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Theorem List for Metamath Proof Explorer - 9201-9300   *Has distinct variable group(s)
TypeLabelDescription
Statement

Theoremacni 9201* The property of being a choice set of length 𝐴. (Contributed by Mario Carneiro, 31-Aug-2015.)
((𝑋AC 𝐴𝐹:𝐴⟶(𝒫 𝑋 ∖ {∅})) → ∃𝑔𝑥𝐴 (𝑔𝑥) ∈ (𝐹𝑥))

Theoremacni2 9202* The property of being a choice set of length 𝐴. (Contributed by Mario Carneiro, 31-Aug-2015.)
((𝑋AC 𝐴 ∧ ∀𝑥𝐴 (𝐵𝑋𝐵 ≠ ∅)) → ∃𝑔(𝑔:𝐴𝑋 ∧ ∀𝑥𝐴 (𝑔𝑥) ∈ 𝐵))

Theoremacni3 9203* The property of being a choice set of length 𝐴. (Contributed by Mario Carneiro, 31-Aug-2015.)
(𝑦 = (𝑔𝑥) → (𝜑𝜓))       ((𝑋AC 𝐴 ∧ ∀𝑥𝐴𝑦𝑋 𝜑) → ∃𝑔(𝑔:𝐴𝑋 ∧ ∀𝑥𝐴 𝜓))

Theoremacnlem 9204* Construct a mapping satisfying the consequent of isacn 9200. (Contributed by Mario Carneiro, 31-Aug-2015.)
((𝐴𝑉 ∧ ∀𝑥𝐴 𝐵 ∈ (𝑓𝑥)) → ∃𝑔𝑥𝐴 (𝑔𝑥) ∈ (𝑓𝑥))

Theoremnumacn 9205 A well-orderable set has choice sequences of every length. (Contributed by Mario Carneiro, 31-Aug-2015.)
(𝐴𝑉 → (𝑋 ∈ dom card → 𝑋AC 𝐴))

Theoremfinacn 9206 Every set has finite choice sequences. (Contributed by Mario Carneiro, 31-Aug-2015.)
(𝐴 ∈ Fin → AC 𝐴 = V)

Theoremacndom 9207 A set with long choice sequences also has shorter choice sequences, where "shorter" here means the new index set is dominated by the old index set. (Contributed by Mario Carneiro, 31-Aug-2015.)
(𝐴𝐵 → (𝑋AC 𝐵𝑋AC 𝐴))

Theoremacnnum 9208 A set 𝑋 which has choice sequences on it of length 𝒫 𝑋 is well-orderable (and hence has choice sequences of every length). (Contributed by Mario Carneiro, 31-Aug-2015.)
(𝑋AC 𝒫 𝑋𝑋 ∈ dom card)

Theoremacnen 9209 The class of choice sets of length 𝐴 is a cardinal invariant. (Contributed by Mario Carneiro, 31-Aug-2015.)
(𝐴𝐵AC 𝐴 = AC 𝐵)

Theoremacndom2 9210 A set smaller than one with choice sequences of length 𝐴 also has choice sequences of length 𝐴. (Contributed by Mario Carneiro, 31-Aug-2015.)
(𝑋𝑌 → (𝑌AC 𝐴𝑋AC 𝐴))

Theoremacnen2 9211 The class of sets with choice sequences of length 𝐴 is a cardinal invariant. (Contributed by Mario Carneiro, 31-Aug-2015.)
(𝑋𝑌 → (𝑋AC 𝐴𝑌AC 𝐴))

Theoremfodomacn 9212 A version of fodom 9679 that doesn't require the Axiom of Choice ax-ac 9616. If 𝐴 has choice sequences of length 𝐵, then any surjection from 𝐴 to 𝐵 can be inverted to an injection the other way. (Contributed by Mario Carneiro, 31-Aug-2015.)
(𝐴AC 𝐵 → (𝐹:𝐴onto𝐵𝐵𝐴))

Theoremfodomnum 9213 A version of fodom 9679 that doesn't require the Axiom of Choice ax-ac 9616. (Contributed by Mario Carneiro, 28-Feb-2013.) (Revised by Mario Carneiro, 28-Apr-2015.)
(𝐴 ∈ dom card → (𝐹:𝐴onto𝐵𝐵𝐴))

Theoremfonum 9214 A surjection maps numerable sets to numerable sets. (Contributed by Mario Carneiro, 30-Apr-2015.)
((𝐴 ∈ dom card ∧ 𝐹:𝐴onto𝐵) → 𝐵 ∈ dom card)

Theoremnumwdom 9215 A surjection maps numerable sets to numerable sets. (Contributed by Mario Carneiro, 27-Aug-2015.)
((𝐴 ∈ dom card ∧ 𝐵* 𝐴) → 𝐵 ∈ dom card)

Theoremfodomfi2 9216 Onto functions define dominance when a finite number of choices need to be made. (Contributed by Stefan O'Rear, 28-Feb-2015.)
((𝐴𝑉𝐵 ∈ Fin ∧ 𝐹:𝐴onto𝐵) → 𝐵𝐴)

