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Theorem List for Intuitionistic Logic Explorer - 4401-4500   *Has distinct variable group(s)
TypeLabelDescription
Statement
 
Theoremonn0 4401 The class of all ordinal numbers is not empty. (Contributed by NM, 17-Sep-1995.)
 |- 
 On  =/=  (/)
 
Theoremonm 4402 The class of all ordinal numbers is inhabited. (Contributed by Jim Kingdon, 6-Mar-2019.)
 |- 
 E. x  x  e. 
 On
 
Theoremsuceq 4403 Equality of successors. (Contributed by NM, 30-Aug-1993.) (Proof shortened by Andrew Salmon, 25-Jul-2011.)
 |-  ( A  =  B  ->  suc  A  =  suc  B )
 
Theoremelsuci 4404 Membership in a successor. This one-way implication does not require that either  A or  B be sets. (Contributed by NM, 6-Jun-1994.)
 |-  ( A  e.  suc  B 
 ->  ( A  e.  B  \/  A  =  B ) )
 
Theoremelsucg 4405 Membership in a successor. Exercise 5 of [TakeutiZaring] p. 17. (Contributed by NM, 15-Sep-1995.)
 |-  ( A  e.  V  ->  ( A  e.  suc  B  <-> 
 ( A  e.  B  \/  A  =  B ) ) )
 
Theoremelsuc2g 4406 Variant of membership in a successor, requiring that  B rather than  A be a set. (Contributed by NM, 28-Oct-2003.)
 |-  ( B  e.  V  ->  ( A  e.  suc  B  <-> 
 ( A  e.  B  \/  A  =  B ) ) )
 
Theoremelsuc 4407 Membership in a successor. Exercise 5 of [TakeutiZaring] p. 17. (Contributed by NM, 15-Sep-2003.)
 |-  A  e.  _V   =>    |-  ( A  e.  suc 
 B 
 <->  ( A  e.  B  \/  A  =  B ) )
 
Theoremelsuc2 4408 Membership in a successor. (Contributed by NM, 15-Sep-2003.)
 |-  A  e.  _V   =>    |-  ( B  e.  suc 
 A 
 <->  ( B  e.  A  \/  B  =  A ) )
 
Theoremnfsuc 4409 Bound-variable hypothesis builder for successor. (Contributed by NM, 15-Sep-2003.)
 |-  F/_ x A   =>    |-  F/_ x  suc  A
 
Theoremelelsuc 4410 Membership in a successor. (Contributed by NM, 20-Jun-1998.)
 |-  ( A  e.  B  ->  A  e.  suc  B )
 
Theoremsucel 4411* Membership of a successor in another class. (Contributed by NM, 29-Jun-2004.)
 |-  ( suc  A  e.  B 
 <-> 
 E. x  e.  B  A. y ( y  e.  x  <->  ( y  e.  A  \/  y  =  A ) ) )
 
Theoremsuc0 4412 The successor of the empty set. (Contributed by NM, 1-Feb-2005.)
 |- 
 suc  (/)  =  { (/) }
 
Theoremsucprc 4413 A proper class is its own successor. (Contributed by NM, 3-Apr-1995.)
 |-  ( -.  A  e.  _V 
 ->  suc  A  =  A )
 
Theoremunisuc 4414 A transitive class is equal to the union of its successor. Combines Theorem 4E of [Enderton] p. 72 and Exercise 6 of [Enderton] p. 73. (Contributed by NM, 30-Aug-1993.)
 |-  A  e.  _V   =>    |-  ( Tr  A  <->  U.
 suc  A  =  A )
 
Theoremunisucg 4415 A transitive class is equal to the union of its successor. Combines Theorem 4E of [Enderton] p. 72 and Exercise 6 of [Enderton] p. 73. (Contributed by Jim Kingdon, 18-Aug-2019.)
 |-  ( A  e.  V  ->  ( Tr  A  <->  U. suc  A  =  A ) )
 
Theoremsssucid 4416 A class is included in its own successor. Part of Proposition 7.23 of [TakeutiZaring] p. 41 (generalized to arbitrary classes). (Contributed by NM, 31-May-1994.)
 |-  A  C_  suc  A
 
