P. 89 of Theorem List - Metamath Proof Explorer

Theorem List for Metamath Proof Explorer - 8801-8900   *Has distinct variable group(s)

Type	Label	Description
Statement
Theorem	hsmexlem5 8801*	Lemma for hsmex 8803. Combining the above constraints, along with itunitc 8792 and tcrank 8293, 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 ) ) )
Theorem	hsmexlem6 8802*	Lemma for hsmex 8803. (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
Theorem	hsmex 8803*	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 8009. (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 )
Theorem	hsmex2 8804*	The set of hereditary size-limited sets, assuming ax-reg 8009. (Contributed by Stefan O'Rear, 11-Feb-2015.)
|- ( X e. V -> { s | A. x e. ( TC ` { s } ) x ~<_ X } e. _V )
Theorem	hsmex3 8805*	The set of hereditary size-limited sets, assuming ax-reg 8009, using strict comparison (an easy corollary 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 8830, as well as weaker forms such as the axiom of countable choice ax-cc 8806 and dependent choice ax-dc 8817. 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 Zermelo-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 satisfy intuitionistic 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
3.1.1  Introduce the Axiom of Countable Choice
Axiom	ax-cc 8806*	The axiom of countable choice (CC), also known as the axiom of denumerable choice. It is clearly a special case of ac5 8848, but is weak enough that it can be proven using DC (see axcc 8829). It is, however, strictly stronger than ZF and cannot be proven in ZF. It states that any countable collection of nonempty 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 ) )
Theorem	axcc2lem 8807*	Lemma for axcc2 8808. (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 ) ) )
Theorem	axcc2 8808*	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 ) ) )
Theorem	axcc3 8809*	A possibly more useful version of ax-cc 8806 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 ) )
Theorem	axcc4 8810*	A version of axcc3 8809 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 ) )
Theorem	acncc 8811	An ax-cc 8806 equivalent: every set has choice sets of length om. (Contributed by Mario Carneiro, 31-Aug-2015.)
|- AC om = _V
Theorem	axcc4dom 8812*	Relax the constraint on axcc4 8810 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 ) )
Theorem	domtriomlem 8813*	Lemma for domtriom 8814. (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 )
Theorem	domtriom 8814	Trichotomy of equinumerosity for om, proven using CC. Equivalently, all Dedekind-finite sets (as in isfin4-2 8685) are finite in the usual sense and conversely. (Contributed by Mario Carneiro, 9-Feb-2013.)
|- A e. _V   =>    |- ( om ~<_ A <-> -. A ~< om )
Theorem	fin41 8815	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
Theorem	dominf 8816	A nonempty set that is a subset of its union is infinite. This version is proved from ax-cc 8806. See dominfac 8939 for a version proved from ax-ac 8830. The axiom of Regularity is used for this proof, via inf3lem6 8041, 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 )
3.1.2  Introduce the Axiom of Dependent Choice
Axiom	ax-dc 8817*	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 8892. 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 ) )
Theorem	dcomex 8818	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
Theorem	axdc2lem 8819*	Lemma for axdc2 8820. 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 8817 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 ) ) ) )
Theorem	axdc2 8820*	An apparent strengthening of ax-dc 8817 (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 ) ) ) )
Theorem	axdc3lem 8821*	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
Theorem	axdc3lem2 8822*	Lemma for axdc3 8825. 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 8817 (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 8817 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 ) ) ) )
Theorem	axdc3lem3 8823*	Simple substitution lemma for axdc3 8825. (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 ) ) ) )
Theorem	axdc3lem4 8824*	Lemma for axdc3 8825. 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 8817 (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 8817 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 8820. See axdc3lem2 8822 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 ) ) ) )
Theorem	axdc3 8825*	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 ) ) ) )
Theorem	axdc4lem 8826*	Lemma for axdc4 8827. (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 ) ) ) )
Theorem	axdc4 8827*	A more general version of axdc3 8825 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 ) ) ) )
Theorem	axcclem 8828*	Lemma for axcc 8829. (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 ) )
Theorem	axcc 8829*	Although CC can be proven trivially using ac5 8848, 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
