P. 101 of Theorem List - Metamath Proof Explorer

Theorem List for Metamath Proof Explorer - 10001-10100   *Has distinct variable group(s)

Type	Label	Description
Statement
Theorem	ackbij1lem17 10001*	Lemma for ackbij1 10003. (Contributed by Stefan O'Rear, 18-Nov-2014.)
⊢ 𝐹 = (𝑥 ∈ (𝒫 ω ∩ Fin) ↦ (card‘∪ 𝑦 ∈ 𝑥 ({𝑦} × 𝒫 𝑦)))    ⇒   ⊢ 𝐹:(𝒫 ω ∩ Fin)–1-1→ω
Theorem	ackbij1lem18 10002*	Lemma for ackbij1 10003. (Contributed by Stefan O'Rear, 18-Nov-2014.)
⊢ 𝐹 = (𝑥 ∈ (𝒫 ω ∩ Fin) ↦ (card‘∪ 𝑦 ∈ 𝑥 ({𝑦} × 𝒫 𝑦)))    ⇒   ⊢ (𝐴 ∈ (𝒫 ω ∩ Fin) → ∃𝑏 ∈ (𝒫 ω ∩ Fin)(𝐹‘𝑏) = suc (𝐹‘𝐴))
Theorem	ackbij1 10003*	The Ackermann bijection, part 1: each natural number can be uniquely coded in binary as a finite set of natural numbers and conversely. (Contributed by Stefan O'Rear, 18-Nov-2014.)
⊢ 𝐹 = (𝑥 ∈ (𝒫 ω ∩ Fin) ↦ (card‘∪ 𝑦 ∈ 𝑥 ({𝑦} × 𝒫 𝑦)))    ⇒   ⊢ 𝐹:(𝒫 ω ∩ Fin)–1-1-onto→ω
Theorem	ackbij1b 10004*	The Ackermann bijection, part 1b: the bijection from ackbij1 10003 restricts naturally to the powers of particular naturals. (Contributed by Stefan O'Rear, 18-Nov-2014.)
⊢ 𝐹 = (𝑥 ∈ (𝒫 ω ∩ Fin) ↦ (card‘∪ 𝑦 ∈ 𝑥 ({𝑦} × 𝒫 𝑦)))    ⇒   ⊢ (𝐴 ∈ ω → (𝐹 “ 𝒫 𝐴) = (card‘𝒫 𝐴))
Theorem	ackbij2lem2 10005*	Lemma for ackbij2 10008. (Contributed by Stefan O'Rear, 18-Nov-2014.)
⊢ 𝐹 = (𝑥 ∈ (𝒫 ω ∩ Fin) ↦ (card‘∪ 𝑦 ∈ 𝑥 ({𝑦} × 𝒫 𝑦)))    &   ⊢ 𝐺 = (𝑥 ∈ V ↦ (𝑦 ∈ 𝒫 dom 𝑥 ↦ (𝐹‘(𝑥 “ 𝑦))))    ⇒   ⊢ (𝐴 ∈ ω → (rec(𝐺, ∅)‘𝐴):(𝑅1‘𝐴)–1-1-onto→(card‘(𝑅1‘𝐴)))
Theorem	ackbij2lem3 10006*	Lemma for ackbij2 10008. (Contributed by Stefan O'Rear, 18-Nov-2014.)
⊢ 𝐹 = (𝑥 ∈ (𝒫 ω ∩ Fin) ↦ (card‘∪ 𝑦 ∈ 𝑥 ({𝑦} × 𝒫 𝑦)))    &   ⊢ 𝐺 = (𝑥 ∈ V ↦ (𝑦 ∈ 𝒫 dom 𝑥 ↦ (𝐹‘(𝑥 “ 𝑦))))    ⇒   ⊢ (𝐴 ∈ ω → (rec(𝐺, ∅)‘𝐴) ⊆ (rec(𝐺, ∅)‘suc 𝐴))
Theorem	ackbij2lem4 10007*	Lemma for ackbij2 10008. (Contributed by Stefan O'Rear, 18-Nov-2014.)
