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Theorem List for Metamath Proof Explorer - 8001-8100   *Has distinct variable group(s)
TypeLabelDescription
Statement

Theoremisf34lem3 8001* Lemma for isfin3-4 8008. (Contributed by Stefan O'Rear, 7-Nov-2014.) (Revised by Mario Carneiro, 17-May-2015.)

Theoremcompss 8002* Express image under of the complementation isomorphism. (Contributed by Stefan O'Rear, 5-Nov-2014.) (Proof shortened by Mario Carneiro, 17-May-2015.)

Theoremisf34lem4 8003* Lemma for isfin3-4 8008. (Contributed by Stefan O'Rear, 7-Nov-2014.) (Revised by Mario Carneiro, 17-May-2015.)

Theoremisf34lem5 8004* Lemma for isfin3-4 8008. (Contributed by Stefan O'Rear, 7-Nov-2014.) (Revised by Mario Carneiro, 17-May-2015.)

Theoremisf34lem7 8005* Lemma for isfin3-4 8008. (Contributed by Stefan O'Rear, 7-Nov-2014.)
FinIII

Theoremisf34lem6 8006* Lemma for isfin3-4 8008. (Contributed by Stefan O'Rear, 7-Nov-2014.) (Revised by Mario Carneiro, 17-May-2015.)
FinIII

Theoremfin34i 8007* Inference from isfin3-4 8008. (Contributed by Mario Carneiro, 17-May-2015.)
FinIII

Theoremisfin3-4 8008* Weakly Dedekind-infinite sets are exactly those with an -indexed ascending chain of subsets. (Contributed by Stefan O'Rear, 7-Nov-2014.) (Proof shortened by Mario Carneiro, 17-May-2015.)
FinIII

Theoremfin11a 8009 Every I-finite set is Ia-finite. (Contributed by Stefan O'Rear, 30-Oct-2014.) (Revised by Mario Carneiro, 17-May-2015.)
FinIa

Theoremenfin1ai 8010 Ia-finiteness is a cardinal property. (Contributed by Mario Carneiro, 18-May-2015.)
FinIa FinIa

Theoremisfin1-2 8011 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.)
FinIV

Theoremisfin1-3 8012 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.)
[]

Theoremisfin1-4 8013 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.)
[]

Theoremdffin1-5 8014 Compact quantifier-free version of the standard definition df-fin 6867. (Contributed by Stefan O'Rear, 6-Jan-2015.)

Theoremfin23 8015 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 is a set which is II-finite but not III-finite.

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

By II-finiteness (as fin2i2 7944) this sequence contains its intersection, call it ; since by induction every subset in the sequence is non-empty, the intersection must be non-empty.

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

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

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 of the sets from each stage. Great caution is required to avoid ax-dc 8072 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 without the axiom.

This 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.)

FinII FinIII

Theoremfin34 8016 Every III-finite set is IV-finite. (Contributed by Stefan O'Rear, 30-Oct-2014.)
FinIII FinIV

Theoremisfin5-2 8017 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.)
FinV

Theoremfin45 8018 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.)
FinIV FinV

Theoremfin56 8019 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.)
FinV FinVI

Theoremfin17 8020 Every I-finite set is VII-finite. (Contributed by Mario Carneiro, 17-May-2015.)
FinVII

Theoremfin67 8021 Every VI-finite set is VII-finite. (Contributed by Stefan O'Rear, 29-Oct-2014.) (Revised by Mario Carneiro, 17-May-2015.)
FinVI FinVII

Theoremisfin7-2 8022 A set is VII-finite iff it is non-well-orderable or finite. (Contributed by Mario Carneiro, 17-May-2015.)
FinVII

Theoremfin71num 8023 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.)
FinVII

Theoremdffin7-2 8024 Class form of isfin7-2 8022. (Contributed by Mario Carneiro, 17-May-2015.)
FinVII

Theoremdfacfin7 8025 Axiom of Choice equivalent: the VII-finite sets are the same as I-finite sets. (Contributed by Mario Carneiro, 18-May-2015.)
CHOICE FinVII

Theoremfin1a2lem1 8026 Lemma for fin1a2 8041. (Contributed by Stefan O'Rear, 7-Nov-2014.)