Theoremsucidg 4417 Part of Proposition 7.23 of [TakeutiZaring] p. 41 (generalized). (Contributed by NM, 25-Mar-1995.) (Proof shortened by Scott Fenton, 20-Feb-2012.)
 |-  ( A  e.  V  ->  A  e.  suc  A )
 
Theoremsucid 4418 A set belongs to its successor. (Contributed by NM, 22-Jun-1994.) (Proof shortened by Alan Sare, 18-Feb-2012.) (Proof shortened by Scott Fenton, 20-Feb-2012.)
 |-  A  e.  _V   =>    |-  A  e.  suc  A
 
Theoremnsuceq0g 4419 No successor is empty. (Contributed by Jim Kingdon, 14-Oct-2018.)
 |-  ( A  e.  V  ->  suc  A  =/=  (/) )
 
Theoremeqelsuc 4420 A set belongs to the successor of an equal set. (Contributed by NM, 18-Aug-1994.)
 |-  A  e.  _V   =>    |-  ( A  =  B  ->  A  e.  suc  B )
 
Theoremiunsuc 4421* Inductive definition for the indexed union at a successor. (Contributed by Mario Carneiro, 4-Feb-2013.) (Proof shortened by Mario Carneiro, 18-Nov-2016.)
 |-  A  e.  _V   &    |-  ( x  =  A  ->  B  =  C )   =>    |-  U_ x  e.  suc  A B  =  ( U_ x  e.  A  B  u.  C )
 
Theoremsuctr 4422 The successor of a transitive class is transitive. (Contributed by Alan Sare, 11-Apr-2009.)
 |-  ( Tr  A  ->  Tr 
 suc  A )
 
Theoremtrsuc 4423 A set whose successor belongs to a transitive class also belongs. (Contributed by NM, 5-Sep-2003.) (Proof shortened by Andrew Salmon, 12-Aug-2011.)
 |-  ( ( Tr  A  /\  suc  B  e.  A )  ->  B  e.  A )
 
Theoremtrsucss 4424 A member of the successor of a transitive class is a subclass of it. (Contributed by NM, 4-Oct-2003.)
 |-  ( Tr  A  ->  ( B  e.  suc  A  ->  B  C_  A )
 )
 
Theoremsucssel 4425 A set whose successor is a subset of another class is a member of that class. (Contributed by NM, 16-Sep-1995.)
 |-  ( A  e.  V  ->  ( suc  A  C_  B  ->  A  e.  B ) )
 
Theoremorduniss 4426 An ordinal class includes its union. (Contributed by NM, 13-Sep-2003.)
 |-  ( Ord  A  ->  U. A  C_  A )
 
Theoremonordi 4427 An ordinal number is an ordinal class. (Contributed by NM, 11-Jun-1994.)
 |-  A  e.  On   =>    |-  Ord  A
 
Theoremontrci 4428 An ordinal number is a transitive class. (Contributed by NM, 11-Jun-1994.)
 |-  A  e.  On   =>    |-  Tr  A
 
Theoremoneli 4429 A member of an ordinal number is an ordinal number. Theorem 7M(a) of [Enderton] p. 192. (Contributed by NM, 11-Jun-1994.)
 |-  A  e.  On   =>    |-  ( B  e.  A  ->  B  e.  On )
 
Theoremonelssi 4430 A member of an ordinal number is a subset of it. (Contributed by NM, 11-Aug-1994.)
 |-  A  e.  On   =>    |-  ( B  e.  A  ->  B  C_  A )
 
Theoremonelini 4431 An element of an ordinal number equals the intersection with it. (Contributed by NM, 11-Jun-1994.)
 |-  A  e.  On   =>    |-  ( B  e.  A  ->  B  =  ( B  i^i  A ) )
 
Theoremoneluni 4432 An ordinal number equals its union with any element. (Contributed by NM, 13-Jun-1994.)
 |-  A  e.  On   =>    |-  ( B  e.  A  ->  ( A  u.  B )  =  A )
 
Theoremonunisuci 4433 An ordinal number is equal to the union of its successor. (Contributed by NM, 12-Jun-1994.)
 |-  A  e.  On   =>    |-  U. suc  A  =  A
 
2.4  IZF Set Theory - add the Axiom of Union
 
2.4.1  Introduce the Axiom of Union
 
Axiomax-un 4434* Axiom of Union. An axiom of Intuitionistic Zermelo-Fraenkel set theory. It states that a set  y exists that includes the union of a given set  x i.e. the collection of all members of the members of  x. The variant axun2 4436 states that the union itself exists. A version with the standard abbreviation for union is uniex2 4437. A version using class notation is uniex 4438.