⊢ 𝐹 = (𝑥 ∈ (𝒫 ω ∩ Fin) ↦ (card‘∪ 𝑦 ∈ 𝑥 ({𝑦} × 𝒫 𝑦)))    &   ⊢ 𝐺 = (𝑥 ∈ V ↦ (𝑦 ∈ 𝒫 dom 𝑥 ↦ (𝐹‘(𝑥 “ 𝑦))))    ⇒   ⊢ (((𝐴 ∈ ω ∧ 𝐵 ∈ ω) ∧ 𝐵 ⊆ 𝐴) → (rec(𝐺, ∅)‘𝐵) ⊆ (rec(𝐺, ∅)‘𝐴))
Theorem	ackbij2 10008*	The Ackermann bijection, part 2: hereditarily finite sets can be represented by recursive binary notation. (Contributed by Stefan O'Rear, 18-Nov-2014.)
⊢ 𝐹 = (𝑥 ∈ (𝒫 ω ∩ Fin) ↦ (card‘∪ 𝑦 ∈ 𝑥 ({𝑦} × 𝒫 𝑦)))    &   ⊢ 𝐺 = (𝑥 ∈ V ↦ (𝑦 ∈ 𝒫 dom 𝑥 ↦ (𝐹‘(𝑥 “ 𝑦))))    &   ⊢ 𝐻 = ∪ (rec(𝐺, ∅) “ ω)    ⇒   ⊢ 𝐻:∪ (𝑅1 “ ω)–1-1-onto→ω
Theorem	r1om 10009	The set of hereditarily finite sets is countable. See ackbij2 10008 for an explicit bijection that works without Infinity. See also r1omALT 10541. (Contributed by Stefan O'Rear, 18-Nov-2014.)
⊢ (𝑅1‘ω) ≈ ω
Theorem	fictb 10010	A set is countable iff its collection of finite intersections is countable. (Contributed by Jeff Hankins, 24-Aug-2009.) (Proof shortened by Mario Carneiro, 17-May-2015.)
⊢ (𝐴 ∈ 𝐵 → (𝐴 ≼ ω ↔ (fi‘𝐴) ≼ ω))
2.6.15  Cofinality (without Axiom of Choice)
Theorem	cflem 10011*	A lemma used to simplify cofinality computations, showing the existence of the cardinal of an unbounded subset of a set 𝐴. (Contributed by NM, 24-Apr-2004.)
⊢ (𝐴 ∈ 𝑉 → ∃𝑥∃𝑦(𝑥 = (card‘𝑦) ∧ (𝑦 ⊆ 𝐴 ∧ ∀𝑧 ∈ 𝐴 ∃𝑤 ∈ 𝑦 𝑧 ⊆ 𝑤)))
Theorem	cfval 10012*	Value of the cofinality function. Definition B of Saharon Shelah, Cardinal Arithmetic (1994), p. xxx (Roman numeral 30). The cofinality of an ordinal number 𝐴 is the cardinality (size) of the smallest unbounded subset 𝑦 of the ordinal number. Unbounded means that for every member of 𝐴, there is a member of 𝑦 that is at least as large. Cofinality is a measure of how "reachable from below" an ordinal is. (Contributed by NM, 1-Apr-2004.) (Revised by Mario Carneiro, 15-Sep-2013.)
⊢ (𝐴 ∈ On → (cf‘𝐴) = ∩ {𝑥 ∣ ∃𝑦(𝑥 = (card‘𝑦) ∧ (𝑦 ⊆ 𝐴 ∧ ∀𝑧 ∈ 𝐴 ∃𝑤 ∈ 𝑦 𝑧 ⊆ 𝑤))})
Theorem	cff 10013	Cofinality is a function on the class of ordinal numbers to the class of cardinal numbers. (Contributed by Mario Carneiro, 15-Sep-2013.)
⊢ cf:On⟶On
Theorem	cfub 10014*	An upper bound on cofinality. (Contributed by NM, 25-Apr-2004.) (Revised by Mario Carneiro, 15-Sep-2013.)
