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Theorem List for Metamath Proof Explorer - 8001-8100   *Has distinct variable group(s)
TypeLabelDescription
Statement
 
Theoremisf34lem6 8001* Lemma for isfin3-4 8003. (Contributed by Stefan O'Rear, 7-Nov-2014.) (Revised by Mario Carneiro, 17-May-2015.)
 |-  F  =  ( x  e.  ~P A  |->  ( A  \  x ) )   =>    |-  ( A  e.  V  ->  ( A  e. FinIII  <->  A. f  e.  ( ~P A  ^m  om )
 ( A. y  e.  om  ( f `  y
 )  C_  ( f `  suc  y )  ->  U. ran  f  e.  ran  f ) ) )
 
Theoremfin34i 8002* Inference from isfin3-4 8003. (Contributed by Mario Carneiro, 17-May-2015.)
 |-  ( ( A  e. FinIII  /\  G : om --> ~P A  /\  A. x  e.  om  ( G `
  x )  C_  ( G `  suc  x ) )  ->  U. ran  G  e.  ran  G )
 
Theoremisfin3-4 8003* Weakly Dedekind-infinite sets are exactly those with an  om-indexed ascending chain of subsets. (Contributed by Stefan O'Rear, 7-Nov-2014.) (Proof shortened by Mario Carneiro, 17-May-2015.)
 |-  ( A  e.  V  ->  ( A  e. FinIII  <->  A. f  e.  ( ~P A  ^m  om )
 ( A. x  e.  om  ( f `  x )  C_  ( f `  suc  x )  ->  U. ran  f  e.  ran  f ) ) )
 
Theoremfin11a 8004 Every I-finite set is Ia-finite. (Contributed by Stefan O'Rear, 30-Oct-2014.) (Revised by Mario Carneiro, 17-May-2015.)
 |-  ( A  e.  Fin  ->  A  e. FinIa )
 
Theoremenfin1ai 8005 Ia-finiteness is a cardinal property. (Contributed by Mario Carneiro, 18-May-2015.)
 |-  ( A  ~~  B  ->  ( A  e. FinIa  ->  B  e. FinIa ) )
 
Theoremisfin1-2 8006 A set is finite in the usual sense iff the power set of its power set is Dedekind finite. (Contributed by Stefan O'Rear, 3-Nov-2014.) (Revised by Mario Carneiro, 17-May-2015.)
 |-  ( A  e.  Fin  <->  ~P ~P A  e. FinIV )
 
Theoremisfin1-3 8007 A set is I-finite iff every system of subsets contains a maximal subset. Definition I of [Levy58] p. 2. (Contributed by Stefan O'Rear, 4-Nov-2014.) (Proof shortened by Mario Carneiro, 17-May-2015.)
 |-  ( A  e.  V  ->  ( A  e.  Fin  <->  `' [ C.] 
 Fr  ~P A ) )
 
Theoremisfin1-4 8008 A set is I-finite iff every system of subsets contains a minimal subset. (Contributed by Stefan O'Rear, 4-Nov-2014.) (Revised by Mario Carneiro, 17-May-2015.)
 |-  ( A  e.  V  ->  ( A  e.  Fin  <-> [ C.]  Fr 
 ~P A ) )
 
Theoremdffin1-5 8009 Compact quantifier-free version of the standard definition df-fin 6862. (Contributed by Stefan O'Rear, 6-Jan-2015.)
 |- 
 Fin  =  (  ~~  " om )
 
Theoremfin23 8010 Every II-finite set (every chain of subsets has a maximal element) is III-finite (has no denumerable collection of subsets). The proof here is the only one I could find, from http://matwbn.icm.edu.pl/ksiazki/fm/fm6/fm619.pdf p.94 (writeup by Tarski, credited to Kuratowski). Translated into English and modern notation, the proof proceeds as follows (variables renamed for uniqueness):

Suppose for a contradiction that  A is a set which is II-finite but not III-finite.

For any countable sequence of distinct subsets  T of  A, we can form a decreasing sequence of non-empty subsets  ( U `  T ) by taking finite intersections of initial segments of  T while skipping over any element of  T which would cause the intersection to be empty.

By II-finiteness (as fin2i2 7939) this sequence contains its intersection, call it  Y; since by induction every subset in the sequence  U is non-empty, the intersection must be non-empty.

Suppose that an element  X of  T has non-empty intersection with  Y. Thus said element has a non-empty intersection with the corresponding element of  U, therefore it was used in the construction of  U and all further elements of  U are subsets of  X, thus  X contains the  Y. That is, all elements of  X either contain  Y or are disjoint from it.

Since there are only two cases, there must exist an infinite subset of  T which uniformly either contain  Y or are disjoint from it. In the former case we can create an infinite set by subtracting  Y from each element. In either case, call the result  Z; this is an infinite set of subsets of 
A, each of which is disjoint from  Y and contained in the union of  T; the union of 
Z is strictly contained in the union of  T, because only the latter is a superset of the non-empty set  Y.

The preceeding four steps may be iterated a countable number of times starting from the assumed denumerable set of subsets to produce a denumerable sequence  B of the  T sets from each stage. Great caution is required to avoid ax-dc 8067 here; in particular an effective version of the pigeonhole principle (for aleph-null pigeons and 2 holes) is required. Since a denumerable set of subsets is assumed to exist, we can conclude  om  e.  _V without the axiom.

This  B sequence is strictly decreasing, thus it has no minimum, contradicting the first assumption. (Contributed by Stefan O'Rear, 2-Nov-2014.) (Proof shortened by Mario Carneiro, 17-May-2015.)

 |-  ( A  e. FinII  ->  A  e. FinIII )
 
Theoremfin34 8011 Every III-finite set is IV-finite. (Contributed by Stefan O'Rear, 30-Oct-2014.)
 |-  ( A  e. FinIII  ->  A  e. FinIV )
 
Theoremisfin5-2 8012 Alternate definition of V-finite which emphasizes the idempotent behavior of V-infinite sets. (Contributed by Stefan O'Rear, 30-Oct-2014.) (Revised by Mario Carneiro, 17-May-2015.)
 |-  ( A  e.  V  ->  ( A  e. FinV  <->  -.  ( A  =/=  (/)  /\  A  ~~  ( A  +c  A ) ) ) )
 
Theoremfin45 8013 Every IV-finite set is V-finite: if we can pack two copies of the set into itself, we can certainly leave space. (Contributed by Stefan O'Rear, 30-Oct-2014.) (Proof shortened by Mario Carneiro, 18-May-2015.)
 |-  ( A  e. FinIV  ->  A  e. FinV
 )
 
Theoremfin56 8014 Every V-finite set is VI-finite because multiplication dominates addition for cardinals. (Contributed by Stefan O'Rear, 29-Oct-2014.) (Revised by Mario Carneiro, 17-May-2015.)
 |-  ( A  e. FinV  ->  A  e. FinVI )
 
Theoremfin17 8015 Every I-finite set is VII-finite. (Contributed by Mario Carneiro, 17-May-2015.)
 |-  ( A  e.  Fin  ->  A  e. FinVII )
 
