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Theorem List for Metamath Proof Explorer - 8301-8400   *Has distinct variable group(s)
TypeLabelDescription
Statement

2.6.13  Hereditarily size-limited sets without Choice

Theoremitunifval 8301* 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 8302* Functionality of the iterated union. (Contributed by Stefan O'Rear, 11-Feb-2015.)

Theoremituni0 8303* A zero-fold iterated union. (Contributed by Stefan O'Rear, 11-Feb-2015.)

Theoremitunisuc 8304* Successor iterated union. (Contributed by Stefan O'Rear, 11-Feb-2015.)

Theoremitunitc1 8305* Each union iterate is a member of the transitive closure. (Contributed by Stefan O'Rear, 11-Feb-2015.)

Theoremitunitc 8306* The union of all union iterates creates the transitive closure; compare trcl 7667. (Contributed by Stefan O'Rear, 11-Feb-2015.)

Theoremituniiun 8307* Unwrap an iterated union from the "other end". (Contributed by Stefan O'Rear, 11-Feb-2015.)

Theoremhsmexlem7 8308* Lemma for hsmex 8317. Properties of the recurrent sequence of ordinals. (Contributed by Stefan O'Rear, 14-Feb-2015.)
har har        har

Theoremhsmexlem8 8309* Lemma for hsmex 8317. Properties of the recurrent sequence of ordinals. (Contributed by Stefan O'Rear, 14-Feb-2015.)
har har        har

Theoremhsmexlem9 8310* Lemma for hsmex 8317. Properties of the recurrent sequence of ordinals. (Contributed by Stefan O'Rear, 14-Feb-2015.)
har har

Theoremhsmexlem1 8311 Lemma for hsmex 8317. 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 8312* Lemma for hsmex 8317. Bound the order type of a union of sets of ordinals, each of limited order type. Vaguely reminiscent of unictb 8455 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 8313* Lemma for hsmex 8317. 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 8314* Lemma for hsmex 8317. 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 8315* Lemma for hsmex 8317. Combining the above constraints, along with itunitc 8306 and tcrank 7813, gives an effective constraint on the rank of . (Contributed by Stefan O'Rear, 14-Feb-2015.)
har har                      OrdIso        har

Theoremhsmexlem6 8316* Lemmr for hsmex 8317. (Contributed by Stefan O'Rear, 14-Feb-2015.)
har har                      OrdIso

Theoremhsmex 8317* 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 7563. (Contributed by Stefan O'Rear, 14-Feb-2015.)

Theoremhsmex2 8318* The set of hereditary size-limited sets, assuming ax-reg 7563. (Contributed by Stefan O'Rear, 11-Feb-2015.)

Theoremhsmex3 8319* The set of hereditary size-limited sets, assuming ax-reg 7563, 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 8344, as well as weaker forms such as the axiom of countable choice ax-cc 8320 and dependent choice ax-dc 8331. 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

3.1.1  Introduce the Axiom of Countable Choice

Axiomax-cc 8320* The axiom of countable choice (CC), also known as the axiom of denumerable choice. It is clearly a special case of ac5 8362, but is weak enough that it can be proven using DC (see axcc 8343). 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 8321* Lemma for axcc2 8322. (Contributed by Mario Carneiro, 8-Feb-2013.)

Theoremaxcc2 8322* 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 8323* A possibly more useful version of ax-cc 8320 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 8324* A version of axcc3 8323 that uses wffs instead of classes. (Contributed by Mario Carneiro, 7-Apr-2013.)

Theoremacncc 8325 An ax-cc 8320 equivalent: every set has choice sets of length . (Contributed by Mario Carneiro, 31-Aug-2015.)
AC

Theoremaxcc4dom 8326* Relax the constraint on axcc4 8324 to dominance instead of equinumerosity. (Contributed by Mario Carneiro, 18-Jan-2014.)

Theoremdomtriomlem 8327* Lemma for domtriom 8328. (Contributed by Mario Carneiro, 9-Feb-2013.)

Theoremdomtriom 8328 Trichotomy of equinumerosity for , proven using CC. Equivalently, all Dedekind-finite sets (as in isfin4-2 8199) are finite in the usual sense and conversely. (Contributed by Mario Carneiro, 9-Feb-2013.)

Theoremfin41 8329 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 8330 A nonempty set that is a subset of its union is infinite. This version is proved from ax-cc 8320. See dominfac 8453 for a version proved from ax-ac 8344. The axiom of Regularity is used for this proof, via inf3lem6 7591, 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.)