⊢ (cf‘𝐴) ⊆ ∩ {𝑥 ∣ ∃𝑦(𝑥 = (card‘𝑦) ∧ (𝑦 ⊆ 𝐴 ∧ 𝐴 ⊆ ∪ 𝑦))}
Theorem	cflm 10015*	Value of the cofinality function at a limit ordinal. Part of Definition of cofinality of [Enderton] p. 257. (Contributed by NM, 26-Apr-2004.)
⊢ ((𝐴 ∈ 𝐵 ∧ Lim 𝐴) → (cf‘𝐴) = ∩ {𝑥 ∣ ∃𝑦(𝑥 = (card‘𝑦) ∧ (𝑦 ⊆ 𝐴 ∧ 𝐴 = ∪ 𝑦))})
Theorem	cf0 10016	Value of the cofinality function at 0. Exercise 2 of [TakeutiZaring] p. 102. (Contributed by NM, 16-Apr-2004.)
⊢ (cf‘∅) = ∅
Theorem	cardcf 10017	Cofinality is a cardinal number. Proposition 11.11 of [TakeutiZaring] p. 103. (Contributed by NM, 24-Apr-2004.) (Revised by Mario Carneiro, 15-Sep-2013.)
⊢ (card‘(cf‘𝐴)) = (cf‘𝐴)
Theorem	cflecard 10018	Cofinality is bounded by the cardinality of its argument. (Contributed by NM, 24-Apr-2004.) (Revised by Mario Carneiro, 15-Sep-2013.)
⊢ (cf‘𝐴) ⊆ (card‘𝐴)
Theorem	cfle 10019	Cofinality is bounded by its argument. Exercise 1 of [TakeutiZaring] p. 102. (Contributed by NM, 26-Apr-2004.) (Revised by Mario Carneiro, 15-Sep-2013.)
⊢ (cf‘𝐴) ⊆ 𝐴
Theorem	cfon 10020	The cofinality of any set is an ordinal (although it only makes sense when 𝐴 is an ordinal). (Contributed by Mario Carneiro, 9-Mar-2013.)
⊢ (cf‘𝐴) ∈ On
Theorem	cfeq0 10021	Only the ordinal zero has cofinality zero. (Contributed by NM, 24-Apr-2004.) (Revised by Mario Carneiro, 12-Feb-2013.)
⊢ (𝐴 ∈ On → ((cf‘𝐴) = ∅ ↔ 𝐴 = ∅))
Theorem	cfsuc 10022	Value of the cofinality function at a successor ordinal. Exercise 3 of [TakeutiZaring] p. 102. (Contributed by NM, 23-Apr-2004.) (Revised by Mario Carneiro, 12-Feb-2013.)
⊢ (𝐴 ∈ On → (cf‘suc 𝐴) = 1o)
Theorem	cff1 10023*	There is always a map from (cf‘𝐴) to 𝐴 (this is a stronger condition than the definition, which only presupposes a map from some 𝑦 ≈ (cf‘𝐴). (Contributed by Mario Carneiro, 28-Feb-2013.)
⊢ (𝐴 ∈ On → ∃𝑓(𝑓:(cf‘𝐴)–1-1→𝐴 ∧ ∀𝑧 ∈ 𝐴 ∃𝑤 ∈ (cf‘𝐴)𝑧 ⊆ (𝑓‘𝑤)))
Theorem	cfflb 10024*	If there is a cofinal map from 𝐵 to 𝐴, then 𝐵 is at least (cf‘𝐴). This theorem and cff1 10023 motivate the picture of (cf‘𝐴) as the greatest lower bound of the domain of cofinal maps into 𝐴. (Contributed by Mario Carneiro, 28-Feb-2013.)
⊢ ((𝐴 ∈ On ∧ 𝐵 ∈ On) → (∃𝑓(𝑓:𝐵⟶𝐴 ∧ ∀𝑧 ∈ 𝐴 ∃𝑤 ∈ 𝐵 𝑧 ⊆ (𝑓‘𝑤)) → (cf‘𝐴) ⊆ 𝐵))
Theorem	cfval2 10025*	Another expression for the cofinality function. (Contributed by Mario Carneiro, 28-Feb-2013.)