Theoremfin1a2lem2 8027 Lemma for fin1a2 8041. (Contributed by Stefan O'Rear, 7-Nov-2014.)

Theoremfin1a2lem3 8028 Lemma for fin1a2 8041. (Contributed by Stefan O'Rear, 7-Nov-2014.)

Theoremfin1a2lem4 8029 Lemma for fin1a2 8041. (Contributed by Stefan O'Rear, 7-Nov-2014.)

Theoremfin1a2lem5 8030 Lemma for fin1a2 8041. (Contributed by Stefan O'Rear, 7-Nov-2014.)

Theoremfin1a2lem6 8031 Lemma for fin1a2 8041. Establish that can be broken into two equipollent pieces. (Contributed by Stefan O'Rear, 7-Nov-2014.)

Theoremfin1a2lem7 8032* Lemma for fin1a2 8041. Split a III-infinite set in two pieces. (Contributed by Stefan O'Rear, 7-Nov-2014.)
FinIII FinIII FinIII

Theoremfin1a2lem8 8033* Lemma for fin1a2 8041. Split a III-infinite set in two pieces. (Contributed by Stefan O'Rear, 7-Nov-2014.)
FinIII FinIII FinIII

Theoremfin1a2lem9 8034* Lemma for fin1a2 8041. In a chain of finite sets, initial segments are finite. (Contributed by Stefan O'Rear, 8-Nov-2014.)
[]

Theoremfin1a2lem10 8035 Lemma for fin1a2 8041. A nonempty finite union of members of a chain is a member of the chain. (Contributed by Stefan O'Rear, 8-Nov-2014.)
[]

Theoremfin1a2lem11 8036* Lemma for fin1a2 8041. (Contributed by Stefan O'Rear, 8-Nov-2014.)
[]

Theoremfin1a2lem12 8037 Lemma for fin1a2 8041. (Contributed by Stefan O'Rear, 8-Nov-2014.) (Revised by Mario Carneiro, 17-May-2015.)
[] FinIII

Theoremfin1a2lem13 8038 Lemma for fin1a2 8041. (Contributed by Stefan O'Rear, 8-Nov-2014.) (Revised by Mario Carneiro, 17-May-2015.)
[] FinII

Theoremfin12 8039 Weak theorem which skips Ia but has a trivial proof, needed to prove fin1a2 8041. (Contributed by Stefan O'Rear, 8-Nov-2014.) (Revised by Mario Carneiro, 17-May-2015.)
FinII

Theoremfin1a2s 8040* 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.)
FinII FinII

Theoremfin1a2 8041 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.)
FinIa FinII

2.6.13  Hereditarily size-limited sets without Choice

Theoremitunifval 8042* 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.)

Theoremitunifn 8043* Functionality of the iterated union. (Contributed by Stefan O'Rear, 11-Feb-2015.)

Theoremituni0 8044* A zero-fold iterated union. (Contributed by Stefan O'Rear, 11-Feb-2015.)

Theoremitunisuc 8045* Successor iterated union. (Contributed by Stefan O'Rear, 11-Feb-2015.)

Theoremitunitc1 8046* Each union iterate is a member of the transitive closure. (Contributed by Stefan O'Rear, 11-Feb-2015.)

Theoremitunitc 8047* The union of all union iterates creates the transitive closure; compare trcl 7410. (Contributed by Stefan O'Rear, 11-Feb-2015.)

Theoremituniiun 8048* Unwrap an iterated union from the "other end". (Contributed by Stefan O'Rear, 11-Feb-2015.)