Theoremwdomfil 9217 Weak dominance agrees with normal for finite left sets. (Contributed by Stefan O'Rear, 28-Feb-2015.) (Revised by Mario Carneiro, 5-May-2015.)
(𝑋 ∈ Fin → (𝑋* 𝑌𝑋𝑌))

Theoreminfpwfien 9218 Any infinite well-orderable set is equinumerous to its set of finite subsets. (Contributed by Mario Carneiro, 18-May-2015.)
((𝐴 ∈ dom card ∧ ω ≼ 𝐴) → (𝒫 𝐴 ∩ Fin) ≈ 𝐴)

Theoreminffien 9219 The set of finite intersections of an infinite well-orderable set is equinumerous to the set itself. (Contributed by Mario Carneiro, 18-May-2015.)
((𝐴 ∈ dom card ∧ ω ≼ 𝐴) → (fi‘𝐴) ≈ 𝐴)

Theoremwdomnumr 9220 Weak dominance agrees with normal for numerable right sets. (Contributed by Stefan O'Rear, 28-Feb-2015.) (Revised by Mario Carneiro, 5-May-2015.)
(𝐵 ∈ dom card → (𝐴* 𝐵𝐴𝐵))

Theoremalephfnon 9221 The aleph function is a function on the class of ordinal numbers. (Contributed by NM, 21-Oct-2003.) (Revised by Mario Carneiro, 13-Sep-2013.)
ℵ Fn On

Theoremaleph0 9222 The first infinite cardinal number, discovered by Georg Cantor in 1873, has the same size as the set of natural numbers ω (and under our particular definition is also equal to it). In the literature, the argument of the aleph function is often written as a subscript, and the first aleph is written _0. Exercise 3 of [TakeutiZaring] p. 91. Also Definition 12(i) of [Suppes] p. 228. From Moshé Machover, Set Theory, Logic, and Their Limitations, p. 95: "Aleph...the first letter in the Hebrew alphabet...is also the first letter of the Hebrew word...(einsoph, meaning infinity), which is a cabbalistic appellation of the deity. The notation is due to Cantor, who was deeply interested in mysticism." (Contributed by NM, 21-Oct-2003.) (Revised by Mario Carneiro, 13-Sep-2013.)
(ℵ‘∅) = ω

Theoremalephlim 9223* Value of the aleph function at a limit ordinal. Definition 12(iii) of [Suppes] p. 91. (Contributed by NM, 21-Oct-2003.) (Revised by Mario Carneiro, 13-Sep-2013.)
((𝐴𝑉 ∧ Lim 𝐴) → (ℵ‘𝐴) = 𝑥𝐴 (ℵ‘𝑥))

Theoremalephsuc 9224 Value of the aleph function at a successor ordinal. Definition 12(ii) of [Suppes] p. 91. Here we express the successor aleph in terms of the Hartogs function df-har 8752, which gives the smallest ordinal that strictly dominates its argument (or the supremum of all ordinals that are dominated by the argument). (Contributed by Mario Carneiro, 13-Sep-2013.) (Revised by Mario Carneiro, 15-May-2015.)
(𝐴 ∈ On → (ℵ‘suc 𝐴) = (har‘(ℵ‘𝐴)))

Theoremalephon 9225 An aleph is an ordinal number. (Contributed by NM, 10-Nov-2003.) (Revised by Mario Carneiro, 13-Sep-2013.)
(ℵ‘𝐴) ∈ On

Theoremalephcard 9226 Every aleph is a cardinal number. Theorem 65 of [Suppes] p. 229. (Contributed by NM, 25-Oct-2003.) (Revised by Mario Carneiro, 2-Feb-2013.)
(card‘(ℵ‘𝐴)) = (ℵ‘𝐴)

Theoremalephnbtwn 9227 No cardinal can be sandwiched between an aleph and its successor aleph. Theorem 67 of [Suppes] p. 229. (Contributed by NM, 10-Nov-2003.) (Revised by Mario Carneiro, 15-May-2015.)
((card‘𝐵) = 𝐵 → ¬ ((ℵ‘𝐴) ∈ 𝐵𝐵 ∈ (ℵ‘suc 𝐴)))

Theoremalephnbtwn2 9228 No set has equinumerosity between an aleph and its successor aleph. (Contributed by NM, 3-Nov-2003.) (Revised by Mario Carneiro, 2-Feb-2013.)
¬ ((ℵ‘𝐴) ≺ 𝐵𝐵 ≺ (ℵ‘suc 𝐴))

Theoremalephordilem1 9229 Lemma for alephordi 9230. (Contributed by NM, 23-Oct-2009.) (Revised by Mario Carneiro, 15-May-2015.)
(𝐴 ∈ On → (ℵ‘𝐴) ≺ (ℵ‘suc 𝐴))

Theoremalephordi 9230 Strict ordering property of the aleph function. (Contributed by Mario Carneiro, 2-Feb-2013.)
(𝐵 ∈ On → (𝐴𝐵 → (ℵ‘𝐴) ≺ (ℵ‘𝐵)))