This is Axiom 3 of [Crosilla] p. "Axioms of CZF and IZF", except (a) unnecessary quantifiers are removed, (b) Crosilla has a biconditional rather than an implication (but the two are equivalent by bm1.3ii 4125), and (c) the order of the conjuncts is swapped (which is equivalent by ancom 266).

The union of a class df-uni 3811 should not be confused with the union of two classes df-un 3134. Their relationship is shown in unipr 3824. (Contributed by NM, 23-Dec-1993.)

 |- 
 E. y A. z
 ( E. w ( z  e.  w  /\  w  e.  x )  ->  z  e.  y )
 
Theoremzfun 4435* Axiom of Union expressed with the fewest number of different variables. (Contributed by NM, 14-Aug-2003.)
 |- 
 E. x A. y
 ( E. x ( y  e.  x  /\  x  e.  z )  ->  y  e.  x )
 
Theoremaxun2 4436* A variant of the Axiom of Union ax-un 4434. For any set  x, there exists a set  y whose members are exactly the members of the members of  x i.e. the union of  x. Axiom Union of [BellMachover] p. 466. (Contributed by NM, 4-Jun-2006.)
 |- 
 E. y A. z
 ( z  e.  y  <->  E. w ( z  e.  w  /\  w  e.  x ) )
 
Theoremuniex2 4437* The Axiom of Union using the standard abbreviation for union. Given any set  x, its union  y exists. (Contributed by NM, 4-Jun-2006.)
 |- 
 E. y  y  = 
 U. x
 
Theoremuniex 4438 The Axiom of Union in class notation. This says that if  A is a set i.e.  A  e.  _V (see isset 2744), then the union of  A is also a set. Same as Axiom 3 of [TakeutiZaring] p. 16. (Contributed by NM, 11-Aug-1993.)
 |-  A  e.  _V   =>    |-  U. A  e.  _V
 
Theoremvuniex 4439 The union of a setvar is a set. (Contributed by BJ, 3-May-2021.)
 |- 
 U. x  e.  _V
 
Theoremuniexg 4440 The ZF Axiom of Union in class notation, in the form of a theorem instead of an inference. We use the antecedent  A  e.  V instead of  A  e.  _V to make the theorem more general and thus shorten some proofs; obviously the universal class constant  _V is one possible substitution for class variable  V. (Contributed by NM, 25-Nov-1994.)
 |-  ( A  e.  V  ->  U. A  e.  _V )
 
Theoremuniexd 4441 Deduction version of the ZF Axiom of Union in class notation. (Contributed by Glauco Siliprandi, 26-Jun-2021.)
 |-  ( ph  ->  A  e.  V )   =>    |-  ( ph  ->  U. A  e.  _V )
 
Theoremunex 4442 The union of two sets is a set. Corollary 5.8 of [TakeutiZaring] p. 16. (Contributed by NM, 1-Jul-1994.)
 |-  A  e.  _V   &    |-  B  e.  _V   =>    |-  ( A  u.  B )  e.  _V
 
Theoremunexb 4443 Existence of union is equivalent to existence of its components. (Contributed by NM, 11-Jun-1998.)
 |-  ( ( A  e.  _V 
 /\  B  e.  _V ) 
 <->  ( A  u.  B )  e.  _V )
 
Theoremunexg 4444 A union of two sets is a set. Corollary 5.8 of [TakeutiZaring] p. 16. (Contributed by NM, 18-Sep-2006.)
 |-  ( ( A  e.  V  /\  B  e.  W )  ->  ( A  u.  B )  e.  _V )
 