Axiom	ax-ac 8830*	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 nonempty 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 8833 for a more detailed explanation. Theorem ac2 8832 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 8836 is slightly shorter when the biconditional of ax-ac 8830 is expanded into implication and negation. In axac3 8835 we allow the constant CHOICE to represent the Axiom of Choice; this simplifies the representation of theorems like gchac 9050 (the Generalized Continuum Hypothesis implies the Axiom of Choice). Standard textbook versions of AC are derived as ac8 8863, ac5 8848, and ac7 8844. The Axiom of Regularity ax-reg 8009 (among others) is used to derive our version from the standard ones; this reverse derivation is shown as theorem dfac2 8502. Equivalents to AC are the well-ordering theorem weth 8866 and Zorn's lemma zorn 8878. See ac4 8846 for comments about stronger versions of AC. In order to avoid uses of ax-reg 8009 for derivation of AC equivalents, we provide ax-ac2 8834 (due to Kurt Maes), which is equivalent to the standard AC of textbooks. The derivation of ax-ac2 8834 from ax-ac 8830 is shown by theorem axac2 8837, and the reverse derivation by axac 8838. Therefore, new proofs should normally use ax-ac2 8834 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 ) )
Theorem	zfac 8831*	Axiom of Choice expressed with the fewest number of different variables. The penultimate step shows the logical equivalence to ax-ac 8830. (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 ) )
Theorem	ac2 8832*	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 8833 is easier to understand.) Note: aceq0 8490 shows the logical equivalence to ax-ac 8830. (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 )
Theorem	ac3 8833*	Axiom of Choice using abbreviations. The logical equivalence to ax-ac 8830 can be established by chaining aceq0 8490 and aceq2 8491. A standard textbook version of AC is derived from this one in dfac2a 8501, and this version of AC is derived from the textbook version in dfac2 8502. 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 nonempty 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 8026. The key theorem for this (used in the proof of dfac2 8502) is preleq 8025. 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 ) )
Axiom	ax-ac2 8834*	In order to avoid uses of ax-reg 8009 for derivation of AC equivalents, we provide ax-ac2 8834, 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 8836. We removed the leading quantifier to make it slightly shorter, since we have ax-gen 1596 available. The derivation of ax-ac2 8834 from ax-ac 8830 is shown by theorem axac2 8837, and the reverse derivation by axac 8838. Note that we use ax-reg 8009 to derive ax-ac 8830 from ax-ac2 8834, but not to derive ax-ac2 8834 from ax-ac 8830. (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 ) ) ) ) )
Theorem	axac3 8835	This theorem asserts that the constant CHOICE is a theorem, thus eliminating it as a hypothesis while assuming ax-ac2 8834 as an axiom. (Contributed by Mario Carneiro, 6-May-2015.) (Revised by NM, 20-Dec-2016.) (Proof modification is discouraged.)
|- CHOICE
Theorem	ackm 8836*	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 8537. 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 ) ) ) ) )
Theorem	axac2 8837*	Derive ax-ac2 8834 from ax-ac 8830. (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 ) ) ) ) )
Theorem	axac 8838*	Derive ax-ac 8830 from ax-ac2 8834. Note that ax-reg 8009 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 ) )
Theorem	axaci 8839	Apply a choice equivalent. (Contributed by Mario Carneiro, 17-May-2015.)
|- (CHOICE <-> A. x ph )   =>    |- ph
Theorem	cardeqv 8840	All sets are well-orderable under choice. (Contributed by Mario Carneiro, 28-Apr-2015.)
|- dom card = _V
Theorem	numth3 8841	All sets are well-orderable under choice. (Contributed by Stefan O'Rear, 28-Feb-2015.)
|- ( A e. V -> A e. dom card )
Theorem	numth2 8842*	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
Theorem	numth 8843*	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
Theorem	ac7 8844*	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 )
Theorem	ac7g 8845*	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 ) )
Theorem	ac4 8846*	Equivalent of Axiom of Choice. We do not insist that f be a function. However, theorem ac5 8848, 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 8862. (Contributed by NM, 21-Jul-1996.)
|- E. f A. z e. x ( z =/= (/) -> ( f ` z ) e. z )
Theorem	ac4c 8847*	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 )
Theorem	ac5 8848*	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 8846. (Contributed by NM, 29-Aug-1999.)
|- A e. _V   =>    |- E. f ( f Fn A /\ A. x e. A ( x =/= (/) -> ( f ` x ) e. x ) )
Theorem	ac5b 8849*	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 ) )
Theorem	ac6num 8850*	A version of ac6 8851 which takes the choice as a hypothesis. (Contributed by Mario Carneiro, 27-Aug-2015.)