Theoremalephord 9231 Ordering property of the aleph function. (Contributed by NM, 26-Oct-2003.) (Revised by Mario Carneiro, 9-Feb-2013.)
((𝐴 ∈ On ∧ 𝐵 ∈ On) → (𝐴𝐵 ↔ (ℵ‘𝐴) ≺ (ℵ‘𝐵)))

Theoremalephord2 9232 Ordering property of the aleph function. Theorem 8A(a) of [Enderton] p. 213 and its converse. (Contributed by NM, 3-Nov-2003.) (Revised by Mario Carneiro, 9-Feb-2013.)
((𝐴 ∈ On ∧ 𝐵 ∈ On) → (𝐴𝐵 ↔ (ℵ‘𝐴) ∈ (ℵ‘𝐵)))

Theoremalephord2i 9233 Ordering property of the aleph function. Theorem 66 of [Suppes] p. 229. (Contributed by NM, 25-Oct-2003.)
(𝐵 ∈ On → (𝐴𝐵 → (ℵ‘𝐴) ∈ (ℵ‘𝐵)))

Theoremalephord3 9234 Ordering property of the aleph function. (Contributed by NM, 11-Nov-2003.)
((𝐴 ∈ On ∧ 𝐵 ∈ On) → (𝐴𝐵 ↔ (ℵ‘𝐴) ⊆ (ℵ‘𝐵)))

Theoremalephsucdom 9235 A set dominated by an aleph is strictly dominated by its successor aleph and vice-versa. (Contributed by NM, 3-Nov-2003.) (Revised by Mario Carneiro, 2-Feb-2013.)
(𝐵 ∈ On → (𝐴 ≼ (ℵ‘𝐵) ↔ 𝐴 ≺ (ℵ‘suc 𝐵)))

Theoremalephsuc2 9236* An alternate representation of a successor aleph. The aleph function is the function obtained from the hartogs 8738 function by transfinite recursion, starting from ω. Using this theorem we could define the aleph function with {𝑧 ∈ On ∣ 𝑧𝑥} in place of {𝑧 ∈ On ∣ 𝑥𝑧} in df-aleph 9099. (Contributed by NM, 3-Nov-2003.) (Revised by Mario Carneiro, 2-Feb-2013.)
(𝐴 ∈ On → (ℵ‘suc 𝐴) = {𝑥 ∈ On ∣ 𝑥 ≼ (ℵ‘𝐴)})

Theoremalephdom 9237 Relationship between inclusion of ordinal numbers and dominance of infinite initial ordinals. (Contributed by Jeff Hankins, 23-Oct-2009.)
((𝐴 ∈ On ∧ 𝐵 ∈ On) → (𝐴𝐵 ↔ (ℵ‘𝐴) ≼ (ℵ‘𝐵)))

Theoremalephgeom 9238 Every aleph is greater than or equal to the set of natural numbers. (Contributed by NM, 11-Nov-2003.)
(𝐴 ∈ On ↔ ω ⊆ (ℵ‘𝐴))

Theoremalephislim 9239 Every aleph is a limit ordinal. (Contributed by NM, 11-Nov-2003.)
(𝐴 ∈ On ↔ Lim (ℵ‘𝐴))

Theoremaleph11 9240 The aleph function is one-to-one. (Contributed by NM, 3-Aug-2004.)
((𝐴 ∈ On ∧ 𝐵 ∈ On) → ((ℵ‘𝐴) = (ℵ‘𝐵) ↔ 𝐴 = 𝐵))

Theoremalephf1 9241 The aleph function is a one-to-one mapping from the ordinals to the infinite cardinals. See also alephf1ALT 9259. (Contributed by Mario Carneiro, 2-Feb-2013.)
ℵ:On–1-1→On

Theoremalephsdom 9242 If an ordinal is smaller than an initial ordinal, it is strictly dominated by it. (Contributed by Jeff Hankins, 24-Oct-2009.) (Proof shortened by Mario Carneiro, 20-Sep-2014.)
((𝐴 ∈ On ∧ 𝐵 ∈ On) → (𝐴 ∈ (ℵ‘𝐵) ↔ 𝐴 ≺ (ℵ‘𝐵)))

Theoremalephdom2 9243 A dominated initial ordinal is included. (Contributed by Jeff Hankins, 24-Oct-2009.)
((𝐴 ∈ On ∧ 𝐵 ∈ On) → ((ℵ‘𝐴) ⊆ 𝐵 ↔ (ℵ‘𝐴) ≼ 𝐵))

Theoremalephle 9244 The argument of the aleph function is less than or equal to its value. Exercise 2 of [TakeutiZaring] p. 91. (Later, in alephfp2 9265, we will that equality can sometimes hold.) (Contributed by NM, 9-Nov-2003.) (Proof shortened by Mario Carneiro, 22-Feb-2013.)
(𝐴 ∈ On → 𝐴 ⊆ (ℵ‘𝐴))