Theoremtpexg 4445 An unordered triple of classes exists. (Contributed by NM, 10-Apr-1994.)
 |-  ( ( A  e.  U  /\  B  e.  V  /\  C  e.  W ) 
 ->  { A ,  B ,  C }  e.  _V )
 
Theoremunisn3 4446* Union of a singleton in the form of a restricted class abstraction. (Contributed by NM, 3-Jul-2008.)
 |-  ( A  e.  B  ->  U. { x  e.  B  |  x  =  A }  =  A )
 
Theoremabnexg 4447* Sufficient condition for a class abstraction to be a proper class. The class  F can be thought of as an expression in  x and the abstraction appearing in the statement as the class of values  F as  x varies through  A. Assuming the antecedents, if that class is a set, then so is the "domain"  A. The converse holds without antecedent, see abrexexg 6119. Note that the second antecedent  A. x  e.  A x  e.  F cannot be translated to  A  C_  F since  F may depend on  x. In applications, one may take  F  =  { x } or  F  =  ~P x (see snnex 4449 and pwnex 4450 respectively, proved from abnex 4448, which is a consequence of abnexg 4447 with  A  =  _V). (Contributed by BJ, 2-Dec-2021.)
 |-  ( A. x  e.  A  ( F  e.  V  /\  x  e.  F )  ->  ( { y  |  E. x  e.  A  y  =  F }  e.  W  ->  A  e.  _V ) )
 
Theoremabnex 4448* Sufficient condition for a class abstraction to be a proper class. Lemma for snnex 4449 and pwnex 4450. See the comment of abnexg 4447. (Contributed by BJ, 2-May-2021.)
 |-  ( A. x ( F  e.  V  /\  x  e.  F )  ->  -.  { y  | 
 E. x  y  =  F }  e.  _V )
 
Theoremsnnex 4449* The class of all singletons is a proper class. (Contributed by NM, 10-Oct-2008.) (Proof shortened by Eric Schmidt, 7-Dec-2008.)
 |- 
 { x  |  E. y  x  =  {
 y } }  e/  _V
 
Theorempwnex 4450* The class of all power sets is a proper class. See also snnex 4449. (Contributed by BJ, 2-May-2021.)
 |- 
 { x  |  E. y  x  =  ~P y }  e/  _V
 
Theoremopeluu 4451 Each member of an ordered pair belongs to the union of the union of a class to which the ordered pair belongs. Lemma 3D of [Enderton] p. 41. (Contributed by NM, 31-Mar-1995.) (Revised by Mario Carneiro, 27-Feb-2016.)
 |-  A  e.  _V   &    |-  B  e.  _V   =>    |-  ( <. A ,  B >.  e.  C  ->  ( A  e.  U. U. C  /\  B  e.  U. U. C ) )
 
Theoremuniuni 4452* Expression for double union that moves union into a class builder. (Contributed by FL, 28-May-2007.)
 |- 
 U. U. A  =  U. { x  |  E. y
 ( x  =  U. y  /\  y  e.  A ) }
 
Theoremeusv1 4453* Two ways to express single-valuedness of a class expression  A ( x ). (Contributed by NM, 14-Oct-2010.)
 |-  ( E! y A. x  y  =  A  <->  E. y A. x  y  =  A )
 
Theoremeusvnf 4454* Even if  x is free in  A, it is effectively bound when  A ( x ) is single-valued. (Contributed by NM, 14-Oct-2010.) (Revised by Mario Carneiro, 14-Oct-2016.)
 |-  ( E! y A. x  y  =  A  -> 
 F/_ x A )
 
Theoremeusvnfb 4455* Two ways to say that  A ( x ) is a set expression that does not depend on  x. (Contributed by Mario Carneiro, 18-Nov-2016.)
 |-  ( E! y A. x  y  =  A  <->  (
 F/_ x A  /\  A  e.  _V )
 )
 
Theoremeusv2i 4456* Two ways to express single-valuedness of a class expression  A ( x ). (Contributed by NM, 14-Oct-2010.) (Revised by Mario Carneiro, 18-Nov-2016.)
 |-  ( E! y A. x  y  =  A  ->  E! y E. x  y  =  A )
 