|- ( y = ( f ` x ) -> ( ph <-> ps ) )   =>    |- ( ( A e. V /\ U_ x e. A { y e. B | ph } e. dom card /\ A. x e. A E. y e. B ph ) -> E. f ( f : A --> B /\ A. x e. A ps ) )
Theorem	ac6 8851*	Equivalent of Axiom of Choice. This is useful for proving that there exists, for example, a sequence mapping natural numbers to members of a larger set B, where ph depends on x (the natural number) and y (to specify a member of B). A stronger version of this theorem, ac6s 8855, allows B to be a proper class. (Contributed by NM, 18-Oct-1999.) (Revised by Mario Carneiro, 27-Aug-2015.)
|- A e. _V   &    |- B e. _V   &    |- ( y = ( f ` x ) -> ( ph <-> ps ) )   =>    |- ( A. x e. A E. y e. B ph -> E. f ( f : A --> B /\ A. x e. A ps ) )
Theorem	ac6c4 8852*	Equivalent of Axiom of Choice. B is a collection B ( x ) of nonempty sets. (Contributed by Mario Carneiro, 22-Mar-2013.)
|- A e. _V   &    |- B e. _V   =>    |- ( A. x e. A B =/= (/) -> E. f ( f Fn A /\ A. x e. A ( f ` x ) e. B ) )
Theorem	ac6c5 8853*	Equivalent of Axiom of Choice. B is a collection B ( x ) of nonempty sets. Remark after Theorem 10.46 of [TakeutiZaring] p. 98. (Contributed by Mario Carneiro, 22-Mar-2013.)
|- A e. _V   &    |- B e. _V   =>    |- ( A. x e. A B =/= (/) -> E. f A. x e. A ( f ` x ) e. B )
Theorem	ac9 8854*	An Axiom of Choice equivalent: the infinite Cartesian product of nonempty classes is nonempty. Axiom of Choice (second form) of [Enderton] p. 55 and its converse. (Contributed by Mario Carneiro, 22-Mar-2013.)
|- A e. _V   &    |- B e. _V   =>    |- ( A. x e. A B =/= (/) <-> X_ x e. A B =/= (/) )
Theorem	ac6s 8855*	Equivalent of Axiom of Choice. Using the Boundedness Axiom bnd2 8302, we derive this strong version of ac6 8851 that doesn't require B to be a set. (Contributed by NM, 4-Feb-2004.)
|- A e. _V   &    |- ( y = ( f ` x ) -> ( ph <-> ps ) )   =>    |- ( A. x e. A E. y e. B ph -> E. f ( f : A --> B /\ A. x e. A ps ) )
Theorem	ac6n 8856*	Equivalent of Axiom of Choice. Contrapositive of ac6s 8855. (Contributed by NM, 10-Jun-2007.)
|- A e. _V   &    |- ( y = ( f ` x ) -> ( ph <-> ps ) )   =>    |- ( A. f ( f : A --> B -> E. x e. A ps ) -> E. x e. A A. y e. B ph )
Theorem	ac6s2 8857*	Generalization of the Axiom of Choice to classes. Slightly strengthened version of ac6s3 8858. (Contributed by NM, 29-Sep-2006.)
|- A e. _V   &    |- ( y = ( f ` x ) -> ( ph <-> ps ) )   =>    |- ( A. x e. A E. y ph -> E. f ( f Fn A /\ A. x e. A ps ) )
Theorem	ac6s3 8858*	Generalization of the Axiom of Choice to classes. Theorem 10.46 of [TakeutiZaring] p. 97. (Contributed by NM, 3-Nov-2004.)
|- A e. _V   &    |- ( y = ( f ` x ) -> ( ph <-> ps ) )   =>    |- ( A. x e. A E. y ph -> E. f A. x e. A ps )
Theorem	ac6sg 8859*	ac6s 8855 with sethood as antecedent. (Contributed by FL, 3-Aug-2009.)
|- ( y = ( f ` x ) -> ( ph <-> ps ) )   =>    |- ( A e. V -> ( A. x e. A E. y e. B ph -> E. f ( f : A --> B /\ A. x e. A ps ) ) )
Theorem	ac6sf 8860*	Version of ac6 8851 with bound-variable hypothesis. (Contributed by NM, 2-Mar-2008.)