Theoremcardaleph 9245* Given any transfinite cardinal number 𝐴, there is exactly one aleph that is equal to it. Here we compute that aleph explicitly. (Contributed by NM, 9-Nov-2003.) (Revised by Mario Carneiro, 2-Feb-2013.)
((ω ⊆ 𝐴 ∧ (card‘𝐴) = 𝐴) → 𝐴 = (ℵ‘ {𝑥 ∈ On ∣ 𝐴 ⊆ (ℵ‘𝑥)}))

Theoremcardalephex 9246* Every transfinite cardinal is an aleph and vice-versa. Theorem 8A(b) of [Enderton] p. 213 and its converse. (Contributed by NM, 5-Nov-2003.)
(ω ⊆ 𝐴 → ((card‘𝐴) = 𝐴 ↔ ∃𝑥 ∈ On 𝐴 = (ℵ‘𝑥)))

Theoreminfenaleph 9247* An infinite numerable set is equinumerous to an infinite initial ordinal. (Contributed by Jeff Hankins, 23-Oct-2009.) (Revised by Mario Carneiro, 29-Apr-2015.)
((𝐴 ∈ dom card ∧ ω ≼ 𝐴) → ∃𝑥 ∈ ran ℵ𝑥𝐴)

Theoremisinfcard 9248 Two ways to express the property of being a transfinite cardinal. (Contributed by NM, 9-Nov-2003.)
((ω ⊆ 𝐴 ∧ (card‘𝐴) = 𝐴) ↔ 𝐴 ∈ ran ℵ)

Theoremiscard3 9249 Two ways to express the property of being a cardinal number. (Contributed by NM, 9-Nov-2003.)
((card‘𝐴) = 𝐴𝐴 ∈ (ω ∪ ran ℵ))

Theoremcardnum 9250 Two ways to express the class of all cardinal numbers, which consists of the finite ordinals in ω plus the transfinite alephs. (Contributed by NM, 10-Sep-2004.)
{𝑥 ∣ (card‘𝑥) = 𝑥} = (ω ∪ ran ℵ)

Theoremalephinit 9251* An infinite initial ordinal is characterized by the property of being initial - that is, it is a subset of any dominating ordinal. (Contributed by Jeff Hankins, 29-Oct-2009.) (Proof shortened by Mario Carneiro, 20-Sep-2014.)
((𝐴 ∈ On ∧ ω ⊆ 𝐴) → (𝐴 ∈ ran ℵ ↔ ∀𝑥 ∈ On (𝐴𝑥𝐴𝑥)))

Theoremcarduniima 9252 The union of the image of a mapping to cardinals is a cardinal. Proposition 11.16 of [TakeutiZaring] p. 104. (Contributed by NM, 4-Nov-2004.)
(𝐴𝐵 → (𝐹:𝐴⟶(ω ∪ ran ℵ) → (𝐹𝐴) ∈ (ω ∪ ran ℵ)))

Theoremcardinfima 9253* If a mapping to cardinals has an infinite value, then the union of its image is an infinite cardinal. Corollary 11.17 of [TakeutiZaring] p. 104. (Contributed by NM, 4-Nov-2004.)
(𝐴𝐵 → ((𝐹:𝐴⟶(ω ∪ ran ℵ) ∧ ∃𝑥𝐴 (𝐹𝑥) ∈ ran ℵ) → (𝐹𝐴) ∈ ran ℵ))

Theoremalephiso 9254 Aleph is an order isomorphism of the class of ordinal numbers onto the class of infinite cardinals. Definition 10.27 of [TakeutiZaring] p. 90. (Contributed by NM, 3-Aug-2004.)
ℵ Isom E , E (On, {𝑥 ∣ (ω ⊆ 𝑥 ∧ (card‘𝑥) = 𝑥)})

Theoremalephprc 9255 The class of all transfinite cardinal numbers (the range of the aleph function) is a proper class. Proposition 10.26 of [TakeutiZaring] p. 90. (Contributed by NM, 11-Nov-2003.)
¬ ran ℵ ∈ V

Theoremalephsson 9256 The class of transfinite cardinals (the range of the aleph function) is a subclass of the class of ordinal numbers. (Contributed by NM, 11-Nov-2003.)
ran ℵ ⊆ On

Theoremunialeph 9257 The union of the class of transfinite cardinals (the range of the aleph function) is the class of ordinal numbers. (Contributed by NM, 11-Nov-2003.)
ran ℵ = On

Theoremalephsmo 9258 The aleph function is strictly monotone. (Contributed by Mario Carneiro, 15-Mar-2013.)
Smo ℵ

Theoremalephf1ALT 9259 Alternate proof of alephf1 9241. (Contributed by Mario Carneiro, 15-Mar-2013.) (Proof modification is discouraged.) (New usage is discouraged.)
ℵ:On–1-1→On

Theoremalephfplem1 9260 Lemma for alephfp 9264. (Contributed by NM, 6-Nov-2004.)
𝐻 = (rec(ℵ, ω) ↾ ω)       (𝐻‘∅) ∈ ran ℵ

