238	Variable substitution in description binder. Compare sb8eu 2066. (Contributed by NM, 18-Mar-2013.)
|- F/ y ph   =>    |- ( iota x ph ) = ( iota y [ y / x ] ph )
Theorem	iotaeq 5239	Equality theorem for descriptions. (Contributed by Andrew Salmon, 30-Jun-2011.)
|- ( A. x x = y -> ( iota x ph ) = ( iota y ph ) )
Theorem	iotabi 5240	Equivalence theorem for descriptions. (Contributed by Andrew Salmon, 30-Jun-2011.)
|- ( A. x ( ph <-> ps ) -> ( iota x ph ) = ( iota x ps ) )
Theorem	uniabio 5241*	Part of Theorem 8.17 in [Quine] p. 56. This theorem serves as a lemma for the fundamental property of iota. (Contributed by Andrew Salmon, 11-Jul-2011.)
|- ( A. x ( ph <-> x = y ) -> U. { x | ph } = y )
Theorem	iotaval 5242*	Theorem 8.19 in [Quine] p. 57. This theorem is the fundamental property of iota. (Contributed by Andrew Salmon, 11-Jul-2011.)
|- ( A. x ( ph <-> x = y ) -> ( iota x ph ) = y )
Theorem	iotauni 5243	Equivalence between two different forms of iota. (Contributed by Andrew Salmon, 12-Jul-2011.)
|- ( E! x ph -> ( iota x ph ) = U. { x | ph } )
Theorem	iotaint 5244	Equivalence between two different forms of iota. (Contributed by Mario Carneiro, 24-Dec-2016.)
|- ( E! x ph -> ( iota x ph ) = |^| { x | ph } )
Theorem	iota1 5245	Property of iota. (Contributed by NM, 23-Aug-2011.) (Revised by Mario Carneiro, 23-Dec-2016.)
|- ( E! x ph -> ( ph <-> ( iota x ph ) = x ) )
Theorem	iotanul 5246	Theorem 8.22 in [Quine] p. 57. This theorem is the result if there isn't exactly one x that satisfies ph. (Contributed by Andrew Salmon, 11-Jul-2011.)
|- ( -. E! x ph -> ( iota x ph ) = (/) )
Theorem	euiotaex 5247	Theorem 8.23 in [Quine] p. 58, with existential uniqueness condition added. This theorem proves the existence of the iota class under our definition. (Contributed by Jim Kingdon, 21-Dec-2018.)
|- ( E! x ph -> ( iota x ph ) e. _V )
Theorem	iotass 5248*	Value of iota based on a proposition which holds only for values which are subsets of a given class. (Contributed by Mario Carneiro and Jim Kingdon, 21-Dec-2018.)
|- ( A. x ( ph -> x C_ A ) -> ( iota x ph ) C_ A )
Theorem	iotaexab 5249	Existence of the iota class when all the possible values are contained in a set. (Contributed by Jim Kingdon, 27-May-2025.)
|- ( { x | ph } e. V -> ( iota x ph ) e. _V )
Theorem	iota4 5250	Theorem *14.22 in [WhiteheadRussell] p. 190. (Contributed by Andrew Salmon, 12-Jul-2011.)
|- ( E! x ph -> [. ( iota x ph ) / x ]. ph )
Theorem	iota4an 5251	Theorem *14.23 in [WhiteheadRussell] p. 191. (Contributed by Andrew Salmon, 12-Jul-2011.)
|- ( E! x ( ph /\ ps ) -> [. ( iota x ( ph /\ ps ) ) / x ]. ph )
Theorem	iota5 5252*	A method for computing iota. (Contributed by NM, 17-Sep-2013.)
|- ( ( ph /\ A e. V ) -> ( ps <-> x = A ) )   =>    |- ( ( ph /\ A e. V ) -> ( iota x ps ) = A )
Theorem	iotabidv 5253*	Formula-building deduction for iota. (Contributed by NM, 20-Aug-2011.)
|- ( ph -> ( ps <-> ch ) )   =>    |- ( ph -> ( iota x ps ) = ( iota x ch ) )
Theorem	iotabii 5254	Formula-building deduction for iota. (Contributed by Mario Carneiro, 2-Oct-2015.)
|- ( ph <-> ps )   =>    |- ( iota x ph ) = ( iota x ps )
Theorem	iotacl 5255	Membership law for descriptions. This can useful for expanding an unbounded iota-based definition (see df-iota 5231). (Contributed by Andrew Salmon, 1-Aug-2011.)