Theoremfin67 8016 Every VI-finite set is VII-finite. (Contributed by Stefan O'Rear, 29-Oct-2014.) (Revised by Mario Carneiro, 17-May-2015.)
 |-  ( A  e. FinVI  ->  A  e. FinVII )
 
Theoremisfin7-2 8017 A set is VII-finite iff it is non-well-orderable or finite. (Contributed by Mario Carneiro, 17-May-2015.)
 |-  ( A  e.  V  ->  ( A  e. FinVII  <->  ( A  e.  dom  card  ->  A  e.  Fin ) ) )
 
Theoremfin71num 8018 A well-orderable set is VII-finite iff it is I-finite. Thus even without choice, on the class of well-orderable sets all eight definitions of finite set coincide. (Contributed by Mario Carneiro, 18-May-2015.)
 |-  ( A  e.  dom  card 
 ->  ( A  e. FinVII  <->  A  e.  Fin ) )
 
Theoremdffin7-2 8019 Class form of isfin7-2 8017. (Contributed by Mario Carneiro, 17-May-2015.)
 |- FinVII  =  ( Fin  u.  ( _V  \  dom  card )
 )
 
Theoremdfacfin7 8020 Axiom of Choice equivalent: the VII-finite sets are the same as I-finite sets. (Contributed by Mario Carneiro, 18-May-2015.)
 |-  (CHOICE  <-> FinVII  =  Fin )
 
Theoremfin1a2lem1 8021 Lemma for fin1a2 8036. (Contributed by Stefan O'Rear, 7-Nov-2014.)
 |-  S  =  ( x  e.  On  |->  suc  x )   =>    |-  ( A  e.  On  ->  ( S `  A )  =  suc  A )
 
Theoremfin1a2lem2 8022 Lemma for fin1a2 8036. (Contributed by Stefan O'Rear, 7-Nov-2014.)
 |-  S  =  ( x  e.  On  |->  suc  x )   =>    |-  S : On -1-1-> On
 
Theoremfin1a2lem3 8023 Lemma for fin1a2 8036. (Contributed by Stefan O'Rear, 7-Nov-2014.)
 |-  E  =  ( x  e.  om  |->  ( 2o 
 .o  x ) )   =>    |-  ( A  e.  om  ->  ( E `  A )  =  ( 2o  .o  A ) )
 
Theoremfin1a2lem4 8024 Lemma for fin1a2 8036. (Contributed by Stefan O'Rear, 7-Nov-2014.)
 |-  E  =  ( x  e.  om  |->  ( 2o 
 .o  x ) )   =>    |-  E : om -1-1-> om
 
Theoremfin1a2lem5 8025 Lemma for fin1a2 8036. (Contributed by Stefan O'Rear, 7-Nov-2014.)
 |-  E  =  ( x  e.  om  |->  ( 2o 
 .o  x ) )   =>    |-  ( A  e.  om  ->  ( A  e.  ran  E  <->  -. 
 suc  A  e.  ran  E ) )
 
Theoremfin1a2lem6 8026 Lemma for fin1a2 8036. Establish that  om can be broken into two equipollent pieces. (Contributed by Stefan O'Rear, 7-Nov-2014.)
 |-  E  =  ( x  e.  om  |->  ( 2o 
 .o  x ) )   &    |-  S  =  ( x  e.  On  |->  suc  x )   =>    |-  ( S  |`  ran  E ) : ran  E -1-1-onto-> ( om  \  ran  E )
 
Theoremfin1a2lem7 8027* Lemma for fin1a2 8036. Split a III-infinite set in two pieces. (Contributed by Stefan O'Rear, 7-Nov-2014.)
 |-  E  =  ( x  e.  om  |->  ( 2o 
 .o  x ) )   &    |-  S  =  ( x  e.  On  |->  suc  x )   =>    |-  (
 ( A  e.  V  /\  A. y  e.  ~P  A ( y  e. FinIII  \/  ( A  \  y
 )  e. FinIII ) )  ->  A  e. FinIII )
 
Theoremfin1a2lem8 8028* Lemma for fin1a2 8036. Split a III-infinite set in two pieces. (Contributed by Stefan O'Rear, 7-Nov-2014.)
 |-  ( ( A  e.  V  /\  A. x  e. 
 ~P  A ( x  e. FinIII  \/  ( A  \  x )  e. FinIII ) )  ->  A  e. FinIII )
 
Theoremfin1a2lem9 8029* Lemma for fin1a2 8036. In a chain of finite sets, initial segments are finite. (Contributed by Stefan O'Rear, 8-Nov-2014.)
 |-  ( ( [ C.]  Or  X  /\  X  C_  Fin  /\  A  e.  om )  ->  { b  e.  X  |  b  ~<_  A }  e.  Fin )
 
Theoremfin1a2lem10 8030 Lemma for fin1a2 8036. A nonempty finite union of members of a chain is a member of the chain. (Contributed by Stefan O'Rear, 8-Nov-2014.)
 |-  ( ( A  =/=  (/)  /\  A  e.  Fin  /\ [ C.] 
 Or  A )  ->  U. A  e.  A )
 
Theoremfin1a2lem11 8031* Lemma for fin1a2 8036. (Contributed by Stefan O'Rear, 8-Nov-2014.)
 |-  ( ( [ C.]  Or  A  /\  A  C_  Fin )  ->  ran  (  b  e. 
 om  |->  U. { c  e.  A  |  c  ~<_  b } )  =  ( A  u.  { (/) } )
 )
 
Theoremfin1a2lem12 8032 Lemma for fin1a2 8036. (Contributed by Stefan O'Rear, 8-Nov-2014.) (Revised by Mario Carneiro, 17-May-2015.)
 |-  ( ( ( A 
 C_  ~P B  /\ [ C.]  Or  A  /\  -.  U. A  e.  A )  /\  ( A  C_  Fin  /\  A  =/=  (/) ) )  ->  -.  B  e. FinIII )
 
Theoremfin1a2lem13 8033 Lemma for fin1a2 8036. (Contributed by Stefan O'Rear, 8-Nov-2014.) (Revised by Mario Carneiro, 17-May-2015.)
 |-  ( ( ( A 
 C_  ~P B  /\ [ C.]  Or  A  /\  -.  U. A  e.  A )  /\  ( -.  C  e.  Fin  /\  C  e.  A )
 )  ->  -.  ( B  \  C )  e. FinII )
 
Theoremfin12 8034 Weak theorem which skips Ia but has a trivial proof, needed to prove fin1a2 8036. (Contributed by Stefan O'Rear, 8-Nov-2014.) (Revised by Mario Carneiro, 17-May-2015.)
 |-  ( A  e.  Fin  ->  A  e. FinII )
 
Theoremfin1a2s 8035* An II-infinite set can have an I-infinite part broken off and remain II-infinite. (Contributed by Stefan O'Rear, 8-Nov-2014.) (Proof shortened by Mario Carneiro, 17-May-2015.)
 |-  ( ( A  e.  V  /\  A. x  e. 
 ~P  A ( x  e.  Fin  \/  ( A  \  x )  e. FinII ) )  ->  A  e. FinII )
 