|- F/ y ps   &    |- A e. _V   &    |- ( y = ( f ` x ) -> ( ph <-> ps ) )   =>    |- ( A. x e. A E. y e. B ph -> E. f ( f : A --> B /\ A. x e. A ps ) )
Theorem	ac6s4 8861*	Generalization of the Axiom of Choice to proper classes. B is a collection B ( x ) of nonempty, possible proper classes. (Contributed by NM, 29-Sep-2006.)
|- A e. _V   =>    |- ( A. x e. A B =/= (/) -> E. f ( f Fn A /\ A. x e. A ( f ` x ) e. B ) )
Theorem	ac6s5 8862*	Generalization of the Axiom of Choice to proper classes. B is a collection B ( x ) of nonempty, possible proper classes. Remark after Theorem 10.46 of [TakeutiZaring] p. 98. (Contributed by NM, 27-Mar-2006.)
|- A e. _V   =>    |- ( A. x e. A B =/= (/) -> E. f A. x e. A ( f ` x ) e. B )
Theorem	ac8 8863*	An Axiom of Choice equivalent. Given a family x of mutually disjoint nonempty sets, there exists a set y containing exactly one member from each set in the family. Theorem 6M(4) of [Enderton] p. 151. (Contributed by NM, 14-May-2004.)
|- ( ( A. z e. x z =/= (/) /\ A. z e. x A. w e. x ( z =/= w -> ( z i^i w ) = (/) ) ) -> E. y A. z e. x E! v v e. ( z i^i y ) )
Theorem	ac9s 8864*	An Axiom of Choice equivalent: the infinite Cartesian product of nonempty classes is nonempty. Axiom of Choice (second form) of [Enderton] p. 55 and its converse. This is a stronger version of the axiom in Enderton, with no existence requirement for the family of classes B ( x ) (achieved via the Collection Principle cp 8300). (Contributed by NM, 29-Sep-2006.)
|- A e. _V   =>    |- ( A. x e. A B =/= (/) <-> X_ x e. A B =/= (/) )
3.2.2  AC equivalents: well-ordering, Zorn's lemma
Theorem	numthcor 8865*	Any set is strictly dominated by some ordinal. (Contributed by NM, 22-Oct-2003.)
|- ( A e. V -> E. x e. On A ~< x )
Theorem	weth 8866*	Well-ordering theorem: any set A can be well-ordered. This is an equivalent of the Axiom of Choice. Theorem 6 of [Suppes] p. 242. First proved by Ernst Zermelo (the "Z" in ZFC) in 1904. (Contributed by Mario Carneiro, 5-Jan-2013.)
|- ( A e. V -> E. x x We A )
Theorem	zorn2lem1 8867*	Lemma for zorn2 8877. (Contributed by NM, 3-Apr-1997.) (Revised by Mario Carneiro, 9-May-2015.)
|- F = recs ( ( f e. _V |-> ( iota_ v e. C A. u e. C -. u w v ) ) )   &    |- C = { z e. A | A. g e. ran f g R z }   &    |- D = { z e. A | A. g e. ( F " x ) g R z }   =>    |- ( ( x e. On /\ ( w We A /\ D =/= (/) ) ) -> ( F ` x ) e. D )
Theorem	zorn2lem2 8868*	Lemma for zorn2 8877. (Contributed by NM, 3-Apr-1997.) (Revised by Mario Carneiro, 9-May-2015.)
|- F = recs ( ( f e. _V |-> ( iota_ v e. C A. u e. C -. u w v ) ) )   &    |- C = { z e. A | A. g e. ran f g R z }   &    |- D = { z e. A | A. g e. ( F " x ) g R z }   =>    |- ( ( x e. On /\ ( w We A /\ D =/= (/) ) ) -> ( y e. x -> ( F ` y ) R ( F ` x ) ) )
Theorem	zorn2lem3 8869*	Lemma for zorn2 8877. (Contributed by NM, 3-Apr-1997.) (Revised by Mario Carneiro, 9-May-2015.)
|- F = recs ( ( f e. _V |-> ( iota_ v e. C A. u e. C -. u w v ) ) )   &    |- C = { z e. A | A. g e. ran f g R z }   &    |- D = { z e. A | A. g e. ( F " x ) g R z }   =>    |- ( ( R Po A /\ ( x e. On /\ ( w We A /\ D =/= (/) ) ) ) -> ( y e. x -> -. ( F ` x ) = ( F ` y ) ) )
Theorem	zorn2lem4 8870*	Lemma for zorn2 8877. (Contributed by NM, 3-Apr-1997.) (Revised by Mario Carneiro, 9-May-2015.)