3.1.2  Introduce the Axiom of Dependent Choice

Axiomax-dc 8331* 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 8406. 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 8332 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 8333* Lemma for axdc2 8334. We construct a relation based on such that iff , and show that the "function" described by ax-dc 8331 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 8334* An apparent strengthening of ax-dc 8331 (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 8335* 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 8336* Lemma for axdc3 8339. 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 8331 (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 8331 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 8337* Simple substitution lemma for axdc3 8339. (Contributed by Mario Carneiro, 27-Jan-2013.)

Theoremaxdc3lem4 8338* Lemma for axdc3 8339. 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 8331 (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 8331 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 8334. See axdc3lem2 8336 for the rest of the proof. (Contributed by Mario Carneiro, 27-Jan-2013.)

Theoremaxdc3 8339* 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 8340* Lemma for axdc4 8341. (Contributed by Mario Carneiro, 31-Jan-2013.) (Revised by Mario Carneiro, 16-Nov-2013.)

Theoremaxdc4 8341* A more general version of axdc3 8339 that allows the function to vary with . (Contributed by Mario Carneiro, 31-Jan-2013.)

Theoremaxcclem 8342* Lemma for axcc 8343. (Contributed by Mario Carneiro, 2-Feb-2013.) (Revised by Mario Carneiro, 16-Nov-2013.)

Theoremaxcc 8343* Although CC can be proven trivially using ac5 8362, 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 8344* 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 8347 for a more detailed explanation. Theorem ac2 8346 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 8350 is slightly shorter when the biconditional of ax-ac 8344 is expanded into implication and negation. In axac3 8349 we allow the constant CHOICE to represent the Axiom of Choice; this simplifies the representation of theorems like gchac 8561 (the Generalized Continuum Hypothesis implies the Axiom of Choice).

Standard textbook versions of AC are derived as ac8 8377, ac5 8362, and ac7 8358. The Axiom of Regularity ax-reg 7563 (among others) is used to derive our version from the standard ones; this reverse derivation is shown as theorem dfac2 8016. Equivalents to AC are the well-ordering theorem weth 8380 and Zorn's lemma zorn 8392. See ac4 8360 for comments about stronger versions of AC.

In order to avoid uses of ax-reg 7563 for derivation of AC equivalents, we provide ax-ac2 8348 (due to Kurt Maes), which is equivalent to the standard AC of textbooks. The derivation of ax-ac2 8348 from ax-ac 8344 is shown by theorem axac2 8351, and the reverse derivation by axac 8352. Therefore, new proofs should normally use ax-ac2 8348 instead. (New usage is discouraged.) (Contributed by NM, 18-Jul-1996.)

Theoremzfac 8345* Axiom of Choice expressed with the fewest number of different variables. The penultimate step shows the logical equivalence to ax-ac 8344. (New usage is discouraged.) (Contributed by NM, 14-Aug-2003.)

Theoremac2 8346* 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 8347 is easier to understand.) Note: aceq0 8004 shows the logical equivalence to ax-ac 8344. (New usage is discouraged.) (Contributed by NM, 18-Jul-1996.)

Theoremac3 8347* Axiom of Choice using abbreviations. The logical equivalence to ax-ac 8344 can be established by chaining aceq0 8004 and aceq2 8005. A standard textbook version of AC is derived from this one in dfac2a 8015, and this version of AC is derived from the textbook version in dfac2 8016.

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 7576. The key theorem for this (used in the proof of dfac2 8016) is preleq 7575. 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 8348* In order to avoid uses of ax-reg 7563 for derivation of AC equivalents, we provide ax-ac2 8348, 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 8350. We removed the leading quantifier to make it slightly shorter, since we have ax-gen 1556 available. The derivation of ax-ac2 8348 from ax-ac 8344 is shown by theorem axac2 8351, and the reverse derivation by axac 8352. Note that we use ax-reg 7563 to derive ax-ac 8344 from ax-ac2 8348, but not to derive ax-ac2 8348 from ax-ac 8344. (Contributed by NM, 19-Dec-2016.)

Theoremaxac3 8349 This theorem asserts that the constant CHOICE is a theorem, thus eliminating it as a hypothesis while assuming ax-ac2 8348 as an axiom. (Contributed by Mario Carneiro, 6-May-2015.) (Revised by NM, 20-Dec-2016.) (Proof modification is discouraged.)
CHOICE

Theoremackm 8350* 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 8051. 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 8351* Derive ax-ac2 8348 from ax-ac 8344. (Contributed by NM, 19-Dec-2016.) (New usage is discouraged.) (Proof modification is discouraged.)