Theoremalephfplem2 9261* Lemma for alephfp 9264. (Contributed by NM, 6-Nov-2004.)
𝐻 = (rec(ℵ, ω) ↾ ω)       (𝑤 ∈ ω → (𝐻‘suc 𝑤) = (ℵ‘(𝐻𝑤)))

Theoremalephfplem3 9262* Lemma for alephfp 9264. (Contributed by NM, 6-Nov-2004.)
𝐻 = (rec(ℵ, ω) ↾ ω)       (𝑣 ∈ ω → (𝐻𝑣) ∈ ran ℵ)

Theoremalephfplem4 9263 Lemma for alephfp 9264. (Contributed by NM, 5-Nov-2004.)
𝐻 = (rec(ℵ, ω) ↾ ω)        (𝐻 “ ω) ∈ ran ℵ

Theoremalephfp 9264 The aleph function has a fixed point. Similar to Proposition 11.18 of [TakeutiZaring] p. 104, except that we construct an actual example of a fixed point rather than just showing its existence. See alephfp2 9265 for an abbreviated version just showing existence. (Contributed by NM, 6-Nov-2004.) (Proof shortened by Mario Carneiro, 15-May-2015.)
𝐻 = (rec(ℵ, ω) ↾ ω)       (ℵ‘ (𝐻 “ ω)) = (𝐻 “ ω)

Theoremalephfp2 9265 The aleph function has at least one fixed point. Proposition 11.18 of [TakeutiZaring] p. 104. See alephfp 9264 for an actual example of a fixed point. Compare the inequality alephle 9244 that holds in general. Note that if 𝑥 is a fixed point, then ℵ‘ℵ‘ℵ‘... ℵ‘𝑥 = 𝑥. (Contributed by NM, 6-Nov-2004.) (Revised by Mario Carneiro, 15-May-2015.)
𝑥 ∈ On (ℵ‘𝑥) = 𝑥

Theoremalephval3 9266* An alternate way to express the value of the aleph function: it is the least infinite cardinal different from all values at smaller arguments. Definition of aleph in [Enderton] p. 212 and definition of aleph in [BellMachover] p. 490 . (Contributed by NM, 16-Nov-2003.)
(𝐴 ∈ On → (ℵ‘𝐴) = {𝑥 ∣ ((card‘𝑥) = 𝑥 ∧ ω ⊆ 𝑥 ∧ ∀𝑦𝐴 ¬ 𝑥 = (ℵ‘𝑦))})

Theoremalephsucpw2 9267 The power set of an aleph is not strictly dominated by the successor aleph. (The Generalized Continuum Hypothesis says they are equinumerous, see gch3 9833 or gchaleph2 9829.) The transposed form alephsucpw 9727 cannot be proven without the AC, and is in fact equivalent to it. (Contributed by Mario Carneiro, 2-Feb-2013.)
¬ 𝒫 (ℵ‘𝐴) ≺ (ℵ‘suc 𝐴)

Theoremmappwen 9268 Power rule for cardinal arithmetic. Theorem 11.21 of [TakeutiZaring] p. 106. (Contributed by Mario Carneiro, 9-Mar-2013.) (Revised by Mario Carneiro, 27-Apr-2015.)
(((𝐵 ∈ dom card ∧ ω ≼ 𝐵) ∧ (2o𝐴𝐴 ≼ 𝒫 𝐵)) → (𝐴𝑚 𝐵) ≈ 𝒫 𝐵)

Theoremfinnisoeu 9269* A finite totally ordered set has a unique order isomorphism to a finite ordinal. (Contributed by Stefan O'Rear, 16-Nov-2014.) (Proof shortened by Mario Carneiro, 26-Jun-2015.)
((𝑅 Or 𝐴𝐴 ∈ Fin) → ∃!𝑓 𝑓 Isom E , 𝑅 ((card‘𝐴), 𝐴))

Theoremiunfictbso 9270 Countability of a countable union of finite sets with a strict (not globally well) order fulfilling the choice role. (Contributed by Stefan O'Rear, 16-Nov-2014.)
((𝐴 ≼ ω ∧ 𝐴 ⊆ Fin ∧ 𝐵 Or 𝐴) → 𝐴 ≼ ω)

2.6.9  Axiom of Choice equivalents

Syntaxwac 9271 Wff for an abbreviation of the axiom of choice.
wff CHOICE

Definitiondf-ac 9272* The expression CHOICE will be used as a readable shorthand for any form of the axiom of choice; all concrete forms are long, cryptic, have dummy variables, or all three, making it useful to have a short name. Similar to the Axiom of Choice (first form) of [Enderton] p. 49.

There is a slight problem with taking the exact form of ax-ac 9616 as our definition, because the equivalence to more standard forms (dfac2 9287) requires the Axiom of Regularity, which we often try to avoid. Thus, we take the first of the "textbook forms" as the definition and derive the form of ax-ac 9616 itself as dfac0 9290. (Contributed by Mario Carneiro, 22-Feb-2015.)