Theoremfin1a2 8036 Every Ia-finite set is II-finite. Theorem 1 of [Levy58], p. 3. (Contributed by Stefan O'Rear, 8-Nov-2014.) (Proof shortened by Mario Carneiro, 17-May-2015.)
 |-  ( A  e. FinIa  ->  A  e. FinII )
 
2.6.13  Hereditarily size-limited sets without Choice
 
Theoremitunifval 8037* Function value of iterated unions. EDITORIAL: The iterated unions and order types of ordered sets are split out here because they could concievably be independently useful. (Contributed by Stefan O'Rear, 11-Feb-2015.)
 |-  U  =  ( x  e.  _V  |->  ( rec ( ( y  e. 
 _V  |->  U. y ) ,  x )  |`  om )
 )   =>    |-  ( A  e.  V  ->  ( U `  A )  =  ( rec ( ( y  e. 
 _V  |->  U. y ) ,  A )  |`  om )
 )
 
Theoremitunifn 8038* Functionality of the iterated union. (Contributed by Stefan O'Rear, 11-Feb-2015.)
 |-  U  =  ( x  e.  _V  |->  ( rec ( ( y  e. 
 _V  |->  U. y ) ,  x )  |`  om )
 )   =>    |-  ( A  e.  V  ->  ( U `  A )  Fn  om )
 
Theoremituni0 8039* A zero-fold iterated union. (Contributed by Stefan O'Rear, 11-Feb-2015.)
 |-  U  =  ( x  e.  _V  |->  ( rec ( ( y  e. 
 _V  |->  U. y ) ,  x )  |`  om )
 )   =>    |-  ( A  e.  V  ->  ( ( U `  A ) `  (/) )  =  A )
 
Theoremitunisuc 8040* Successor iterated union. (Contributed by Stefan O'Rear, 11-Feb-2015.)
 |-  U  =  ( x  e.  _V  |->  ( rec ( ( y  e. 
 _V  |->  U. y ) ,  x )  |`  om )
 )   =>    |-  ( ( U `  A ) `  suc  B )  =  U. (
 ( U `  A ) `  B )
 
Theoremitunitc1 8041* Each union iterate is a member of the transitive closure. (Contributed by Stefan O'Rear, 11-Feb-2015.)
 |-  U  =  ( x  e.  _V  |->  ( rec ( ( y  e. 
 _V  |->  U. y ) ,  x )  |`  om )
 )   =>    |-  ( ( U `  A ) `  B )  C_  ( TC `  A )
 
Theoremitunitc 8042* The union of all union iterates creates the transitive closure; compare trcl 7405. (Contributed by Stefan O'Rear, 11-Feb-2015.)
 |-  U  =  ( x  e.  _V  |->  ( rec ( ( y  e. 
 _V  |->  U. y ) ,  x )  |`  om )
 )   =>    |-  ( TC `  A )  =  U. ran  (  U `  A )
 
Theoremituniiun 8043* Unwrap an iterated union from the "other end". (Contributed by Stefan O'Rear, 11-Feb-2015.)
 |-  U  =  ( x  e.  _V  |->  ( rec ( ( y  e. 
 _V  |->  U. y ) ,  x )  |`  om )
 )   =>    |-  ( A  e.  V  ->  ( ( U `  A ) `  suc  B )  =  U_ a  e.  A  ( ( U `
  a ) `  B ) )
 
Theoremhsmexlem7 8044* Lemma for hsmex 8053. Properties of the recurrent sequence of ordinals. (Contributed by Stefan O'Rear, 14-Feb-2015.)
 |-  H  =  ( rec ( ( z  e. 
 _V  |->  (har `  ~P ( X  X.  z
 ) ) ) ,  (har `  ~P X ) )  |`  om )   =>    |-  ( H `  (/) )  =  (har `  ~P X )
 
Theoremhsmexlem8 8045* Lemma for hsmex 8053. Properties of the recurrent sequence of ordinals. (Contributed by Stefan O'Rear, 14-Feb-2015.)
 |-  H  =  ( rec ( ( z  e. 
 _V  |->  (har `  ~P ( X  X.  z
 ) ) ) ,  (har `  ~P X ) )  |`  om )   =>    |-  (
 a  e.  om  ->  ( H `  suc  a
 )  =  (har `  ~P ( X  X.  ( H `  a ) ) ) )
 
Theoremhsmexlem9 8046* Lemma for hsmex 8053. Properties of the recurrent sequence of ordinals. (Contributed by Stefan O'Rear, 14-Feb-2015.)
 |-  H  =  ( rec ( ( z  e. 
 _V  |->  (har `  ~P ( X  X.  z
 ) ) ) ,  (har `  ~P X ) )  |`  om )   =>    |-  (
 a  e.  om  ->  ( H `  a )  e.  On )
 
Theoremhsmexlem1 8047 Lemma for hsmex 8053. Bound the order type of a limited-cardinality set of ordinals. (Contributed by Stefan O'Rear, 14-Feb-2015.) (Revised by Mario Carneiro, 26-Jun-2015.)
 |-  O  = OrdIso (  _E 
 ,  A )   =>    |-  ( ( A 
 C_  On  /\  A  ~<_*  B )  ->  dom  O  e.  (har `  ~P B ) )
 
Theoremhsmexlem2 8048* Lemma for hsmex 8053. Bound the order type of a union of sets of ordinals, each of limited order type. Vaguely reminiscent of unictb 8192 but use of order types allows to canonically choose the sub-bijections, removing the choice requirement. (Contributed by Stefan O'Rear, 14-Feb-2015.) (Revised by Mario Carneiro, 26-Jun-2015.)
 |-  F  = OrdIso (  _E 
 ,  B )   &    |-  G  = OrdIso (  _E  ,  U_ a  e.  A  B )   =>    |-  ( ( A  e.  _V 
 /\  C  e.  On  /\ 
 A. a  e.  A  ( B  e.  ~P On  /\  dom  F  e.  C ) )  ->  dom  G  e.  (har `  ~P ( A  X.  C ) ) )
 
Theoremhsmexlem3 8049* Lemma for hsmex 8053. Clear  I hypothesis and extend previous result by dominance. Note that this could be substantially strengthened, e.g. using the weak Hartogs function, but all we need here is that there be *some* dominating ordinal. (Contributed by Stefan O'Rear, 14-Feb-2015.) (Revised by Mario Carneiro, 26-Jun-2015.)
 |-  F  = OrdIso (  _E 
 ,  B )   &    |-  G  = OrdIso (  _E  ,  U_ a  e.  A  B )   =>    |-  ( ( ( A  ~<_*  D 
 /\  C  e.  On )  /\  A. a  e.  A  ( B  e.  ~P On  /\  dom  F  e.  C ) )  ->  dom  G  e.  (har `  ~P ( D  X.  C ) ) )
 