Theoremeusv2nf 4457* Two ways to express single-valuedness of a class expression  A ( x ). (Contributed by Mario Carneiro, 18-Nov-2016.)
 |-  A  e.  _V   =>    |-  ( E! y E. x  y  =  A 
 <-> 
 F/_ x A )
 
Theoremeusv2 4458* Two ways to express single-valuedness of a class expression  A ( x ). (Contributed by NM, 15-Oct-2010.) (Proof shortened by Mario Carneiro, 18-Nov-2016.)
 |-  A  e.  _V   =>    |-  ( E! y E. x  y  =  A 
 <->  E! y A. x  y  =  A )
 
Theoremreusv1 4459* Two ways to express single-valuedness of a class expression  C ( y ). (Contributed by NM, 16-Dec-2012.) (Proof shortened by Mario Carneiro, 18-Nov-2016.)
 |-  ( E. y  e.  B  ph  ->  ( E! x  e.  A  A. y  e.  B  ( ph  ->  x  =  C ) 
 <-> 
 E. x  e.  A  A. y  e.  B  (
 ph  ->  x  =  C ) ) )
 
Theoremreusv3i 4460* Two ways of expressing existential uniqueness via an indirect equality. (Contributed by NM, 23-Dec-2012.)
 |-  ( y  =  z 
 ->  ( ph  <->  ps ) )   &    |-  (
 y  =  z  ->  C  =  D )   =>    |-  ( E. x  e.  A  A. y  e.  B  (
 ph  ->  x  =  C )  ->  A. y  e.  B  A. z  e.  B  ( ( ph  /\  ps )  ->  C  =  D ) )
 
Theoremreusv3 4461* Two ways to express single-valuedness of a class expression  C ( y ). See reusv1 4459 for the connection to uniqueness. (Contributed by NM, 27-Dec-2012.)
 |-  ( y  =  z 
 ->  ( ph  <->  ps ) )   &    |-  (
 y  =  z  ->  C  =  D )   =>    |-  ( E. y  e.  B  ( ph  /\  C  e.  A )  ->  ( A. y  e.  B  A. z  e.  B  ( ( ph  /\ 
 ps )  ->  C  =  D )  <->  E. x  e.  A  A. y  e.  B  (
 ph  ->  x  =  C ) ) )
 
Theoremalxfr 4462* Transfer universal quantification from a variable  x to another variable  y contained in expression  A. (Contributed by NM, 18-Feb-2007.)
 |-  ( x  =  A  ->  ( ph  <->  ps ) )   =>    |-  ( ( A. y  A  e.  B  /\  A. x E. y  x  =  A )  ->  ( A. x ph  <->  A. y ps ) )
 
Theoremralxfrd 4463* Transfer universal quantification from a variable  x to another variable  y contained in expression  A. (Contributed by NM, 15-Aug-2014.) (Proof shortened by Mario Carneiro, 19-Nov-2016.)
 |-  ( ( ph  /\  y  e.  C )  ->  A  e.  B )   &    |-  ( ( ph  /\  x  e.  B ) 
 ->  E. y  e.  C  x  =  A )   &    |-  (
 ( ph  /\  x  =  A )  ->  ( ps 
 <->  ch ) )   =>    |-  ( ph  ->  (
 A. x  e.  B  ps 
 <-> 
 A. y  e.  C  ch ) )
 
Theoremrexxfrd 4464* Transfer universal quantification from a variable  x to another variable  y contained in expression  A. (Contributed by FL, 10-Apr-2007.) (Revised by Mario Carneiro, 15-Aug-2014.)
 |-  ( ( ph  /\  y  e.  C )  ->  A  e.  B )   &    |-  ( ( ph  /\  x  e.  B ) 
 ->  E. y  e.  C  x  =  A )   &    |-  (
 ( ph  /\  x  =  A )  ->  ( ps 
 <->  ch ) )   =>    |-  ( ph  ->  ( E. x  e.  B  ps 
 <-> 
 E. y  e.  C  ch ) )
 