(CHOICE ↔ ∀𝑥𝑓(𝑓𝑥𝑓 Fn dom 𝑥))

Theoremaceq1 9273* Equivalence of two versions of the Axiom of Choice ax-ac 9616. The proof uses neither AC nor the Axiom of Regularity. The right-hand side expresses our AC with the fewest number of different variables. (Contributed by NM, 5-Apr-2004.)
(∃𝑦𝑧𝑥𝑤𝑧 ∃!𝑣𝑧𝑢𝑦 (𝑧𝑢𝑣𝑢) ↔ ∃𝑦𝑧𝑤((𝑧𝑤𝑤𝑥) → ∃𝑥𝑧(∃𝑥((𝑧𝑤𝑤𝑥) ∧ (𝑧𝑥𝑥𝑦)) ↔ 𝑧 = 𝑥)))

Theoremaceq0 9274* Equivalence of two versions of the Axiom of Choice. The proof uses neither AC nor the Axiom of Regularity. The right-hand side is our original ax-ac 9616. (Contributed by NM, 5-Apr-2004.)
(∃𝑦𝑧𝑥𝑤𝑧 ∃!𝑣𝑧𝑢𝑦 (𝑧𝑢𝑣𝑢) ↔ ∃𝑦𝑧𝑤((𝑧𝑤𝑤𝑥) → ∃𝑣𝑢(∃𝑡((𝑢𝑤𝑤𝑡) ∧ (𝑢𝑡𝑡𝑦)) ↔ 𝑢 = 𝑣)))

Theoremaceq2 9275* Equivalence of two versions of the Axiom of Choice. The proof uses neither AC nor the Axiom of Regularity. (Contributed by NM, 5-Apr-2004.)
(∃𝑦𝑧𝑥𝑤𝑧 ∃!𝑣𝑧𝑢𝑦 (𝑧𝑢𝑣𝑢) ↔ ∃𝑦𝑧𝑥 (𝑧 ≠ ∅ → ∃!𝑤𝑧𝑣𝑦 (𝑧𝑣𝑤𝑣)))

Theoremaceq3lem 9276* Lemma for dfac3 9277. (Contributed by NM, 2-Apr-2004.) (Revised by Mario Carneiro, 26-Jun-2015.)
𝐹 = (𝑤 ∈ dom 𝑦 ↦ (𝑓‘{𝑢𝑤𝑦𝑢}))       (∀𝑥𝑓𝑧𝑥 (𝑧 ≠ ∅ → (𝑓𝑧) ∈ 𝑧) → ∃𝑓(𝑓𝑦𝑓 Fn dom 𝑦))

Theoremdfac3 9277* Equivalence of two versions of the Axiom of Choice. The left-hand side is defined as the Axiom of Choice (first form) of [Enderton] p. 49. The right-hand side is the Axiom of Choice of [TakeutiZaring] p. 83. The proof does not depend on AC. (Contributed by NM, 24-Mar-2004.) (Revised by Stefan O'Rear, 22-Feb-2015.)
(CHOICE ↔ ∀𝑥𝑓𝑧𝑥 (𝑧 ≠ ∅ → (𝑓𝑧) ∈ 𝑧))

Theoremdfac4 9278* Equivalence of two versions of the Axiom of Choice. The right-hand side is Axiom AC of [BellMachover] p. 488. The proof does not depend on AC. (Contributed by NM, 24-Mar-2004.) (Revised by Mario Carneiro, 26-Jun-2015.)
(CHOICE ↔ ∀𝑥𝑓(𝑓 Fn 𝑥 ∧ ∀𝑧𝑥 (𝑧 ≠ ∅ → (𝑓𝑧) ∈ 𝑧)))

Theoremdfac5lem1 9279* Lemma for dfac5 9284. (Contributed by NM, 12-Apr-2004.)
(∃!𝑣 𝑣 ∈ (({𝑤} × 𝑤) ∩ 𝑦) ↔ ∃!𝑔(𝑔𝑤 ∧ ⟨𝑤, 𝑔⟩ ∈ 𝑦))

Theoremdfac5lem2 9280* Lemma for dfac5 9284. (Contributed by NM, 12-Apr-2004.)
𝐴 = {𝑢 ∣ (𝑢 ≠ ∅ ∧ ∃𝑡 𝑢 = ({𝑡} × 𝑡))}       (⟨𝑤, 𝑔⟩ ∈ 𝐴 ↔ (𝑤𝑔𝑤))

Theoremdfac5lem3 9281* Lemma for dfac5 9284. (Contributed by NM, 12-Apr-2004.)
𝐴 = {𝑢 ∣ (𝑢 ≠ ∅ ∧ ∃𝑡 𝑢 = ({𝑡} × 𝑡))}       (({𝑤} × 𝑤) ∈ 𝐴 ↔ (𝑤 ≠ ∅ ∧ 𝑤))