Theoremhsmexlem4 8050* Lemma for hsmex 8053. The core induction, establishing bounds on the order types of iterated unions of the initial set. (Contributed by Stefan O'Rear, 14-Feb-2015.)
 |-  X  e.  _V   &    |-  H  =  ( rec ( ( z  e.  _V  |->  (har `  ~P ( X  X.  z ) ) ) ,  (har `  ~P X ) )  |`  om )   &    |-  U  =  ( x  e.  _V  |->  ( rec ( ( y  e.  _V  |->  U. y
 ) ,  x )  |`  om ) )   &    |-  S  =  { a  e.  U. ( R1 " On )  |  A. b  e.  ( TC `  { a }
 ) b  ~<_  X }   &    |-  O  = OrdIso (  _E  ,  ( rank " ( ( U `
  d ) `  c ) ) )   =>    |-  ( ( c  e. 
 om  /\  d  e.  S )  ->  dom  O  e.  ( H `  c
 ) )
 
Theoremhsmexlem5 8051* Lemma for hsmex 8053. Combining the above constraints, along with itunitc 8042 and tcrank 7549, gives an effective constraint on the rank of  S. (Contributed by Stefan O'Rear, 14-Feb-2015.)
 |-  X  e.  _V   &    |-  H  =  ( rec ( ( z  e.  _V  |->  (har `  ~P ( X  X.  z ) ) ) ,  (har `  ~P X ) )  |`  om )   &    |-  U  =  ( x  e.  _V  |->  ( rec ( ( y  e.  _V  |->  U. y
 ) ,  x )  |`  om ) )   &    |-  S  =  { a  e.  U. ( R1 " On )  |  A. b  e.  ( TC `  { a }
 ) b  ~<_  X }   &    |-  O  = OrdIso (  _E  ,  ( rank " ( ( U `
  d ) `  c ) ) )   =>    |-  ( d  e.  S  ->  ( rank `  d )  e.  (har `  ~P ( om  X. 
 U. ran  H )
 ) )
 
Theoremhsmexlem6 8052* Lemmr for hsmex 8053. (Contributed by Stefan O'Rear, 14-Feb-2015.)
 |-  X  e.  _V   &    |-  H  =  ( rec ( ( z  e.  _V  |->  (har `  ~P ( X  X.  z ) ) ) ,  (har `  ~P X ) )  |`  om )   &    |-  U  =  ( x  e.  _V  |->  ( rec ( ( y  e.  _V  |->  U. y
 ) ,  x )  |`  om ) )   &    |-  S  =  { a  e.  U. ( R1 " On )  |  A. b  e.  ( TC `  { a }
 ) b  ~<_  X }   &    |-  O  = OrdIso (  _E  ,  ( rank " ( ( U `
  d ) `  c ) ) )   =>    |-  S  e.  _V
 
Theoremhsmex 8053* The collection of hereditarily size-limited well-founded sets comprise a set. The proof is that of Randall Holmes at http://math.boisestate.edu/~holmes/holmes/hereditary.pdf, with modifications to use Hartogs' theorem instead of the weak variant (inconsequentially weakening some intermediate results), and making the well-foundedness condition explicit to avoid a direct dependence on ax-reg 7301. (Contributed by Stefan O'Rear, 14-Feb-2015.)
 |-  ( X  e.  V  ->  { s  e.  U. ( R1 " On )  |  A. x  e.  ( TC `  { s }
 ) x  ~<_  X }  e.  _V )
 
Theoremhsmex2 8054* The set of hereditary size-limited sets, assuming ax-reg 7301. (Contributed by Stefan O'Rear, 11-Feb-2015.)
 |-  ( X  e.  V  ->  { s  |  A. x  e.  ( TC ` 
 { s } ) x 
 ~<_  X }  e.  _V )
 
Theoremhsmex3 8055* The set of hereditary size-limited sets, assuming ax-reg 7301, using strict comparison (an easy corrolary by separation). (Contributed by Stefan O'Rear, 11-Feb-2015.)
 |-  ( X  e.  V  ->  { s  |  A. x  e.  ( TC ` 
 { s } ) x  ~<  X }  e.  _V )
 
PART 3  ZFC (ZERMELO-FRAENKEL WITH CHOICE) SET THEORY

In this section we add the Axiom of Choice ax-ac 8080, as well as weaker forms such as the axiom of countable choice ax-cc 8056 and dependent choice ax-dc 8067. We introduce these weaker forms so that theorems that do not need the full power of the axiom of choice, but need more than simple ZF, can use these intermediate axioms instead.

The combination of the Zermel-Fraenkel axioms and the axiom of choice is often abbreviated as ZFC. The axiom of choice is widely accepted, and ZFC is the most commonly-accepted fundamental set of axioms for mathematics.

However, there have been and still are some lingering controversies about the Axiom of Choice. The axiom of choice does not satisfy those who wish to have a constructive proof (e.g., it will not satify intuitionist logic). Thus, we make it easy to identify which proofs depend on the axiom of choice or its weaker forms.

 
3.1  ZFC Set Theory - add Countable Choice and Dependent Choice
 
Axiomax-cc 8056* The axiom of countable choice (CC), also known as the axiom of denumerable choice. It is clearly a special case of ac5 8099, but is weak enough that it can be proven using DC (see axcc 8079). It is, however, strictly stronger than ZF and cannot be proven in ZF. It states that any countable collection of non-empty sets must have a choice function. (Contributed by Mario Carneiro, 9-Feb-2013.)
 |-  ( x  ~~  om  ->  E. f A. z  e.  x  ( z  =/= 
 (/)  ->  ( f `  z )  e.  z
 ) )
 
Theoremaxcc2lem 8057* Lemma for axcc2 8058. (Contributed by Mario Carneiro, 8-Feb-2013.)
 |-  K  =  ( n  e.  om  |->  if (
 ( F `  n )  =  (/) ,  { (/)
 } ,  ( F `
  n ) ) )   &    |-  A  =  ( n  e.  om  |->  ( { n }  X.  ( K `  n ) ) )   &    |-  G  =  ( n  e.  om  |->  ( 2nd `  ( f `  ( A `  n ) ) ) )   =>    |-  E. g ( g  Fn 
 om  /\  A. n  e. 
 om  ( ( F `
  n )  =/=  (/)  ->  ( g `  n )  e.  ( F `  n ) ) )
 
Theoremaxcc2 8058* A possibly more useful version of ax-cc using sequences instead of countable sets. The Axiom of Infinity is needed to prove this, and indeed this implies the Axiom of Infinity. (Contributed by Mario Carneiro, 8-Feb-2013.)
 |- 
 E. g ( g  Fn  om  /\  A. n  e.  om  ( ( F `  n )  =/=  (/)  ->  ( g `  n )  e.  ( F `  n ) ) )
 
Theoremaxcc3 8059* A possibly more useful version of ax-cc 8056 using sequences  F
( n ) instead of countable sets. The Axiom of Infinity is needed to prove this, and indeed this implies the Axiom of Infinity. (Contributed by Mario Carneiro, 8-Feb-2013.) (Revised by Mario Carneiro, 26-Dec-2014.)
 |-  F  e.  _V   &    |-  N  ~~ 
 om   =>    |- 
 E. f ( f  Fn  N  /\  A. n  e.  N  ( F  =/=  (/)  ->  ( f `  n )  e.  F ) )
 