Theoremralxfr2d 4465* Transfer universal quantification from a variable  x to another variable  y contained in expression  A. (Contributed by Mario Carneiro, 20-Aug-2014.)
 |-  ( ( ph  /\  y  e.  C )  ->  A  e.  V )   &    |-  ( ph  ->  ( x  e.  B  <->  E. y  e.  C  x  =  A )
 )   &    |-  ( ( ph  /\  x  =  A )  ->  ( ps 
 <->  ch ) )   =>    |-  ( ph  ->  (
 A. x  e.  B  ps 
 <-> 
 A. y  e.  C  ch ) )
 
Theoremrexxfr2d 4466* Transfer universal quantification from a variable  x to another variable  y contained in expression  A. (Contributed by Mario Carneiro, 20-Aug-2014.) (Proof shortened by Mario Carneiro, 19-Nov-2016.)
 |-  ( ( ph  /\  y  e.  C )  ->  A  e.  V )   &    |-  ( ph  ->  ( x  e.  B  <->  E. y  e.  C  x  =  A )
 )   &    |-  ( ( ph  /\  x  =  A )  ->  ( ps 
 <->  ch ) )   =>    |-  ( ph  ->  ( E. x  e.  B  ps 
 <-> 
 E. y  e.  C  ch ) )
 
Theoremralxfr 4467* Transfer universal quantification from a variable  x to another variable  y contained in expression  A. (Contributed by NM, 10-Jun-2005.) (Revised by Mario Carneiro, 15-Aug-2014.)
 |-  ( y  e.  C  ->  A  e.  B )   &    |-  ( x  e.  B  ->  E. y  e.  C  x  =  A )   &    |-  ( x  =  A  ->  (
 ph 
 <->  ps ) )   =>    |-  ( A. x  e.  B  ph  <->  A. y  e.  C  ps )
 
TheoremralxfrALT 4468* Transfer universal quantification from a variable  x to another variable  y contained in expression  A. This proof does not use ralxfrd 4463. (Contributed by NM, 10-Jun-2005.) (Revised by Mario Carneiro, 15-Aug-2014.) (Proof modification is discouraged.) (New usage is discouraged.)
 |-  ( y  e.  C  ->  A  e.  B )   &    |-  ( x  e.  B  ->  E. y  e.  C  x  =  A )   &    |-  ( x  =  A  ->  (
 ph 
 <->  ps ) )   =>    |-  ( A. x  e.  B  ph  <->  A. y  e.  C  ps )
 
Theoremrexxfr 4469* Transfer existence from a variable 
x to another variable  y contained in expression  A. (Contributed by NM, 10-Jun-2005.) (Revised by Mario Carneiro, 15-Aug-2014.)
 |-  ( y  e.  C  ->  A  e.  B )   &    |-  ( x  e.  B  ->  E. y  e.  C  x  =  A )   &    |-  ( x  =  A  ->  (
 ph 
 <->  ps ) )   =>    |-  ( E. x  e.  B  ph  <->  E. y  e.  C  ps )
 
Theoremrabxfrd 4470* Class builder membership after substituting an expression  A (containing  y) for  x in the class expression  ch. (Contributed by NM, 16-Jan-2012.)
 |-  F/_ y B   &    |-  F/_ y C   &    |-  (
 ( ph  /\  y  e.  D )  ->  A  e.  D )   &    |-  ( x  =  A  ->  ( ps  <->  ch ) )   &    |-  ( y  =  B  ->  A  =  C )   =>    |-  ( ( ph  /\  B  e.  D )  ->  ( C  e.  { x  e.  D  |  ps }  <->  B  e.  { y  e.  D  |  ch }
 ) )
 
Theoremrabxfr 4471* Class builder membership after substituting an expression  A (containing  y) for  x in the class expression  ph. (Contributed by NM, 10-Jun-2005.)
 |-  F/_ y B   &    |-  F/_ y C   &    |-  (
 y  e.  D  ->  A  e.  D )   &    |-  ( x  =  A  ->  (
 ph 
 <->  ps ) )   &    |-  (
 y  =  B  ->  A  =  C )   =>    |-  ( B  e.  D  ->  ( C  e.  { x  e.  D  |  ph
 } 
 <->  B  e.  { y  e.  D  |  ps }
 ) )
 