|- F = recs ( ( f e. _V |-> ( iota_ v e. C A. u e. C -. u w v ) ) )   &    |- C = { z e. A | A. g e. ran f g R z }   &    |- D = { z e. A | A. g e. ( F " x ) g R z }   =>    |- ( ( R Po A /\ w We A ) -> E. x e. On D = (/) )
Theorem	zorn2lem5 8871*	Lemma for zorn2 8877. (Contributed by NM, 4-Apr-1997.) (Revised by Mario Carneiro, 9-May-2015.)
|- F = recs ( ( f e. _V |-> ( iota_ v e. C A. u e. C -. u w v ) ) )   &    |- C = { z e. A | A. g e. ran f g R z }   &    |- D = { z e. A | A. g e. ( F " x ) g R z }   &    |- H = { z e. A | A. g e. ( F " y ) g R z }   =>    |- ( ( ( w We A /\ x e. On ) /\ A. y e. x H =/= (/) ) -> ( F " x ) C_ A )
Theorem	zorn2lem6 8872*	Lemma for zorn2 8877. (Contributed by NM, 4-Apr-1997.) (Revised by Mario Carneiro, 9-May-2015.)
|- F = recs ( ( f e. _V |-> ( iota_ v e. C A. u e. C -. u w v ) ) )   &    |- C = { z e. A | A. g e. ran f g R z }   &    |- D = { z e. A | A. g e. ( F " x ) g R z }   &    |- H = { z e. A | A. g e. ( F " y ) g R z }   =>    |- ( R Po A -> ( ( ( w We A /\ x e. On ) /\ A. y e. x H =/= (/) ) -> R Or ( F " x ) ) )
Theorem	zorn2lem7 8873*	Lemma for zorn2 8877. (Contributed by NM, 6-Apr-1997.) (Revised by Mario Carneiro, 9-May-2015.)
|- F = recs ( ( f e. _V |-> ( iota_ v e. C A. u e. C -. u w v ) ) )   &    |- C = { z e. A | A. g e. ran f g R z }   &    |- D = { z e. A | A. g e. ( F " x ) g R z }   &    |- H = { z e. A | A. g e. ( F " y ) g R z }   =>    |- ( ( A e. dom card /\ R Po A /\ A. s ( ( s C_ A /\ R Or s ) -> E. a e. A A. r e. s ( r R a \/ r = a ) ) ) -> E. a e. A A. b e. A -. a R b )
Theorem	zorn2g 8874*	Zorn's Lemma of [Monk1] p. 117. This version of zorn2 8877 avoids the Axiom of Choice by assuming that A is well-orderable. (Contributed by NM, 6-Apr-1997.) (Revised by Mario Carneiro, 9-May-2015.)
|- ( ( A e. dom card /\ R Po A /\ A. w ( ( w C_ A /\ R Or w ) -> E. x e. A A. z e. w ( z R x \/ z = x ) ) ) -> E. x e. A A. y e. A -. x R y )
Theorem	zorng 8875*	Zorn's Lemma. If the union of every chain (with respect to inclusion) in a set belongs to the set, then the set contains a maximal element. Theorem 6M of [Enderton] p. 151. This version of zorn 8878 avoids the Axiom of Choice by assuming that A is well-orderable. (Contributed by NM, 12-Aug-2004.) (Revised by Mario Carneiro, 9-May-2015.)
|- ( ( A e. dom card /\ A. z ( ( z C_ A /\ [ C.] Or z ) -> U. z e. A ) ) -> E. x e. A A. y e. A -. x C. y )
Theorem	zornn0g 8876*	Variant of Zorn's lemma zorng 8875 in which (/), the union of the empty chain, is not required to be an element of A. (Contributed by Jeff Madsen, 5-Jan-2011.) (Revised by Mario Carneiro, 9-May-2015.)