Theoremaxcc4 8060* A version of axcc3 8059 that uses wffs instead of classes. (Contributed by Mario Carneiro, 7-Apr-2013.)
 |-  A  e.  _V   &    |-  N  ~~ 
 om   &    |-  ( x  =  ( f `  n ) 
 ->  ( ph  <->  ps ) )   =>    |-  ( A. n  e.  N  E. x  e.  A  ph  ->  E. f
 ( f : N --> A  /\  A. n  e.  N  ps ) )
 
Theoremacncc 8061 An ax-cc 8056 equivalent: every set has choice sets of length  om. (Contributed by Mario Carneiro, 31-Aug-2015.)
 |- AC  om  =  _V
 
Theoremaxcc4dom 8062* Relax the constraint on axcc4 8060 to dominance instead of equinumerosity. (Contributed by Mario Carneiro, 18-Jan-2014.)
 |-  A  e.  _V   &    |-  ( x  =  ( f `  n )  ->  ( ph 
 <->  ps ) )   =>    |-  ( ( N  ~<_ 
 om  /\  A. n  e.  N  E. x  e.  A  ph )  ->  E. f ( f : N --> A  /\  A. n  e.  N  ps ) )
 
Theoremdomtriomlem 8063* Lemma for domtriom 8064. (Contributed by Mario Carneiro, 9-Feb-2013.)
 |-  A  e.  _V   &    |-  B  =  { y  |  ( y  C_  A  /\  y  ~~  ~P n ) }   &    |-  C  =  ( n  e.  om  |->  ( ( b `  n )  \  U_ k  e.  n  ( b `  k ) ) )   =>    |-  ( -.  A  e.  Fin  ->  om 
 ~<_  A )
 
Theoremdomtriom 8064 Trichotomy of equinumerosity for 
om, proven using CC. Equivalently, all Dedekind-finite sets (as in isfin4-2 7935) are finite in the usual sense and conversely. (Contributed by Mario Carneiro, 9-Feb-2013.)
 |-  A  e.  _V   =>    |-  ( om  ~<_  A  <->  -.  A  ~<  om )
 
Theoremfin41 8065 Under countable choice, the IV-finite sets (Dedekind-finite) coincide with I-finite (finite in the usual sense) sets. (Contributed by Mario Carneiro, 16-May-2015.)
 |- FinIV  = 
 Fin
 
Theoremdominf 8066 A nonempty set that is a subset of its union is infinite. This version is proved from ax-cc 8056. See dominfac 8190 for a version proved from ax-ac 8080. The axiom of Regularity is used for this proof, via inf3lem6 7329, and its use is necessary: otherwise the set  A  =  { A } or  A  =  { (/)
,  A } (where the second example even has nonempty well-founded part) provides a counterexample. (Contributed by Mario Carneiro, 9-Feb-2013.)
 |-  A  e.  _V   =>    |-  ( ( A  =/=  (/)  /\  A  C_  U. A )  ->  om  ~<_  A )
 
Axiomax-dc 8067* Dependent Choice. Axiom DC1 of [Schechter] p. 149. This theorem is weaker than the Axiom of Choice but is stronger than Countable Choice. It shows the existence of a sequence whose values can only be shown to exist (but cannot be constructed explicitly) and also depend on earlier values in the sequence. Dependent choice is equivalent to the statement that every (nonempty) pruned tree has a branch. This axiom is redundant in ZFC; see axdc 8143. But ZF+DC is strictly weaker than ZF+AC, so this axiom provides for theorems that do not need the full power of AC. (Contributed by Mario Carneiro, 25-Jan-2013.)
 |-  ( ( E. y E. z  y x z  /\  ran  x  C_  dom  x )  ->  E. f A. n  e.  om  ( f `  n ) x ( f `  suc  n ) )
 
Theoremdcomex 8068 The Axiom of Dependent Choice implies Infinity, the way we have stated it. Thus we have Inf+AC implies DC and DC implies Inf, but AC does not imply Inf. (Contributed by Mario Carneiro, 25-Jan-2013.)
 |- 
 om  e.  _V
 
Theoremaxdc2lem 8069* Lemma for axdc2 8070. We construct a relation  R based on  F such that  x R y iff  y  e.  ( F `
 x ), and show that the "function" described by ax-dc 8067 can be restricted so that it is a real function (since the stated properties only show that it is the superset of a function). (Contributed by Mario Carneiro, 25-Jan-2013.) (Revised by Mario Carneiro, 26-Jun-2015.)
 |-  A  e.  _V   &    |-  R  =  { <. x ,  y >.  |  ( x  e.  A  /\  y  e.  ( F `  x ) ) }   &    |-  G  =  ( x  e.  om  |->  ( h `  x ) )   =>    |-  ( ( A  =/=  (/)  /\  F : A --> ( ~P A  \  { (/) } )
 )  ->  E. g
 ( g : om --> A  /\  A. k  e. 
 om  ( g `  suc  k )  e.  ( F `  ( g `  k ) ) ) )
 
Theoremaxdc2 8070* An apparent strengthening of ax-dc 8067 (but derived from it) which shows that there is a denumerable sequence  g for any function that maps elements of a set  A to nonempty subsets of 
A such that  g (
x  +  1 )  e.  F ( g ( x ) ) for all  x  e.  om. The finitistic version of this can be proven by induction, but the infinite version requires this new axiom. (Contributed by Mario Carneiro, 25-Jan-2013.)
 |-  A  e.  _V   =>    |-  ( ( A  =/=  (/)  /\  F : A
 --> ( ~P A  \  { (/) } ) ) 
 ->  E. g ( g : om --> A  /\  A. k  e.  om  (
 g `  suc  k )  e.  ( F `  ( g `  k
 ) ) ) )
 
Theoremaxdc3lem 8071* The class  S of finite approximations to the DC sequence is a set. (We derive here the stronger statement that  S is a subset of a specific set, namely  ~P ( om  X.  A ).) (Unnecessary distinct variable restrictions were removed by David Abernethy, 18-Mar-2014.) (Contributed by Mario Carneiro, 27-Jan-2013.) (Revised by Mario Carneiro, 18-Mar-2014.)
 |-  A  e.  _V   &    |-  S  =  { s  |  E. n  e.  om  ( s : suc  n --> A  /\  ( s `  (/) )  =  C  /\  A. k  e.  n  ( s `  suc  k )  e.  ( F `  (
 s `  k )
 ) ) }   =>    |-  S  e.  _V
 