Theoremdfac5lem4 9282* Lemma for dfac5 9284. (Contributed by NM, 11-Apr-2004.)
𝐴 = {𝑢 ∣ (𝑢 ≠ ∅ ∧ ∃𝑡 𝑢 = ({𝑡} × 𝑡))}    &   𝐵 = ( 𝐴𝑦)    &   (𝜑 ↔ ∀𝑥((∀𝑧𝑥 𝑧 ≠ ∅ ∧ ∀𝑧𝑥𝑤𝑥 (𝑧𝑤 → (𝑧𝑤) = ∅)) → ∃𝑦𝑧𝑥 ∃!𝑣 𝑣 ∈ (𝑧𝑦)))       (𝜑 → ∃𝑦𝑧𝐴 ∃!𝑣 𝑣 ∈ (𝑧𝑦))

Theoremdfac5lem5 9283* Lemma for dfac5 9284. (Contributed by NM, 12-Apr-2004.)
𝐴 = {𝑢 ∣ (𝑢 ≠ ∅ ∧ ∃𝑡 𝑢 = ({𝑡} × 𝑡))}    &   𝐵 = ( 𝐴𝑦)    &   (𝜑 ↔ ∀𝑥((∀𝑧𝑥 𝑧 ≠ ∅ ∧ ∀𝑧𝑥𝑤𝑥 (𝑧𝑤 → (𝑧𝑤) = ∅)) → ∃𝑦𝑧𝑥 ∃!𝑣 𝑣 ∈ (𝑧𝑦)))       (𝜑 → ∃𝑓𝑤 (𝑤 ≠ ∅ → (𝑓𝑤) ∈ 𝑤))

Theoremdfac5 9284* Equivalence of two versions of the Axiom of Choice. The right-hand side is Theorem 6M(4) of [Enderton] p. 151 and asserts that given a family of mutually disjoint nonempty sets, a set exists containing exactly one member from each set in the family. The proof does not depend on AC. (Contributed by NM, 11-Apr-2004.) (Revised by Mario Carneiro, 17-May-2015.)
(CHOICE ↔ ∀𝑥((∀𝑧𝑥 𝑧 ≠ ∅ ∧ ∀𝑧𝑥𝑤𝑥 (𝑧𝑤 → (𝑧𝑤) = ∅)) → ∃𝑦𝑧𝑥 ∃!𝑣 𝑣 ∈ (𝑧𝑦)))

Theoremdfac2a 9285* Our Axiom of Choice (in the form of ac3 9619) implies the Axiom of Choice (first form) of [Enderton] p. 49. The proof uses neither AC nor the Axiom of Regularity. See dfac2b 9286 for the converse (which does use the Axiom of Regularity). (Contributed by NM, 5-Apr-2004.) (Revised by Mario Carneiro, 26-Jun-2015.)
(∀𝑥𝑦𝑧𝑥 (𝑧 ≠ ∅ → ∃!𝑤𝑧𝑣𝑦 (𝑧𝑣𝑤𝑣)) → CHOICE)

Theoremdfac2b 9286* Axiom of Choice (first form) of [Enderton] p. 49 implies our Axiom of Choice (in the form of ac3 9619). The proof does not make use of AC. Note that the Axiom of Regularity is used by the proof. Specifically, elneq 8792 and preleq 8808 that are referenced in the proof each make use of Regularity for their derivations. (The reverse implication can be derived without using Regularity; see dfac2a 9285.) (Contributed by NM, 5-Apr-2004.) (Revised by Mario Carneiro, 26-Jun-2015.) (Revised by AV, 16-Jun-2022.)
(CHOICE → ∀𝑥𝑦𝑧𝑥 (𝑧 ≠ ∅ → ∃!𝑤𝑧𝑣𝑦 (𝑧𝑣𝑤𝑣)))

Theoremdfac2 9287* Axiom of Choice (first form) of [Enderton] p. 49 corresponds to our Axiom of Choice (in the form of ac3 9619). The proof does not make use of AC, but the Axiom of Regularity is used (by applying dfac2b 9286). (Contributed by NM, 5-Apr-2004.) (Revised by Mario Carneiro, 26-Jun-2015.) (Revised by AV, 16-Jun-2022.)
(CHOICE ↔ ∀𝑥𝑦𝑧𝑥 (𝑧 ≠ ∅ → ∃!𝑤𝑧𝑣𝑦 (𝑧𝑣𝑤𝑣)))

Theoremdfac2OLD 9288* Obsolete proof of dfac2 9287 as of 16-Jun-2022. Axiom of Choice (first form) of [Enderton] p. 49 implies of our Axiom of Choice (in the form of ac3 9619). The proof does not make use of AC. Note that the Axiom of Regularity is used by the proof. Specifically, elirrv 8790 and preleqOLD 8811 that are referenced in the proof each make use of Regularity for their derivations. (The reverse implication can be derived without using Regularity; see dfac2a 9285.) (Contributed by NM, 5-Apr-2004.) (Revised by Mario Carneiro, 26-Jun-2015.) (Proof modification is discouraged.) (New usage is discouraged.)
(CHOICE ↔ ∀𝑥𝑦𝑧𝑥 (𝑧 ≠ ∅ → ∃!𝑤𝑧𝑣𝑦 (𝑧𝑣𝑤𝑣)))