Theoremhsmexlem7 8049* Lemma for hsmex 8058. Properties of the recurrent sequence of ordinals. (Contributed by Stefan O'Rear, 14-Feb-2015.)
har har        har

Theoremhsmexlem8 8050* Lemma for hsmex 8058. Properties of the recurrent sequence of ordinals. (Contributed by Stefan O'Rear, 14-Feb-2015.)
har har        har

Theoremhsmexlem9 8051* Lemma for hsmex 8058. Properties of the recurrent sequence of ordinals. (Contributed by Stefan O'Rear, 14-Feb-2015.)
har har

Theoremhsmexlem1 8052 Lemma for hsmex 8058. 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.)
OrdIso        * har

Theoremhsmexlem2 8053* Lemma for hsmex 8058. Bound the order type of a union of sets of ordinals, each of limited order type. Vaguely reminiscent of unictb 8197 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.)
OrdIso        OrdIso        har

Theoremhsmexlem3 8054* Lemma for hsmex 8058. Clear 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.)
OrdIso        OrdIso        * har

Theoremhsmexlem4 8055* Lemma for hsmex 8058. The core induction, establishing bounds on the order types of iterated unions of the initial set. (Contributed by Stefan O'Rear, 14-Feb-2015.)
har har                      OrdIso

Theoremhsmexlem5 8056* Lemma for hsmex 8058. Combining the above constraints, along with itunitc 8047 and tcrank 7554, gives an effective constraint on the rank of . (Contributed by Stefan O'Rear, 14-Feb-2015.)
har har                      OrdIso        har

Theoremhsmexlem6 8057* Lemmr for hsmex 8058. (Contributed by Stefan O'Rear, 14-Feb-2015.)
har har                      OrdIso

Theoremhsmex 8058* 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 7306. (Contributed by Stefan O'Rear, 14-Feb-2015.)

Theoremhsmex2 8059* The set of hereditary size-limited sets, assuming ax-reg 7306. (Contributed by Stefan O'Rear, 11-Feb-2015.)

Theoremhsmex3 8060* The set of hereditary size-limited sets, assuming ax-reg 7306, using strict comparison (an easy corrolary by separation). (Contributed by Stefan O'Rear, 11-Feb-2015.)

PART 3  ZFC (ZERMELO-FRAENKEL WITH CHOICE) SET THEORY

In this section we add the Axiom of Choice ax-ac 8085, as well as weaker forms such as the axiom of countable choice ax-cc 8061 and dependent choice ax-dc 8072. 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 8061* The axiom of countable choice (CC), also known as the axiom of denumerable choice. It is clearly a special case of ac5 8104, but is weak enough that it can be proven using DC (see axcc 8084). 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.)

Theoremaxcc2lem 8062* Lemma for axcc2 8063. (Contributed by Mario Carneiro, 8-Feb-2013.)

Theoremaxcc2 8063* 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.)

Theoremaxcc3 8064* A possibly more useful version of ax-cc 8061 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.) (Revised by Mario Carneiro, 26-Dec-2014.)

Theoremaxcc4 8065* A version of axcc3 8064 that uses wffs instead of classes. (Contributed by Mario Carneiro, 7-Apr-2013.)

Theoremacncc 8066 An ax-cc 8061 equivalent: every set has choice sets of length . (Contributed by Mario Carneiro, 31-Aug-2015.)
AC

Theoremaxcc4dom 8067* Relax the constraint on axcc4 8065 to dominance instead of equinumerosity. (Contributed by Mario Carneiro, 18-Jan-2014.)

Theoremdomtriomlem 8068* Lemma for domtriom 8069. (Contributed by Mario Carneiro, 9-Feb-2013.)

Theoremdomtriom 8069 Trichotomy of equinumerosity for , proven using CC. Equivalently, all Dedekind-finite sets (as in isfin4-2 7940) are finite in the usual sense and conversely. (Contributed by Mario Carneiro, 9-Feb-2013.)

Theoremfin41 8070 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

Theoremdominf 8071 A nonempty set that is a subset of its union is infinite. This version is proved from ax-cc 8061. See dominfac 8195 for a version proved from ax-ac 8085. The axiom of Regularity is used for this proof, via inf3lem6 7334, and its use is necessary: otherwise the set or (where the second example even has nonempty well-founded part) provides a counterexample. (Contributed by Mario Carneiro, 9-Feb-2013.)

Axiomax-dc 8072* 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 8148. 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.)

Theoremdcomex 8073 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.)