Theoremaxdc3lem2 8072* Lemma for axdc3 8075. We have constructed a "candidate set"  S, which consists of all finite sequences  s that satisfy our property of interest, namely  s ( x  + 
1 )  e.  F
( s ( x ) ) on its domain, but with the added constraint that 
s ( 0 )  =  C. These sets are possible "initial segments" of the infinite sequence satisfying these constraints, but we can leverage the standard ax-dc 8067 (with no initial condition) to select a sequence of ever-lengthening finite sequences, namely  ( h `  n ) : m --> A (for some integer  m). We let our "choice" function select a sequence whose domain is one more than the last one, and agrees with the previous one on its domain. Thus, the application of vanilla ax-dc 8067 yields a sequence of sequences whose domains increase without bound, and whose union is a function which has all the properties we want. In this lemma, we show that given the sequence  h, we can construct the sequence  g that we are after. (Contributed by Mario Carneiro, 30-Jan-2013.)
 |-  A  e.  _V   &    |-  S  =  { s  |  E. n  e.  om  ( s : suc  n --> A  /\  ( s `  (/) )  =  C  /\  A. k  e.  n  ( s `  suc  k )  e.  ( F `  (
 s `  k )
 ) ) }   &    |-  G  =  ( x  e.  S  |->  { y  e.  S  |  ( dom  y  =  suc  dom 
 x  /\  ( y  |` 
 dom  x )  =  x ) } )   =>    |-  ( E. h ( h : om
 --> S  /\  A. k  e.  om  ( h `  suc  k )  e.  ( G `  ( h `  k ) ) ) 
 ->  E. g ( g : om --> A  /\  ( g `  (/) )  =  C  /\  A. k  e.  om  ( g `  suc  k )  e.  ( F `  ( g `  k ) ) ) )
 
Theoremaxdc3lem3 8073* Simple substitution lemma for axdc3 8075. (Contributed by Mario Carneiro, 27-Jan-2013.)
 |-  A  e.  _V   &    |-  S  =  { s  |  E. n  e.  om  ( s : suc  n --> A  /\  ( s `  (/) )  =  C  /\  A. k  e.  n  ( s `  suc  k )  e.  ( F `  (
 s `  k )
 ) ) }   &    |-  B  e.  _V   =>    |-  ( B  e.  S  <->  E. m  e.  om  ( B : suc  m --> A  /\  ( B `  (/) )  =  C  /\  A. k  e.  m  ( B ` 
 suc  k )  e.  ( F `  ( B `  k ) ) ) )
 
Theoremaxdc3lem4 8074* Lemma for axdc3 8075. We have constructed a "candidate set"  S, which consists of all finite sequences  s that satisfy our property of interest, namely  s ( x  + 
1 )  e.  F
( s ( x ) ) on its domain, but with the added constraint that 
s ( 0 )  =  C. These sets are possible "initial segments" of the infinite sequence satisfying these constraints, but we can leverage the standard ax-dc 8067 (with no initial condition) to select a sequence of ever-lengthening finite sequences, namely  ( h `  n ) : m --> A (for some integer  m). We let our "choice" function select a sequence whose domain is one more than the last one, and agrees with the previous one on its domain. Thus, the application of vanilla ax-dc 8067 yields a sequence of sequences whose domains increase without bound, and whose union is a function which has all the properties we want. In this lemma, we show that  S is nonempty, and that  G always maps to a nonempty subset of  S, so that we can apply axdc2 8070. See axdc3lem2 8072 for the rest of the proof. (Contributed by Mario Carneiro, 27-Jan-2013.)
 |-  A  e.  _V   &    |-  S  =  { s  |  E. n  e.  om  ( s : suc  n --> A  /\  ( s `  (/) )  =  C  /\  A. k  e.  n  ( s `  suc  k )  e.  ( F `  (
 s `  k )
 ) ) }   &    |-  G  =  ( x  e.  S  |->  { y  e.  S  |  ( dom  y  =  suc  dom 
 x  /\  ( y  |` 
 dom  x )  =  x ) } )   =>    |-  (
 ( C  e.  A  /\  F : A --> ( ~P A  \  { (/) } )
 )  ->  E. g
 ( g : om --> A  /\  ( g `  (/) )  =  C  /\  A. k  e.  om  (
 g `  suc  k )  e.  ( F `  ( g `  k
 ) ) ) )
 
Theoremaxdc3 8075* Dependent Choice. Axiom DC1 of [Schechter] p. 149, with the addition of an initial value  C. This theorem is weaker than the Axiom of Choice but is stronger than Countable Choice. It shows the existence of a sequence whose values can only be shown to exist (but cannot be constructed explicitly) and also depend on earlier values in the sequence. (Contributed by Mario Carneiro, 27-Jan-2013.)
 |-  A  e.  _V   =>    |-  ( ( C  e.  A  /\  F : A --> ( ~P A  \  { (/) } ) ) 
 ->  E. g ( g : om --> A  /\  ( g `  (/) )  =  C  /\  A. k  e.  om  ( g `  suc  k )  e.  ( F `  ( g `  k ) ) ) )
 
Theoremaxdc4lem 8076* Lemma for axdc4 8077. (Contributed by Mario Carneiro, 31-Jan-2013.) (Revised by Mario Carneiro, 16-Nov-2013.)
 |-  A  e.  _V   &    |-  G  =  ( n  e.  om ,  x  e.  A  |->  ( { suc  n }  X.  ( n F x ) ) )   =>    |-  ( ( C  e.  A  /\  F : ( om  X.  A ) --> ( ~P A  \  { (/) } )
 )  ->  E. g
 ( g : om --> A  /\  ( g `  (/) )  =  C  /\  A. k  e.  om  (
 g `  suc  k )  e.  ( k F ( g `  k
 ) ) ) )
 
Theoremaxdc4 8077* A more general version of axdc3 8075 that allows the function  F to vary with  k. (Contributed by Mario Carneiro, 31-Jan-2013.)
 |-  A  e.  _V   =>    |-  ( ( C  e.  A  /\  F : ( om  X.  A ) --> ( ~P A  \  { (/) } )
 )  ->  E. g
 ( g : om --> A  /\  ( g `  (/) )  =  C  /\  A. k  e.  om  (
 g `  suc  k )  e.  ( k F ( g `  k
 ) ) ) )
 
Theoremaxcclem 8078* Lemma for axcc 8079. (Contributed by Mario Carneiro, 2-Feb-2013.) (Revised by Mario Carneiro, 16-Nov-2013.)
 |-  A  =  ( x 
 \  { (/) } )   &    |-  F  =  ( n  e.  om ,  y  e.  U. A  |->  ( f `  n ) )   &    |-  G  =  ( w  e.  A  |->  ( h `  suc  ( `' f `  w ) ) )   =>    |-  ( x  ~~  om  ->  E. g A. z  e.  x  ( z  =/= 
 (/)  ->  ( g `  z )  e.  z
 ) )
 
Theoremaxcc 8079* Although CC can be proven trivially using ac5 8099, we prove it here using DC. (New usage is discouraged.) (Contributed by Mario Carneiro, 2-Feb-2013.)
 |-  ( x  ~~  om  ->  E. f A. z  e.  x  ( z  =/= 
 (/)  ->  ( f `  z )  e.  z
 ) )
 
3.2  ZFC Set Theory - add the Axiom of Choice
 
3.2.1  Introduce the Axiom of Choice
 
Axiomax-ac 8080* Axiom of Choice. The Axiom of Choice (AC) is usually considered an extension of ZF set theory rather than a proper part of it. It is sometimes considered philosophically controversial because it asserts the existence of a set without telling us what the set is. ZF set theory that includes AC is called ZFC.