|- ( ( A e. dom card /\ A =/= (/) /\ A. z ( ( z C_ A /\ z =/= (/) /\ [ C.] Or z ) -> U. z e. A ) ) -> E. x e. A A. y e. A -. x C. y )
Theorem	zorn2 8877*	Zorn's Lemma of [Monk1] p. 117. This theorem is equivalent to the Axiom of Choice and states that every partially ordered set A (with an ordering relation R) in which every totally ordered subset has an upper bound, contains at least one maximal element. The main proof consists of lemmas zorn2lem1 8867 through zorn2lem7 8873; this final piece mainly changes bound variables to eliminate the hypotheses of zorn2lem7 8873. (Contributed by NM, 6-Apr-1997.) (Revised by Mario Carneiro, 9-May-2015.)
|- A e. _V   =>    |- ( ( R Po A /\ A. w ( ( w C_ A /\ R Or w ) -> E. x e. A A. z e. w ( z R x \/ z = x ) ) ) -> E. x e. A A. y e. A -. x R y )
Theorem	zorn 8878*	Zorn's Lemma. If the union of every chain (with respect to inclusion) in a set belongs to the set, then the set contains a maximal element. This theorem is equivalent to the Axiom of Choice. Theorem 6M of [Enderton] p. 151. See zorn2 8877 for a version with general partial orderings. (Contributed by NM, 12-Aug-2004.)
|- A e. _V   =>    |- ( A. z ( ( z C_ A /\ [ C.] Or z ) -> U. z e. A ) -> E. x e. A A. y e. A -. x C. y )
Theorem	zornn0 8879*	Variant of Zorn's lemma zorn 8878 in which (/), the union of the empty chain, is not required to be an element of A. (Contributed by Jeff Madsen, 5-Jan-2011.)
|- A e. _V   =>    |- ( ( A =/= (/) /\ A. z ( ( z C_ A /\ z =/= (/) /\ [ C.] Or z ) -> U. z e. A ) ) -> E. x e. A A. y e. A -. x C. y )
Theorem	ttukeylem1 8880*	Lemma for ttukey 8889. Expand out the property of being an element of a property of finite character. (Contributed by Mario Carneiro, 15-May-2015.)
|- ( ph -> F : ( card ` ( U. A \ B ) ) -1-1-onto-> ( U. A \ B ) )   &    |- ( ph -> B e. A )   &    |- ( ph -> A. x ( x e. A <-> ( ~P x i^i Fin ) C_ A ) )   =>    |- ( ph -> ( C e. A <-> ( ~P C i^i Fin ) C_ A ) )
Theorem	ttukeylem2 8881*	Lemma for ttukey 8889. A property of finite character is closed under subsets. (Contributed by Mario Carneiro, 15-May-2015.)
|- ( ph -> F : ( card ` ( U. A \ B ) ) -1-1-onto-> ( U. A \ B ) )   &    |- ( ph -> B e. A )   &    |- ( ph -> A. x ( x e. A <-> ( ~P x i^i Fin ) C_ A ) )   =>    |- ( ( ph /\ ( C e. A /\ D C_ C ) ) -> D e. A )
Theorem	ttukeylem3 8882*	Lemma for ttukey 8889. (Contributed by Mario Carneiro, 11-May-2015.)
|- ( ph -> F : ( card ` ( U. A \ B ) ) -1-1-onto-> ( U. A \ B ) )   &    |- ( ph -> B e. A )   &    |- ( ph -> A. x ( x e. A <-> ( ~P x i^i Fin ) C_ A ) )   &    |- G = recs ( ( z e. _V |-> if ( dom z = U. dom z , if ( dom z = (/) , B , U. ran z ) , ( ( z ` U. dom z ) u. if ( ( ( z ` U. dom z ) u. { ( F ` U. dom z ) } ) e. A , { ( F ` U. dom z ) } , (/) ) ) ) ) )   =>    |- ( ( ph /\ C e. On ) -> ( G ` C ) = if ( C = U. C , if ( C = (/) , B , U. ( G " C ) ) , ( ( G ` U. C ) u. if ( ( ( G ` U. C ) u. { ( F ` U. C ) } ) e. A , { ( F ` U. C ) } , (/) ) ) ) )
Theorem	ttukeylem4 8883*	Lemma for ttukey 8889. (Contributed by Mario Carneiro, 15-May-2015.)