|- ( E! x ph -> ( iota x ph ) e. { x | ph } )
Theorem	iota2df 5256	A condition that allows us to represent "the unique element such that ph " with a class expression A. (Contributed by NM, 30-Dec-2014.)
|- ( ph -> B e. V )   &    |- ( ph -> E! x ps )   &    |- ( ( ph /\ x = B ) -> ( ps <-> ch ) )   &    |- F/ x ph   &    |- ( ph -> F/ x ch )   &    |- ( ph -> F/_ x B )   =>    |- ( ph -> ( ch <-> ( iota x ps ) = B ) )
Theorem	iota2d 5257*	A condition that allows us to represent "the unique element such that ph " with a class expression A. (Contributed by NM, 30-Dec-2014.)
|- ( ph -> B e. V )   &    |- ( ph -> E! x ps )   &    |- ( ( ph /\ x = B ) -> ( ps <-> ch ) )   =>    |- ( ph -> ( ch <-> ( iota x ps ) = B ) )
Theorem	eliota 5258*	An element of an iota expression. (Contributed by Jim Kingdon, 22-Nov-2024.)
|- ( A e. ( iota x ph ) <-> E. y ( A e. y /\ A. x ( ph <-> x = y ) ) )
Theorem	eliotaeu 5259	An inhabited iota expression has a unique value. (Contributed by Jim Kingdon, 22-Nov-2024.)
|- ( A e. ( iota x ph ) -> E! x ph )
Theorem	iota2 5260*	The unique element such that ph. (Contributed by Jeff Madsen, 1-Jun-2011.) (Revised by Mario Carneiro, 23-Dec-2016.)
|- ( x = A -> ( ph <-> ps ) )   =>    |- ( ( A e. B /\ E! x ph ) -> ( ps <-> ( iota x ph ) = A ) )
Theorem	sniota 5261	A class abstraction with a unique member can be expressed as a singleton. (Contributed by Mario Carneiro, 23-Dec-2016.)
|- ( E! x ph -> { x | ph } = { ( iota x ph ) } )
Theorem	iotam 5262*	Representation of "the unique element such that ph " with a class expression A which is inhabited (that means that "the unique element such that ph " exists). (Contributed by AV, 30-Jan-2024.)
|- ( x = A -> ( ph <-> ps ) )   =>    |- ( ( A e. V /\ E. w w e. A /\ A = ( iota x ph ) ) -> ps )
Theorem	csbiotag 5263*	Class substitution within a description binder. (Contributed by Scott Fenton, 6-Oct-2017.)
|- ( A e. V -> [_ A / x ]_ ( iota y ph ) = ( iota y [. A / x ]. ph ) )
2.6.8  Functions
Syntax	wfun 5264	Extend the definition of a wff to include the function predicate. (Read: A is a function.)
wff Fun A
Syntax	wfn 5265	Extend the definition of a wff to include the function predicate with a domain. (Read: A is a function on B.)
wff A Fn B
Syntax	wf 5266	Extend the definition of a wff to include the function predicate with domain and codomain. (Read: F maps A into B.)
wff F : A --> B
Syntax	wf1 5267	Extend the definition of a wff to include one-to-one functions. (Read: F maps A one-to-one into B.) The notation ("1-1" above the arrow) is from Definition 6.15(5) of [TakeutiZaring] p. 27.
wff F : A -1-1-> B
Syntax	wfo 5268	Extend the definition of a wff to include onto functions. (Read: F maps A onto B.) The notation ("onto" below the arrow) is from Definition 6.15(4) of [TakeutiZaring] p. 27.
wff F : A -onto-> B
Syntax	wf1o 5269	Extend the definition of a wff to include one-to-one onto functions. (Read: F maps A one-to-one onto B.) The notation ("1-1" above the arrow and "onto" below the arrow) is from Definition 6.15(6) of [TakeutiZaring] p. 27.
wff F : A -1-1-onto-> B
Syntax	cfv 5270	Extend the definition of a class to include the value of a function. (Read: The value of F at A, or " F of A.")
class ( F ` A )
Syntax	wiso 5271	Extend the definition of a wff to include the isomorphism property. (Read: H is an R, S isomorphism of A onto B.)