Theoremaxac 8352* Derive ax-ac 8344 from ax-ac2 8348. Note that ax-reg 7563 is used by the proof. (Contributed by NM, 19-Dec-2016.) (Proof modification is discouraged.)

Theoremaxaci 8353 Apply a choice equivalent. (Contributed by Mario Carneiro, 17-May-2015.)
CHOICE

Theoremcardeqv 8354 All sets are well-orderable under choice. (Contributed by Mario Carneiro, 28-Apr-2015.)

Theoremnumth3 8355 All sets are well-orderable under choice. (Contributed by Stefan O'Rear, 28-Feb-2015.)

Theoremnumth2 8356* 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 8357* 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 8358* An Axiom of Choice equivalent similar to the Axiom of Choice (first form) of [Enderton] p. 49. (Contributed by NM, 29-Apr-2004.)

Theoremac7g 8359* An Axiom of Choice equivalent similar to the Axiom of Choice (first form) of [Enderton] p. 49. (Contributed by NM, 23-Jul-2004.)

Theoremac4 8360* Equivalent of Axiom of Choice. We do not insist that be a function. However, theorem ac5 8362, derived from this one, shows that this form of the axiom does imply that at least one such set 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" , which asserts the existence of a universal choice function but requires second-order quantification on (proper) class variable 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 8376. (Contributed by NM, 21-Jul-1996.)

Theoremac4c 8361* Equivalent of Axiom of Choice (class version) (Contributed by NM, 10-Feb-1997.)

Theoremac5 8362* An Axiom of Choice equivalent: there exists a function (called a choice function) with domain 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 be a function is not necessary; see ac4 8360. (Contributed by NM, 29-Aug-1999.)

Theoremac5b 8363* Equivalent of Axiom of Choice. (Contributed by NM, 31-Aug-1999.)

Theoremac6num 8364* A version of ac6 8365 which takes the choice as a hypothesis. (Contributed by Mario Carneiro, 27-Aug-2015.)

Theoremac6 8365* 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 , where depends on (the natural number) and (to specify a member of ). A stronger version of this theorem, ac6s 8369, allows to be a proper class. (Contributed by NM, 18-Oct-1999.) (Revised by Mario Carneiro, 27-Aug-2015.)

Theoremac6c4 8366* Equivalent of Axiom of Choice. is a collection of nonempty sets. (Contributed by Mario Carneiro, 22-Mar-2013.)

Theoremac6c5 8367* Equivalent of Axiom of Choice. is a collection of nonempty sets. Remark after Theorem 10.46 of [TakeutiZaring] p. 98. (Contributed by Mario Carneiro, 22-Mar-2013.)

Theoremac9 8368* 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.)

Theoremac6s 8369* Equivalent of Axiom of Choice. Using the Boundedness Axiom bnd2 7822, we derive this strong version of ac6 8365 that doesn't require to be a set. (Contributed by NM, 4-Feb-2004.)

Theoremac6n 8370* Equivalent of Axiom of Choice. Contrapositive of ac6s 8369. (Contributed by NM, 10-Jun-2007.)

Theoremac6s2 8371* Generalization of the Axiom of Choice to classes. Slightly strengthened version of ac6s3 8372. (Contributed by NM, 29-Sep-2006.)

Theoremac6s3 8372* Generalization of the Axiom of Choice to classes. Theorem 10.46 of [TakeutiZaring] p. 97. (Contributed by NM, 3-Nov-2004.)

Theoremac6sg 8373* ac6s 8369 with sethood as antecedent. (Contributed by FL, 3-Aug-2009.)

Theoremac6sf 8374* Version of ac6 8365 with bound-variable hypothesis. (Contributed by NM, 2-Mar-2008.)

Theoremac6s4 8375* Generalization of the Axiom of Choice to proper classes. is a collection of nonempty, possible proper classes. (Contributed by NM, 29-Sep-2006.)

Theoremac6s5 8376* Generalization of the Axiom of Choice to proper classes. is a collection of nonempty, possible proper classes. Remark after Theorem 10.46 of [TakeutiZaring] p. 98. (Contributed by NM, 27-Mar-2006.)

Theoremac8 8377* An Axiom of Choice equivalent. Given a family of mutually disjoint nonempty sets, there exists a set containing exactly one member from each set in the family. Theorem 6M(4) of [Enderton] p. 151. (Contributed by NM, 14-May-2004.)