Theoremaxdc2lem 8074* Lemma for axdc2 8075. We construct a relation based on such that iff , and show that the "function" described by ax-dc 8072 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.)

Theoremaxdc2 8075* An apparent strengthening of ax-dc 8072 (but derived from it) which shows that there is a denumerable sequence for any function that maps elements of a set to nonempty subsets of such that for all . 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.)

Theoremaxdc3lem 8076* The class of finite approximations to the DC sequence is a set. (We derive here the stronger statement that is a subset of a specific set, namely .) (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.)

Theoremaxdc3lem2 8077* Lemma for axdc3 8080. We have constructed a "candidate set" , which consists of all finite sequences that satisfy our property of interest, namely on its domain, but with the added constraint that . These sets are possible "initial segments" of the infinite sequence satisfying these constraints, but we can leverage the standard ax-dc 8072 (with no initial condition) to select a sequence of ever-lengthening finite sequences, namely (for some integer ). 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 8072 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 , we can construct the sequence that we are after. (Contributed by Mario Carneiro, 30-Jan-2013.)

Theoremaxdc3lem3 8078* Simple substitution lemma for axdc3 8080. (Contributed by Mario Carneiro, 27-Jan-2013.)

Theoremaxdc3lem4 8079* Lemma for axdc3 8080. We have constructed a "candidate set" , which consists of all finite sequences that satisfy our property of interest, namely on its domain, but with the added constraint that . These sets are possible "initial segments" of the infinite sequence satisfying these constraints, but we can leverage the standard ax-dc 8072 (with no initial condition) to select a sequence of ever-lengthening finite sequences, namely (for some integer ). 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 8072 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 is nonempty, and that always maps to a nonempty subset of , so that we can apply axdc2 8075. See axdc3lem2 8077 for the rest of the proof. (Contributed by Mario Carneiro, 27-Jan-2013.)

Theoremaxdc3 8080* Dependent Choice. Axiom DC1 of [Schechter] p. 149, with the addition of an initial value . 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.)

Theoremaxdc4lem 8081* Lemma for axdc4 8082. (Contributed by Mario Carneiro, 31-Jan-2013.) (Revised by Mario Carneiro, 16-Nov-2013.)

Theoremaxdc4 8082* A more general version of axdc3 8080 that allows the function to vary with . (Contributed by Mario Carneiro, 31-Jan-2013.)

Theoremaxcclem 8083* Lemma for axcc 8084. (Contributed by Mario Carneiro, 2-Feb-2013.) (Revised by Mario Carneiro, 16-Nov-2013.)

Theoremaxcc 8084* Although CC can be proven trivially using ac5 8104, we prove it here using DC. (New usage is discouraged.) (Contributed by Mario Carneiro, 2-Feb-2013.)

3.2  ZFC Set Theory - add the Axiom of Choice

3.2.1  Introduce the Axiom of Choice

Axiomax-ac 8085* 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 , there exists a that is a collection of unordered pairs, one pair for each non-empty member of . One entry in the pair is the member of , and the other entry is some arbitrary member of that member of . See the rewritten version ac3 8088 for a more detailed explanation. Theorem ac2 8087 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 8092 is slightly shorter when the biconditional of ax-ac 8085 is expanded into implication and negation. In axac3 8090 we allow the constant CHOICE to represent the Axiom of Choice; this simplifies the representation of theorems like gchac 8295 (the Generalized Continuum Hypothesis implies the Axiom of Choice).

Standard textbook versions of AC are derived as ac8 8119, ac5 8104, and ac7 8100. The Axiom of Regularity ax-reg 7306 (among others) is used to derive our version from the standard ones; this reverse derivation is shown as theorem dfac2 7757. Equivalents to AC are the well-ordering theorem weth 8122 and Zorn's lemma zorn 8134. See ac4 8102 for comments about stronger versions of AC.

In order to avoid uses of ax-reg 7306 for derivation of AC equivalents, we provide ax-ac2 8089 (due to Kurt Maes), which is equivalent to the standard AC of textbooks. The derivation of ax-ac2 8089 from ax-ac 8085 is shown by theorem axac2 8093, and the reverse derivation by axac 8094. Therefore, new proofs should normally use ax-ac2 8089 instead. (New usage is discouraged.) (Contributed by NM, 18-Jul-1996.)