|- ( ph -> F : ( card ` ( U. A \ B ) ) -1-1-onto-> ( U. A \ B ) )   &    |- ( ph -> B e. A )   &    |- ( ph -> A. x ( x e. A <-> ( ~P x i^i Fin ) C_ A ) )   &    |- G = recs ( ( z e. _V |-> if ( dom z = U. dom z , if ( dom z = (/) , B , U. ran z ) , ( ( z ` U. dom z ) u. if ( ( ( z ` U. dom z ) u. { ( F ` U. dom z ) } ) e. A , { ( F ` U. dom z ) } , (/) ) ) ) ) )   =>    |- ( ph -> ( G ` (/) ) = B )
Theorem	ttukeylem5 8884*	Lemma for ttukey 8889. The G function forms a (transfinitely long) chain of inclusions. (Contributed by Mario Carneiro, 15-May-2015.)
|- ( ph -> F : ( card ` ( U. A \ B ) ) -1-1-onto-> ( U. A \ B ) )   &    |- ( ph -> B e. A )   &    |- ( ph -> A. x ( x e. A <-> ( ~P x i^i Fin ) C_ A ) )   &    |- G = recs ( ( z e. _V |-> if ( dom z = U. dom z , if ( dom z = (/) , B , U. ran z ) , ( ( z ` U. dom z ) u. if ( ( ( z ` U. dom z ) u. { ( F ` U. dom z ) } ) e. A , { ( F ` U. dom z ) } , (/) ) ) ) ) )   =>    |- ( ( ph /\ ( C e. On /\ D e. On /\ C C_ D ) ) -> ( G ` C ) C_ ( G ` D ) )
Theorem	ttukeylem6 8885*	Lemma for ttukey 8889. (Contributed by Mario Carneiro, 15-May-2015.)
|- ( ph -> F : ( card ` ( U. A \ B ) ) -1-1-onto-> ( U. A \ B ) )   &    |- ( ph -> B e. A )   &    |- ( ph -> A. x ( x e. A <-> ( ~P x i^i Fin ) C_ A ) )   &    |- G = recs ( ( z e. _V |-> if ( dom z = U. dom z , if ( dom z = (/) , B , U. ran z ) , ( ( z ` U. dom z ) u. if ( ( ( z ` U. dom z ) u. { ( F ` U. dom z ) } ) e. A , { ( F ` U. dom z ) } , (/) ) ) ) ) )   =>    |- ( ( ph /\ C e. suc ( card ` ( U. A \ B ) ) ) -> ( G ` C ) e. A )
Theorem	ttukeylem7 8886*	Lemma for ttukey 8889. (Contributed by Mario Carneiro, 15-May-2015.)
|- ( ph -> F : ( card ` ( U. A \ B ) ) -1-1-onto-> ( U. A \ B ) )   &    |- ( ph -> B e. A )   &    |- ( ph -> A. x ( x e. A <-> ( ~P x i^i Fin ) C_ A ) )   &    |- G = recs ( ( z e. _V |-> if ( dom z = U. dom z , if ( dom z = (/) , B , U. ran z ) , ( ( z ` U. dom z ) u. if ( ( ( z ` U. dom z ) u. { ( F ` U. dom z ) } ) e. A , { ( F ` U. dom z ) } , (/) ) ) ) ) )   =>    |- ( ph -> E. x e. A ( B C_ x /\ A. y e. A -. x C. y ) )
Theorem	ttukey2g 8887*	The Teichmüller-Tukey Lemma ttukey 8889 with a slightly stronger conclusion: we can set up the maximal element of A so that it also contains some given B e. A as a subset. (Contributed by Mario Carneiro, 15-May-2015.)
|- ( ( U. A e. dom card /\ B e. A /\ A. x ( x e. A <-> ( ~P x i^i Fin ) C_ A ) ) -> E. x e. A ( B C_ x /\ A. y e. A -. x C. y ) )
Theorem	ttukeyg 8888*	The Teichmüller-Tukey Lemma ttukey 8889 stated with the "choice" as an antecedent (the hypothesis U. A e. dom card says that U. A is well-orderable). (Contributed by Mario Carneiro, 15-May-2015.)
|- ( ( U. A e. dom card /\ A =/= (/) /\ A. x ( x e. A <-> ( ~P x i^i Fin ) C_ A ) ) -> E. x e. A A. y e. A -. x C. y )
Theorem	ttukey 8889*	The Teichmüller-Tukey Lemma, an Axiom of Choice equivalent. If A is a nonempty collection of finite character, then A has a maximal element with respect to inclusion. Here "finite character" means that x e. A iff every finite subset of x is in A. (Contributed by Mario Carneiro, 15-May-2015.)