The unpublished version given here says that given any set  x, there exists a  y that is a collection of unordered pairs, one pair for each non-empty member of  x. One entry in the pair is the member of  x, and the other entry is some arbitrary member of that member of  x. See the rewritten version ac3 8083 for a more detailed explanation. Theorem ac2 8082 shows an equivalent written compactly with restricted quantifiers.

This version was specifically crafted to be short when expanded to primitives. Kurt Maes' 5-quantifier version ackm 8087 is slightly shorter when the biconditional of ax-ac 8080 is expanded into implication and negation. In axac3 8085 we allow the constant CHOICE to represent the Axiom of Choice; this simplifies the representation of theorems like gchac 8290 (the Generalized Continuum Hypothesis implies the Axiom of Choice).

Standard textbook versions of AC are derived as ac8 8114, ac5 8099, and ac7 8095. The Axiom of Regularity ax-reg 7301 (among others) is used to derive our version from the standard ones; this reverse derivation is shown as theorem dfac2 7752. Equivalents to AC are the well-ordering theorem weth 8117 and Zorn's lemma zorn 8129. See ac4 8097 for comments about stronger versions of AC.

In order to avoid uses of ax-reg 7301 for derivation of AC equivalents, we provide ax-ac2 8084 (due to Kurt Maes), which is equivalent to the standard AC of textbooks. The derivation of ax-ac2 8084 from ax-ac 8080 is shown by theorem axac2 8088, and the reverse derivation by axac 8089. Therefore, new proofs should normally use ax-ac2 8084 instead. (New usage is discouraged.) (Contributed by NM, 18-Jul-1996.)

 |- 
 E. y A. z A. w ( ( z  e.  w  /\  w  e.  x )  ->  E. v A. u ( E. t
 ( ( u  e.  w  /\  w  e.  t )  /\  ( u  e.  t  /\  t  e.  y )
 ) 
 <->  u  =  v ) )
 
Theoremzfac 8081* Axiom of Choice expressed with fewest number of different variables. The penultimate step shows the logical equivalence to ax-ac 8080. (New usage is discouraged.) (Contributed by NM, 14-Aug-2003.)
 |- 
 E. x A. y A. z ( ( y  e.  z  /\  z  e.  w )  ->  E. w A. y ( E. w ( ( y  e.  z  /\  z  e.  w )  /\  (
 y  e.  w  /\  w  e.  x )
 ) 
 <->  y  =  w ) )
 
Theoremac2 8082* Axiom of Choice equivalent. By using restricted quantifiers, we can express the Axiom of Choice with a single explicit conjunction. (If you want to figure it out, the rewritten equivalent ac3 8083 is easier to understand.) Note: aceq0 7740 shows the logical equivalence to ax-ac 8080. (New usage is discouraged.) (Contributed by NM, 18-Jul-1996.)
 |- 
 E. y A. z  e.  x  A. w  e.  z  E! v  e.  z  E. u  e.  y  ( z  e.  u  /\  v  e.  u )
 
Theoremac3 8083* Axiom of Choice using abbreviations. The logical equivalence to ax-ac 8080 can be established by chaining aceq0 7740 and aceq2 7741. A standard textbook version of AC is derived from this one in dfac2a 7751, and this version of AC is derived from the textbook version in dfac2 7752.

The following sketch will help you understand this version of the axiom. Given any set  x, the axiom says that there exists a  y that is a collection of unordered pairs, one pair for each non-empty member of  x. One entry in the pair is the member of 
x, and the other entry is some arbitrary member of that member of  x. Using the Axiom of Regularity, we can show that  y is really a set of ordered pairs, very similar to the ordered pair construction opthreg 7314. The key theorem for this (used in the proof of dfac2 7752) is preleq 7313. With this modified definition of ordered pair, it can be seen that  y is actually a choice function on the members of  x.

For example, suppose  x  =  { { 1 ,  2 } ,  { 1 ,  3 } ,  { 2 ,  3 ,  4 } }. Let us try  y  =  { { { 1 ,  2 } ,  1 } ,  { { 1 ,  3 } , 
1 } ,  { { 2 ,  3 ,  4 } ,  2 } }. For the member (of  x)  z  =  {
1 ,  2 }, the only assignment to  w and  v that satisfies the axiom is  w  =  1 and  v  =  { { 1 ,  2 } , 
1 }, so there is exactly one  w as required. We verify the other two members of  x similarly. Thus  y satisfies the axiom. Using our modified ordered pair definition, we can say that  y corresponds to the choice function  { <. { 1 ,  2 } , 
1 >. ,  <. { 1 ,  3 } , 
1 >. ,  <. { 2 ,  3 ,  4 } ,  2 >. }. Of course other choices for  y will also satisfy the axiom, for example  y  =  { { { 1 ,  2 } ,  2 } ,  { { 1 ,  3 } , 
1 } ,  { { 2 ,  3 ,  4 } ,  4 } }. What AC tells us is that there exists at least one such  y, but it doesn't tell us which one.

(New usage is discouraged.) (Contributed by NM, 19-Jul-1996.)

 |- 
 E. y A. z  e.  x  ( z  =/= 
 (/)  ->  E! w  e.  z  E. v  e.  y  ( z  e.  v  /\  w  e.  v ) )
 
Axiomax-ac2 8084* In order to avoid uses of ax-reg 7301 for derivation of AC equivalents, we provide ax-ac2 8084, which is equivalent to the standard AC of textbooks. This appears to be the shortest known equivalent to the standard AC when expressed in terms of set theory primitives. It was found by Kurt Maes as theorem ackm 8087. We removed the leading quantifier to make it slightly shorter, since we have ax-gen 1534 available. The derivation of ax-ac2 8084 from ax-ac 8080 is shown by theorem axac2 8088, and the reverse derivation by axac 8089. Note that we use ax-reg 7301 to derive ax-ac 8080 from ax-ac2 8084, but not to derive ax-ac2 8084 from ax-ac 8080. (Contributed by NM, 19-Dec-2016.)
 |- 
 E. y A. z E. v A. u ( ( y  e.  x  /\  ( z  e.  y  ->  ( ( v  e.  x  /\  -.  y  =  v )  /\  z  e.  v ) ) )  \/  ( -.  y  e.  x  /\  ( z  e.  x  ->  (
 ( v  e.  z  /\  v  e.  y
 )  /\  ( ( u  e.  z  /\  u  e.  y )  ->  u  =  v ) ) ) ) )
 
Theoremaxac3 8085 This theorem asserts that the constant CHOICE is a theorem, thus eliminating it as a hypothesis while assuming ax-ac2 8084 as an axiom. (Contributed by Mario Carneiro, 6-May-2015.) (Revised by NM, 20-Dec-2016.) (Proof modification is discouraged.)
 |- CHOICE
 
Theoremaxac3OLD 8086 This theorem asserts that the constant CHOICE is a theorem, thus eliminating it as a hypothesis while assuming ax-ac 8080 as an axiom. Obsolete as of 20-Dec-2016. (Contributed by Mario Carneiro, 6-May-2015.) (New usage is discouraged.)
 |- CHOICE
 
Theoremackm 8087* A remarkable equivalent to the Axiom of Choice that has only 5 quantifiers (when expanded to 
e.,  = primitives in prenex form), discovered and proved by Kurt Maes. This establishes a new record, reducing from 6 to 5 the largest number of quantified variables needed by any ZFC axiom. The ZF-equivalence to AC is shown by theorem dfackm 7787. Maes found this version of AC in April, 2004 (replacing a longer version, also with 5 quantifiers, that he found in November, 2003). See Kurt Maes, "A 5-quantifier ( e.,=)-expression ZF-equivalent to the Axiom of Choice" (http://arxiv.org/PS_cache/arxiv/pdf/0705/0705.3162v1.pdf).