Theoremac9s 8378* 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 (achieved via the Collection Principle cp 7820). (Contributed by NM, 29-Sep-2006.)

3.2.2  AC equivalents: well-ordering, Zorn's lemma

Theoremnumthcor 8379* Any set is strictly dominated by some ordinal. (Contributed by NM, 22-Oct-2003.)

Theoremweth 8380* Well-ordering theorem: any set 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.)

Theoremzorn2lem1 8381* Lemma for zorn2 8391. (Contributed by NM, 3-Apr-1997.) (Revised by Mario Carneiro, 9-May-2015.)
recs

Theoremzorn2lem2 8382* Lemma for zorn2 8391. (Contributed by NM, 3-Apr-1997.) (Revised by Mario Carneiro, 9-May-2015.)
recs

Theoremzorn2lem3 8383* Lemma for zorn2 8391. (Contributed by NM, 3-Apr-1997.) (Revised by Mario Carneiro, 9-May-2015.)
recs

Theoremzorn2lem4 8384* Lemma for zorn2 8391. (Contributed by NM, 3-Apr-1997.) (Revised by Mario Carneiro, 9-May-2015.)
recs

Theoremzorn2lem5 8385* Lemma for zorn2 8391. (Contributed by NM, 4-Apr-1997.) (Revised by Mario Carneiro, 9-May-2015.)
recs

Theoremzorn2lem6 8386* Lemma for zorn2 8391. (Contributed by NM, 4-Apr-1997.) (Revised by Mario Carneiro, 9-May-2015.)
recs

Theoremzorn2lem7 8387* Lemma for zorn2 8391. (Contributed by NM, 6-Apr-1997.) (Revised by Mario Carneiro, 9-May-2015.)
recs

Theoremzorn2g 8388* Zorn's Lemma of [Monk1] p. 117. This version of zorn2 8391 avoids the Axiom of Choice by assuming that is well-orderable. (Contributed by NM, 6-Apr-1997.) (Revised by Mario Carneiro, 9-May-2015.)

Theoremzorng 8389* 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 8392 avoids the Axiom of Choice by assuming that is well-orderable. (Contributed by NM, 12-Aug-2004.) (Revised by Mario Carneiro, 9-May-2015.)
[]

Theoremzornn0g 8390* Variant of Zorn's lemma zorng 8389 in which , the union of the empty chain, is not required to be an element of . (Contributed by Jeff Madsen, 5-Jan-2011.) (Revised by Mario Carneiro, 9-May-2015.)
[]

Theoremzorn2 8391* Zorn's Lemma of [Monk1] p. 117. This theorem is equivalent to the Axiom of Choice and states that every partially ordered set (with an ordering relation ) in which every totally ordered subset has an upper bound, contains at least one maximal element. The main proof consists of lemmas zorn2lem1 8381 through zorn2lem7 8387; this final piece mainly changes bound variables to eliminate the hypotheses of zorn2lem7 8387. (Contributed by NM, 6-Apr-1997.) (Revised by Mario Carneiro, 9-May-2015.)

Theoremzorn 8392* 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 8391 for a version with general partial orderings. (Contributed by NM, 12-Aug-2004.)
[]

Theoremzornn0 8393* Variant of Zorn's lemma zorn 8392 in which , the union of the empty chain, is not required to be an element of . (Contributed by Jeff Madsen, 5-Jan-2011.)
[]

Theoremttukeylem1 8394* Lemma for ttukey 8403. Expand out the property of being an element of a property of finite character. (Contributed by Mario Carneiro, 15-May-2015.)

Theoremttukeylem2 8395* Lemma for ttukey 8403. A property of finite character is closed under subsets. (Contributed by Mario Carneiro, 15-May-2015.)

Theoremttukeylem3 8396* Lemma for ttukey 8403. (Contributed by Mario Carneiro, 11-May-2015.)
recs

Theoremttukeylem4 8397* Lemma for ttukey 8403. (Contributed by Mario Carneiro, 15-May-2015.)
recs

Theoremttukeylem5 8398* Lemma for ttukey 8403. The function forms a (transfinitely long) chain of inclusions. (Contributed by Mario Carneiro, 15-May-2015.)
recs

Theoremttukeylem6 8399* Lemma for ttukey 8403. (Contributed by Mario Carneiro, 15-May-2015.)
recs

Theoremttukeylem7 8400* Lemma for ttukey 8403. (Contributed by Mario Carneiro, 15-May-2015.)
recs

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