wff H Isom R , S ( A , B )
Definition	df-fun 5272	Define predicate that determines if some class A is a function. Definition 10.1 of [Quine] p. 65. For example, the expression Fun _I is true (funi 5302). This is not the same as defining a specific function's mapping, which is typically done using the format of cmpt 4104 with the maps-to notation (see df-mpt 4106). Contrast this predicate with the predicates to determine if some class is a function with a given domain (df-fn 5273), a function with a given domain and codomain (df-f 5274), a one-to-one function (df-f1 5275), an onto function (df-fo 5276), or a one-to-one onto function (df-f1o 5277). For alternate definitions, see dffun2 5280, dffun4 5281, dffun6 5284, dffun7 5297, dffun8 5298, and dffun9 5299. (Contributed by NM, 1-Aug-1994.)
|- ( Fun A <-> ( Rel A /\ ( A o. `' A ) C_ _I ) )
Definition	df-fn 5273	Define a function with domain. Definition 6.15(1) of [TakeutiZaring] p. 27. (Contributed by NM, 1-Aug-1994.)
|- ( A Fn B <-> ( Fun A /\ dom A = B ) )
Definition	df-f 5274	Define a function (mapping) with domain and codomain. Definition 6.15(3) of [TakeutiZaring] p. 27. (Contributed by NM, 1-Aug-1994.)
|- ( F : A --> B <-> ( F Fn A /\ ran F C_ B ) )
Definition	df-f1 5275	Define a one-to-one function. Compare Definition 6.15(5) of [TakeutiZaring] p. 27. We use their notation ("1-1" above the arrow). (Contributed by NM, 1-Aug-1994.)
|- ( F : A -1-1-> B <-> ( F : A --> B /\ Fun `' F ) )
Definition	df-fo 5276	Define an onto function. Definition 6.15(4) of [TakeutiZaring] p. 27. We use their notation ("onto" under the arrow). (Contributed by NM, 1-Aug-1994.)
|- ( F : A -onto-> B <-> ( F Fn A /\ ran F = B ) )
Definition	df-f1o 5277	Define a one-to-one onto function. Compare Definition 6.15(6) of [TakeutiZaring] p. 27. We use their notation ("1-1" above the arrow and "onto" below the arrow). (Contributed by NM, 1-Aug-1994.)
|- ( F : A -1-1-onto-> B <-> ( F : A -1-1-> B /\ F : A -onto-> B ) )
Definition	df-fv 5278*	Define the value of a function, ( F ` A ), also known as function application. For example, ( _I ` (/) ) = (/). Typically, function F is defined using maps-to notation (see df-mpt 4106), but this is not required. For example, F = { <. 2 , 6 >. , <. 3 , 9 >. } -> ( F ` 3 ) = 9. We will later define two-argument functions using ordered pairs as ( A F B ) = ( F ` <. A , B >. ). This particular definition is quite convenient: it can be applied to any class and evaluates to the empty set when it is not meaningful. The left apostrophe notation originated with Peano and was adopted in Definition *30.01 of [WhiteheadRussell] p. 235, Definition 10.11 of [Quine] p. 68, and Definition 6.11 of [TakeutiZaring] p. 26. It means the same thing as the more familiar F ( A ) notation for a function's value at A, i.e., " F of A," but without context-dependent notational ambiguity. (Contributed by NM, 1-Aug-1994.) Revised to use iota. (Revised by Scott Fenton, 6-Oct-2017.)
|- ( F ` A ) = ( iota x A F x )
Definition	df-isom 5279*	Define the isomorphism predicate. We read this as " H is an R, S isomorphism of A onto B". Normally, R and S are ordering relations on A and B respectively. Definition 6.28 of [TakeutiZaring] p. 32, whose notation is the same as ours except that R and S are subscripts. (Contributed by NM, 4-Mar-1997.)
|- ( H Isom R , S ( A , B ) <-> ( H : A -1-1-onto-> B /\ A. x e. A A. y e. A ( x R y <-> ( H ` x ) S ( H ` y ) ) ) )
Theorem	dffun2 5280*	Alternate definition of a function. (Contributed by NM, 29-Dec-1996.)
|- ( Fun A <-> ( Rel A /\ A. x A. y A. z ( ( x A y /\ x A z ) -> y = z ) ) )
Theorem	dffun4 5281*	Alternate definition of a function. Definition 6.4(4) of [TakeutiZaring] p. 24. (Contributed by NM, 29-Dec-1996.)
|- ( Fun A <-> ( Rel A /\ A. x A. y A. z ( ( <. x , y >. e. A /\ <. x , z >. e. A ) -> y = z ) ) )
Theorem	dffun5r 5282*	A way of proving a relation is a function, analogous to mo2r 2105. (Contributed by Jim Kingdon, 27-May-2020.)