Theoremreuhypd 4472* A theorem useful for eliminating restricted existential uniqueness hypotheses. (Contributed by NM, 16-Jan-2012.)
 |-  ( ( ph  /\  x  e.  C )  ->  B  e.  C )   &    |-  ( ( ph  /\  x  e.  C  /\  y  e.  C )  ->  ( x  =  A  <->  y  =  B ) )   =>    |-  ( ( ph  /\  x  e.  C )  ->  E! y  e.  C  x  =  A )
 
Theoremreuhyp 4473* A theorem useful for eliminating restricted existential uniqueness hypotheses. (Contributed by NM, 15-Nov-2004.)
 |-  ( x  e.  C  ->  B  e.  C )   &    |-  ( ( x  e.  C  /\  y  e.  C )  ->  ( x  =  A  <->  y  =  B ) )   =>    |-  ( x  e.  C  ->  E! y  e.  C  x  =  A )
 
Theoremuniexb 4474 The Axiom of Union and its converse. A class is a set iff its union is a set. (Contributed by NM, 11-Nov-2003.)
 |-  ( A  e.  _V  <->  U. A  e.  _V )
 
Theorempwexb 4475 The Axiom of Power Sets and its converse. A class is a set iff its power class is a set. (Contributed by NM, 11-Nov-2003.)
 |-  ( A  e.  _V  <->  ~P A  e.  _V )
 
Theoremelpwpwel 4476 A class belongs to a double power class if and only if its union belongs to the power class. (Contributed by BJ, 22-Jan-2023.)
 |-  ( A  e.  ~P ~P B  <->  U. A  e.  ~P B )
 
Theoremuniv 4477 The union of the universe is the universe. Exercise 4.12(c) of [Mendelson] p. 235. (Contributed by NM, 14-Sep-2003.)
 |- 
 U. _V  =  _V
 
Theoremeldifpw 4478 Membership in a power class difference. (Contributed by NM, 25-Mar-2007.)
 |-  C  e.  _V   =>    |-  ( ( A  e.  ~P B  /\  -.  C  C_  B )  ->  ( A  u.  C )  e.  ( ~P ( B  u.  C )  \  ~P B ) )
 
Theoremop1stb 4479 Extract the first member of an ordered pair. Theorem 73 of [Suppes] p. 42. (Contributed by NM, 25-Nov-2003.)
 |-  A  e.  _V   &    |-  B  e.  _V   =>    |- 
 |^| |^| <. A ,  B >.  =  A
 
Theoremop1stbg 4480 Extract the first member of an ordered pair. Theorem 73 of [Suppes] p. 42. (Contributed by Jim Kingdon, 17-Dec-2018.)
 |-  ( ( A  e.  V  /\  B  e.  W )  ->  |^| |^| <. A ,  B >.  =  A )
 
Theoremiunpw 4481* An indexed union of a power class in terms of the power class of the union of its index. Part of Exercise 24(b) of [Enderton] p. 33. (Contributed by NM, 29-Nov-2003.)
 |-  A  e.  _V   =>    |-  ( E. x  e.  A  x  =  U. A 
 <->  ~P U. A  =  U_ x  e.  A  ~P x )
 
Theoremifelpwung 4482 Existence of a conditional class, quantitative version (closed form). (Contributed by BJ, 15-Aug-2024.)
 |-  ( ( A  e.  V  /\  B  e.  W )  ->  if ( ph ,  A ,  B )  e.  ~P ( A  u.  B ) )
 
Theoremifelpwund 4483 Existence of a conditional class, quantitative version (deduction form). (Contributed by BJ, 15-Aug-2024.)
 |-  ( ph  ->  A  e.  V )   &    |-  ( ph  ->  B  e.  W )   =>    |-  ( ph  ->  if ( ps ,  A ,  B )  e.  ~P ( A  u.  B ) )
 
Theoremifelpwun 4484 Existence of a conditional class, quantitative version (inference form). (Contributed by BJ, 15-Aug-2024.)
 |-  A  e.  _V   &    |-  B  e.  _V   =>    |- 
 if ( ph ,  A ,  B )  e.  ~P ( A  u.  B )
 