Theoremdfac7 9289* Equivalence of the Axiom of Choice (first form) of [Enderton] p. 49 and our Axiom of Choice (in the form of ac2 9618). The proof does not depend AC on but does depend on the Axiom of Regularity. (Contributed by Mario Carneiro, 17-May-2015.)
(CHOICE ↔ ∀𝑥𝑦𝑧𝑥𝑤𝑧 ∃!𝑣𝑧𝑢𝑦 (𝑧𝑢𝑣𝑢))

Theoremdfac0 9290* Equivalence of two versions of the Axiom of Choice. The proof uses the Axiom of Regularity. The right-hand side is our original ax-ac 9616. (Contributed by Mario Carneiro, 17-May-2015.)
(CHOICE ↔ ∀𝑥𝑦𝑧𝑤((𝑧𝑤𝑤𝑥) → ∃𝑣𝑢(∃𝑡((𝑢𝑤𝑤𝑡) ∧ (𝑢𝑡𝑡𝑦)) ↔ 𝑢 = 𝑣)))

Theoremdfac1 9291* Equivalence of two versions of the Axiom of Choice ax-ac 9616. The proof uses the Axiom of Regularity. The right-hand side expresses our AC with the fewest number of different variables. (Contributed by Mario Carneiro, 17-May-2015.)
(CHOICE ↔ ∀𝑥𝑦𝑧𝑤((𝑧𝑤𝑤𝑥) → ∃𝑥𝑧(∃𝑥((𝑧𝑤𝑤𝑥) ∧ (𝑧𝑥𝑥𝑦)) ↔ 𝑧 = 𝑥)))

Theoremdfac8 9292* A proof of the equivalency of the well-ordering theorem weth 9652 and the axiom of choice ac7 9630. (Contributed by Mario Carneiro, 5-Jan-2013.)
(CHOICE ↔ ∀𝑥𝑟 𝑟 We 𝑥)

Theoremdfac9 9293* Equivalence of the axiom of choice with a statement related to ac9 9640; definition AC3 of [Schechter] p. 139. (Contributed by Stefan O'Rear, 22-Feb-2015.)
(CHOICE ↔ ∀𝑓((Fun 𝑓 ∧ ∅ ∉ ran 𝑓) → X𝑥 ∈ dom 𝑓(𝑓𝑥) ≠ ∅))

Theoremdfac10 9294 Axiom of Choice equivalent: the cardinality function measures every set. (Contributed by Mario Carneiro, 6-May-2015.)
(CHOICE ↔ dom card = V)

Theoremdfac10c 9295* Axiom of Choice equivalent: every set is equinumerous to an ordinal. (Contributed by Stefan O'Rear, 17-Jan-2015.)
(CHOICE ↔ ∀𝑥𝑦 ∈ On 𝑦𝑥)

Theoremdfac10b 9296 Axiom of Choice equivalent: every set is equinumerous to an ordinal (quantifier-free short cryptic version alluded to in df-ac 9272). (Contributed by Stefan O'Rear, 17-Jan-2015.)
(CHOICE ↔ ( ≈ “ On) = V)

Theoremacacni 9297 A choice equivalent: every set has choice sets of every length. (Contributed by Mario Carneiro, 31-Aug-2015.)
((CHOICE𝐴𝑉) → AC 𝐴 = V)

Theoremdfacacn 9298 A choice equivalent: every set has choice sets of every length. (Contributed by Mario Carneiro, 31-Aug-2015.)
(CHOICE ↔ ∀𝑥AC 𝑥 = V)

Theoremdfac13 9299 The axiom of choice holds iff every set has choice sequences as long as itself. (Contributed by Mario Carneiro, 3-Sep-2015.)
(CHOICE ↔ ∀𝑥 𝑥AC 𝑥)

Theoremdfac12lem1 9300* Lemma for dfac12 9306. (Contributed by Mario Carneiro, 29-May-2015.)
(𝜑𝐴 ∈ On)    &   (𝜑𝐹:𝒫 (har‘(𝑅1𝐴))–1-1→On)    &   𝐺 = recs((𝑥 ∈ V ↦ (𝑦 ∈ (𝑅1‘dom 𝑥) ↦ if(dom 𝑥 = dom 𝑥, ((suc ran ran 𝑥 ·o (rank‘𝑦)) +o ((𝑥‘suc (rank‘𝑦))‘𝑦)), (𝐹‘((OrdIso( E , ran (𝑥 dom 𝑥)) ∘ (𝑥 dom 𝑥)) “ 𝑦))))))    &   (𝜑𝐶 ∈ On)    &   𝐻 = (OrdIso( E , ran (𝐺 𝐶)) ∘ (𝐺 𝐶))       (𝜑 → (𝐺𝐶) = (𝑦 ∈ (𝑅1𝐶) ↦ if(𝐶 = 𝐶, ((suc ran (𝐺𝐶) ·o (rank‘𝑦)) +o ((𝐺‘suc (rank‘𝑦))‘𝑦)), (𝐹‘(𝐻𝑦)))))

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