|- ( ( Rel A /\ A. x E. z A. y ( <. x , y >. e. A -> y = z ) ) -> Fun A )
Theorem	dffun6f 5283*	Definition of function, using bound-variable hypotheses instead of distinct variable conditions. (Contributed by NM, 9-Mar-1995.) (Revised by Mario Carneiro, 15-Oct-2016.)
|- F/_ x A   &    |- F/_ y A   =>    |- ( Fun A <-> ( Rel A /\ A. x E* y x A y ) )
Theorem	dffun6 5284*	Alternate definition of a function using "at most one" notation. (Contributed by NM, 9-Mar-1995.)
|- ( Fun F <-> ( Rel F /\ A. x E* y x F y ) )
Theorem	funmo 5285*	A function has at most one value for each argument. (Contributed by NM, 24-May-1998.)
|- ( Fun F -> E* y A F y )
Theorem	dffun4f 5286*	Definition of function like dffun4 5281 but using bound-variable hypotheses instead of distinct variable conditions. (Contributed by Jim Kingdon, 17-Mar-2019.)
|- F/_ x A   &    |- F/_ y A   &    |- F/_ z A   =>    |- ( Fun A <-> ( Rel A /\ A. x A. y A. z ( ( <. x , y >. e. A /\ <. x , z >. e. A ) -> y = z ) ) )
Theorem	funrel 5287	A function is a relation. (Contributed by NM, 1-Aug-1994.)
|- ( Fun A -> Rel A )
Theorem	0nelfun 5288	A function does not contain the empty set. (Contributed by BJ, 26-Nov-2021.)
|- ( Fun R -> (/) e/ R )
Theorem	funss 5289	Subclass theorem for function predicate. (Contributed by NM, 16-Aug-1994.) (Proof shortened by Mario Carneiro, 24-Jun-2014.)
|- ( A C_ B -> ( Fun B -> Fun A ) )
Theorem	funeq 5290	Equality theorem for function predicate. (Contributed by NM, 16-Aug-1994.)
|- ( A = B -> ( Fun A <-> Fun B ) )
Theorem	funeqi 5291	Equality inference for the function predicate. (Contributed by Jonathan Ben-Naim, 3-Jun-2011.)
|- A = B   =>    |- ( Fun A <-> Fun B )
Theorem	funeqd 5292	Equality deduction for the function predicate. (Contributed by NM, 23-Feb-2013.)
|- ( ph -> A = B )   =>    |- ( ph -> ( Fun A <-> Fun B ) )
Theorem	nffun 5293	Bound-variable hypothesis builder for a function. (Contributed by NM, 30-Jan-2004.)
|- F/_ x F   =>    |- F/ x Fun F
Theorem	sbcfung 5294	Distribute proper substitution through the function predicate. (Contributed by Alexander van der Vekens, 23-Jul-2017.)
|- ( A e. V -> ( [. A / x ]. Fun F <-> Fun [_ A / x ]_ F ) )
Theorem	funeu 5295*	There is exactly one value of a function. (Contributed by NM, 22-Apr-2004.) (Proof shortened by Andrew Salmon, 17-Sep-2011.)
|- ( ( Fun F /\ A F B ) -> E! y A F y )
Theorem	funeu2 5296*	There is exactly one value of a function. (Contributed by NM, 3-Aug-1994.)
|- ( ( Fun F /\ <. A , B >. e. F ) -> E! y <. A , y >. e. F )
Theorem	dffun7 5297*	Alternate definition of a function. One possibility for the definition of a function in [Enderton] p. 42. (Enderton's definition is ambiguous because "there is only one" could mean either "there is at most one" or "there is exactly one". However, dffun8 5298 shows that it does not matter which meaning we pick.) (Contributed by NM, 4-Nov-2002.)
|- ( Fun A <-> ( Rel A /\ A. x e. dom A E* y x A y ) )
Theorem	dffun8 5298*	Alternate definition of a function. One possibility for the definition of a function in [Enderton] p. 42. Compare dffun7 5297. (Contributed by NM, 4-Nov-2002.) (Proof shortened by Andrew Salmon, 17-Sep-2011.)
|- ( Fun A <-> ( Rel A /\ A. x e. dom A E! y x A y ) )
Theorem	dffun9 5299*	Alternate definition of a function. (Contributed by NM, 28-Mar-2007.) (Revised by NM, 16-Jun-2017.)
|- ( Fun A <-> ( Rel A /\ A. x e. dom A E* y e. ran A x A y ) )
Theorem	funfn 5300	An equivalence for the function predicate. (Contributed by NM, 13-Aug-2004.)
|- ( Fun A <-> A Fn dom A )