Theoremifexd 4485 Existence of a conditional class (deduction form). (Contributed by BJ, 15-Aug-2024.)
 |-  ( ph  ->  A  e.  V )   &    |-  ( ph  ->  B  e.  W )   =>    |-  ( ph  ->  if ( ps ,  A ,  B )  e.  _V )
 
2.4.2  Ordinals (continued)
 
Theoremordon 4486 The class of all ordinal numbers is ordinal. Proposition 7.12 of [TakeutiZaring] p. 38, but without using the Axiom of Regularity. (Contributed by NM, 17-May-1994.)
 |- 
 Ord  On
 
Theoremssorduni 4487 The union of a class of ordinal numbers is ordinal. Proposition 7.19 of [TakeutiZaring] p. 40. (Contributed by NM, 30-May-1994.) (Proof shortened by Andrew Salmon, 12-Aug-2011.)
 |-  ( A  C_  On  ->  Ord  U. A )
 
Theoremssonuni 4488 The union of a set of ordinal numbers is an ordinal number. Theorem 9 of [Suppes] p. 132. (Contributed by NM, 1-Nov-2003.)
 |-  ( A  e.  V  ->  ( A  C_  On  ->  U. A  e.  On ) )
 
Theoremssonunii 4489 The union of a set of ordinal numbers is an ordinal number. Corollary 7N(d) of [Enderton] p. 193. (Contributed by NM, 20-Sep-2003.)
 |-  A  e.  _V   =>    |-  ( A  C_  On  ->  U. A  e.  On )
 
Theoremonun2 4490 The union of two ordinal numbers is an ordinal number. (Contributed by Jim Kingdon, 25-Jul-2019.)
 |-  ( ( A  e.  On  /\  B  e.  On )  ->  ( A  u.  B )  e.  On )
 
Theoremonun2i 4491 The union of two ordinal numbers is an ordinal number. (Contributed by NM, 13-Jun-1994.) (Constructive proof by Jim Kingdon, 25-Jul-2019.)
 |-  A  e.  On   &    |-  B  e.  On   =>    |-  ( A  u.  B )  e.  On
 
Theoremordsson 4492 Any ordinal class is a subclass of the class of ordinal numbers. Corollary 7.15 of [TakeutiZaring] p. 38. (Contributed by NM, 18-May-1994.)
 |-  ( Ord  A  ->  A 
 C_  On )
 
Theoremonss 4493 An ordinal number is a subset of the class of ordinal numbers. (Contributed by NM, 5-Jun-1994.)
 |-  ( A  e.  On  ->  A  C_  On )
 
Theoremonuni 4494 The union of an ordinal number is an ordinal number. (Contributed by NM, 29-Sep-2006.)
 |-  ( A  e.  On  ->  U. A  e.  On )
 
Theoremorduni 4495 The union of an ordinal class is ordinal. (Contributed by NM, 12-Sep-2003.)
 |-  ( Ord  A  ->  Ord  U. A )
 
Theorembm2.5ii 4496* Problem 2.5(ii) of [BellMachover] p. 471. (Contributed by NM, 20-Sep-2003.)
 |-  A  e.  _V   =>    |-  ( A  C_  On  ->  U. A  =  |^| { x  e.  On  |  A. y  e.  A  y  C_  x } )
 
Theoremsucexb 4497 A successor exists iff its class argument exists. (Contributed by NM, 22-Jun-1998.)
 |-  ( A  e.  _V  <->  suc  A  e.  _V )
 
Theoremsucexg 4498 The successor of a set is a set (generalization). (Contributed by NM, 5-Jun-1994.)
 |-  ( A  e.  V  ->  suc  A  e.  _V )
 
Theoremsucex 4499 The successor of a set is a set. (Contributed by NM, 30-Aug-1993.)
 |-  A  e.  _V   =>    |-  suc  A  e.  _V
 
Theoremordsucim 4500 The successor of an ordinal class is ordinal. (Contributed by Jim Kingdon, 8-Nov-2018.)
 |-  ( Ord  A  ->  Ord 
 suc  A )
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