The original FOM posts are: http://www.cs.nyu.edu/pipermail/fom/2003-November/007631.html http://www.cs.nyu.edu/pipermail/fom/2003-November/007641.html. (Contributed by NM, 29-Apr-2004.) (Revised by Mario Carneiro, 17-May-2015.) (Proof modification is discouraged.)

 |- 
 A. x E. y A. z E. v A. u ( ( y  e.  x  /\  (
 z  e.  y  ->  ( ( v  e.  x  /\  -.  y  =  v )  /\  z  e.  v ) ) )  \/  ( -.  y  e.  x  /\  ( z  e.  x  ->  (
 ( v  e.  z  /\  v  e.  y
 )  /\  ( ( u  e.  z  /\  u  e.  y )  ->  u  =  v ) ) ) ) )
 
Theoremaxac2 8088* Derive ax-ac2 8084 from ax-ac 8080. (Contributed by NM, 19-Dec-2016.) (New usage is discouraged.) (Proof modification is discouraged.)
 |- 
 E. y A. z E. v A. u ( ( y  e.  x  /\  ( z  e.  y  ->  ( ( v  e.  x  /\  -.  y  =  v )  /\  z  e.  v ) ) )  \/  ( -.  y  e.  x  /\  ( z  e.  x  ->  (
 ( v  e.  z  /\  v  e.  y
 )  /\  ( ( u  e.  z  /\  u  e.  y )  ->  u  =  v ) ) ) ) )
 
Theoremaxac 8089* Derive ax-ac 8080 from ax-ac2 8084. Note that ax-reg 7301 is used by the proof. (Contributed by NM, 19-Dec-2016.) (Proof modification is discouraged.)
 |- 
 E. y A. z A. w ( ( z  e.  w  /\  w  e.  x )  ->  E. v A. u ( E. t
 ( ( u  e.  w  /\  w  e.  t )  /\  ( u  e.  t  /\  t  e.  y )
 ) 
 <->  u  =  v ) )
 
Theoremaxaci 8090 Apply a choice equivalent. (Contributed by Mario Carneiro, 17-May-2015.)
 |-  (CHOICE  <->  A. x ph )   =>    |-  ph
 
Theoremcardeqv 8091 All sets are well-orderable under choice. (Contributed by Mario Carneiro, 28-Apr-2015.)
 |- 
 dom  card  =  _V
 
Theoremnumth3 8092 All sets are well-orderable under choice. (Contributed by Stefan O'Rear, 28-Feb-2015.)
 |-  ( A  e.  V  ->  A  e.  dom  card )
 
Theoremnumth2 8093* Numeration theorem: any set is equinumerous to some ordinal (using AC). Theorem 10.3 of [TakeutiZaring] p. 84. (Contributed by NM, 20-Oct-2003.)
 |-  A  e.  _V   =>    |-  E. x  e. 
 On  x  ~~  A
 
Theoremnumth 8094* Numeration theorem: every set can be put into one-to-one correspondence with some ordinal (using AC). Theorem 10.3 of [TakeutiZaring] p. 84. (Contributed by NM, 10-Feb-1997.) (Proof shortened by Mario Carneiro, 8-Jan-2015.)
 |-  A  e.  _V   =>    |-  E. x  e. 
 On  E. f  f : x -1-1-onto-> A
 
Theoremac7 8095* An Axiom of Choice equivalent similar to the Axiom of Choice (first form) of [Enderton] p. 49. (Contributed by NM, 29-Apr-2004.)
 |- 
 E. f ( f 
 C_  x  /\  f  Fn  dom  x )
 
Theoremac7g 8096* An Axiom of Choice equivalent similar to the Axiom of Choice (first form) of [Enderton] p. 49. (Contributed by NM, 23-Jul-2004.)
 |-  ( R  e.  A  ->  E. f ( f 
 C_  R  /\  f  Fn  dom  R ) )
 
Theoremac4 8097* Equivalent of Axiom of Choice. We do not insist that  f be a function. However, theorem ac5 8099, derived from this one, shows that this form of the axiom does imply that at least one such set  f whose existence we assert is in fact a function. Axiom of Choice of [TakeutiZaring] p. 83.

Takeuti and Zaring call this "weak choice" in contrast to "strong choice"  E. F A. z
( z  =/=  (/)  ->  ( F `  z )  e.  z ), which asserts the existence of a universal choice function but requires second-order quantification on (proper) class variable  F and thus cannot be expressed in our first-order formalization. However, it has been shown that ZF plus strong choice is a conservative extension of ZF plus weak choice. See Ulrich Felgner, "Comparison of the axioms of local and universal choice," Fundamenta Mathematica, 71, 43-62 (1971).

Weak choice can be strengthened in a different direction to choose from a collection of proper classes; see ac6s5 8113. (Contributed by NM, 21-Jul-1996.)

 |- 
 E. f A. z  e.  x  ( z  =/= 
 (/)  ->  ( f `  z )  e.  z
 )
 
Theoremac4c 8098* Equivalent of Axiom of Choice (class version) (Contributed by NM, 10-Feb-1997.)
 |-  A  e.  _V   =>    |-  E. f A. x  e.  A  ( x  =/=  (/)  ->  ( f `  x )  e.  x )
 
Theoremac5 8099* An Axiom of Choice equivalent: there exists a function  f (called a choice function) with domain 
A that maps each nonempty member of the domain to an element of that member. Axiom AC of [BellMachover] p. 488. Note that the assertion that  f be a function is not necessary; see ac4 8097. (Contributed by NM, 29-Aug-1999.)
 |-  A  e.  _V   =>    |-  E. f ( f  Fn  A  /\  A. x  e.  A  ( x  =/=  (/)  ->  (
 f `  x )  e.  x ) )
 
Theoremac5b 8100* Equivalent of Axiom of Choice. (Contributed by NM, 31-Aug-1999.)
 |-  A  e.  _V   =>    |-  ( A. x  e.  A  x  =/=  (/)  ->  E. f
 ( f : A --> U. A  /\  A. x  e.  A  ( f `  x )  e.  x ) )
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