|- A e. _V   =>    |- ( ( A =/= (/) /\ A. x ( x e. A <-> ( ~P x i^i Fin ) C_ A ) ) -> E. x e. A A. y e. A -. x C. y )
Theorem	axdclem 8890*	Lemma for axdc 8892. (Contributed by Mario Carneiro, 25-Jan-2013.)
|- F = ( rec ( ( y e. _V |-> ( g ` { z | y x z } ) ) , s ) |` om )   =>    |- ( ( A. y e. ~P dom x ( y =/= (/) -> ( g ` y ) e. y ) /\ ran x C_ dom x /\ E. z ( F ` K ) x z ) -> ( K e. om -> ( F ` K ) x ( F ` suc K ) ) )
Theorem	axdclem2 8891*	Lemma for axdc 8892. Using the full Axiom of Choice, we can construct a choice function g on ~P dom x. From this, we can build a sequence F starting at any value s e. dom x by repeatedly applying g to the set ( F ` x ) (where x is the value from the previous iteration). (Contributed by Mario Carneiro, 25-Jan-2013.)
|- F = ( rec ( ( y e. _V |-> ( g ` { z | y x z } ) ) , s ) |` om )   =>    |- ( E. z s x z -> ( ran x C_ dom x -> E. f A. n e. om ( f ` n ) x ( f ` suc n ) ) )
Theorem	axdc 8892*	This theorem derives ax-dc 8817 using ax-ac 8830 and ax-inf 8046. Thus, AC implies DC, but not vice-versa (so that ZFC is strictly stronger than ZF+DC). (New usage is discouraged.) (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 ) )
Theorem	fodom 8893	An onto function implies dominance of domain over range. Lemma 10.20 of [Kunen] p. 30. This theorem uses the Axiom of Choice ac7g 8845. AC is not needed for finite sets - see fodomfi 7790. See also fodomnum 8429. (Contributed by NM, 23-Jul-2004.)
|- A e. _V   =>    |- ( F : A -onto-> B -> B ~<_ A )
Theorem	fodomg 8894	An onto function implies dominance of domain over range. (Contributed by NM, 23-Jul-2004.)
|- ( A e. C -> ( F : A -onto-> B -> B ~<_ A ) )
Theorem	fodomb 8895*	Equivalence of an onto mapping and dominance for a nonempty set. Proposition 10.35 of [TakeutiZaring] p. 93. (Contributed by NM, 29-Jul-2004.)
|- ( ( A =/= (/) /\ E. f f : A -onto-> B ) <-> ( (/) ~< B /\ B ~<_ A ) )
Theorem	wdomac 8896	When assuming AC, weak and usual dominance coincide. It is not known if this is an AC equivalent. (Contributed by Stefan O'Rear, 11-Feb-2015.) (Revised by Mario Carneiro, 5-May-2015.)
|- ( X ~<_* Y <-> X ~<_ Y )
Theorem	brdom3 8897*	Equivalence to a dominance relation. (Contributed by NM, 27-Mar-2007.)
|- B e. _V   =>    |- ( A ~<_ B <-> E. f ( A. x E* y x f y /\ A. x e. A E. y e. B y f x ) )
Theorem	brdom5 8898*	An equivalence to a dominance relation. (Contributed by NM, 29-Mar-2007.)
|- B e. _V   =>    |- ( A ~<_ B <-> E. f ( A. x e. B E* y x f y /\ A. x e. A E. y e. B y f x ) )
Theorem	brdom4 8899*	An equivalence to a dominance relation. (Contributed by NM, 28-Mar-2007.) (Revised by NM, 16-Jun-2017.)
|- B e. _V   =>    |- ( A ~<_ B <-> E. f ( A. x e. B E* y e. A x f y /\ A. x e. A E. y e. B y f x ) )
Theorem	brdom7disj 8900*	An equivalence to a dominance relation for disjoint sets. (Contributed by NM, 29-Mar-2007.) (Revised by NM, 16-Jun-2017.)
|- A e. _V   &    |- B e. _V   &    |- ( A i^i B ) = (/)   =>    |- ( A ~<_ B <-> E. f ( A. x e. B E* y e. A { x , y } e. f /\ A. x e. A E. y e. B { y , x } e. f ) )