Theoremzfac 8086* Axiom of Choice expressed with fewest number of different variables. The penultimate step shows the logical equivalence to ax-ac 8085. (New usage is discouraged.) (Contributed by NM, 14-Aug-2003.)

Theoremac2 8087* 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 8088 is easier to understand.) Note: aceq0 7745 shows the logical equivalence to ax-ac 8085. (New usage is discouraged.) (Contributed by NM, 18-Jul-1996.)

Theoremac3 8088* Axiom of Choice using abbreviations. The logical equivalence to ax-ac 8085 can be established by chaining aceq0 7745 and aceq2 7746. A standard textbook version of AC is derived from this one in dfac2a 7756, and this version of AC is derived from the textbook version in dfac2 7757.

The following sketch will help you understand this version of the axiom. Given any set , the axiom says that there exists a that is a collection of unordered pairs, one pair for each non-empty member of . One entry in the pair is the member of , and the other entry is some arbitrary member of that member of . Using the Axiom of Regularity, we can show that is really a set of ordered pairs, very similar to the ordered pair construction opthreg 7319. The key theorem for this (used in the proof of dfac2 7757) is preleq 7318. With this modified definition of ordered pair, it can be seen that is actually a choice function on the members of .

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

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

Axiomax-ac2 8089* In order to avoid uses of ax-reg 7306 for derivation of AC equivalents, we provide ax-ac2 8089, 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 8092. We removed the leading quantifier to make it slightly shorter, since we have ax-gen 1533 available. The derivation of ax-ac2 8089 from ax-ac 8085 is shown by theorem axac2 8093, and the reverse derivation by axac 8094. Note that we use ax-reg 7306 to derive ax-ac 8085 from ax-ac2 8089, but not to derive ax-ac2 8089 from ax-ac 8085. (Contributed by NM, 19-Dec-2016.)

Theoremaxac3 8090 This theorem asserts that the constant CHOICE is a theorem, thus eliminating it as a hypothesis while assuming ax-ac2 8089 as an axiom. (Contributed by Mario Carneiro, 6-May-2015.) (Revised by NM, 20-Dec-2016.) (Proof modification is discouraged.)
CHOICE

Theoremaxac3OLD 8091 This theorem asserts that the constant CHOICE is a theorem, thus eliminating it as a hypothesis while assuming ax-ac 8085 as an axiom. Obsolete as of 20-Dec-2016. (Contributed by Mario Carneiro, 6-May-2015.) (New usage is discouraged.)
CHOICE

Theoremackm 8092* A remarkable equivalent to the Axiom of Choice that has only 5 quantifiers (when expanded to , 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 7792. 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 (,=)-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.)

Theoremaxac2 8093* Derive ax-ac2 8089 from ax-ac 8085. (Contributed by NM, 19-Dec-2016.) (New usage is discouraged.) (Proof modification is discouraged.)

Theoremaxac 8094* Derive ax-ac 8085 from ax-ac2 8089. Note that ax-reg 7306 is used by the proof. (Contributed by NM, 19-Dec-2016.) (Proof modification is discouraged.)

Theoremaxaci 8095 Apply a choice equivalent. (Contributed by Mario Carneiro, 17-May-2015.)
CHOICE

Theoremcardeqv 8096 All sets are well-orderable under choice. (Contributed by Mario Carneiro, 28-Apr-2015.)

Theoremnumth3 8097 All sets are well-orderable under choice. (Contributed by Stefan O'Rear, 28-Feb-2015.)

Theoremnumth2 8098* Numeration theorem: any set is equinumerous to some ordinal (using AC). Theorem 10.3 of [TakeutiZaring] p. 84. (Contributed by NM, 20-Oct-2003.)

Theoremnumth 8099* 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.)

Theoremac7 8100* An Axiom of Choice equivalent similar to the Axiom of Choice (first form) of [Enderton] p. 49. (Contributed by NM, 29-Apr-2004.)

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