⊢ (𝐴 ∈ On → (cf‘𝐴) = ∩ 𝑥 ∈ {𝑥 ∈ 𝒫 𝐴 ∣ ∀𝑧 ∈ 𝐴 ∃𝑤 ∈ 𝑥 𝑧 ⊆ 𝑤} (card‘𝑥))
Theorem	coflim 10026*	A simpler expression for the cofinality predicate, at a limit ordinal. (Contributed by Mario Carneiro, 28-Feb-2013.)
⊢ ((Lim 𝐴 ∧ 𝐵 ⊆ 𝐴) → (∪ 𝐵 = 𝐴 ↔ ∀𝑥 ∈ 𝐴 ∃𝑦 ∈ 𝐵 𝑥 ⊆ 𝑦))
Theorem	cflim3 10027*	Another expression for the cofinality function. (Contributed by Mario Carneiro, 28-Feb-2013.)
⊢ 𝐴 ∈ V    ⇒   ⊢ (Lim 𝐴 → (cf‘𝐴) = ∩ 𝑥 ∈ {𝑥 ∈ 𝒫 𝐴 ∣ ∪ 𝑥 = 𝐴} (card‘𝑥))
Theorem	cflim2 10028	The cofinality function is a limit ordinal iff its argument is. (Contributed by Mario Carneiro, 28-Feb-2013.) (Revised by Mario Carneiro, 15-Sep-2013.)
⊢ 𝐴 ∈ V    ⇒   ⊢ (Lim 𝐴 ↔ Lim (cf‘𝐴))
Theorem	cfom 10029	Value of the cofinality function at omega (the set of natural numbers). Exercise 4 of [TakeutiZaring] p. 102. (Contributed by NM, 23-Apr-2004.) (Proof shortened by Mario Carneiro, 11-Jun-2015.)
⊢ (cf‘ω) = ω
Theorem	cfss 10030*	There is a cofinal subset of 𝐴 of cardinality (cf‘𝐴). (Contributed by Mario Carneiro, 24-Jun-2013.)
⊢ 𝐴 ∈ V    ⇒   ⊢ (Lim 𝐴 → ∃𝑥(𝑥 ⊆ 𝐴 ∧ 𝑥 ≈ (cf‘𝐴) ∧ ∪ 𝑥 = 𝐴))
Theorem	cfslb 10031	Any cofinal subset of 𝐴 is at least as large as (cf‘𝐴). (Contributed by Mario Carneiro, 24-Jun-2013.)
⊢ 𝐴 ∈ V    ⇒   ⊢ ((Lim 𝐴 ∧ 𝐵 ⊆ 𝐴 ∧ ∪ 𝐵 = 𝐴) → (cf‘𝐴) ≼ 𝐵)
Theorem	cfslbn 10032	Any subset of 𝐴 smaller than its cofinality has union less than 𝐴. (This is the contrapositive to cfslb 10031.) (Contributed by Mario Carneiro, 24-Jun-2013.)
⊢ 𝐴 ∈ V    ⇒   ⊢ ((Lim 𝐴 ∧ 𝐵 ⊆ 𝐴 ∧ 𝐵 ≺ (cf‘𝐴)) → ∪ 𝐵 ∈ 𝐴)
Theorem	cfslb2n 10033*	Any small collection of small subsets of 𝐴 cannot have union 𝐴, where "small" means smaller than the cofinality. This is a stronger version of cfslb 10031. This is a common application of cofinality: under AC, (ℵ‘1) is regular, so it is not a countable union of countable sets. (Contributed by Mario Carneiro, 24-Jun-2013.)
⊢ 𝐴 ∈ V    ⇒   ⊢ ((Lim 𝐴 ∧ ∀𝑥 ∈ 𝐵 (𝑥 ⊆ 𝐴 ∧ 𝑥 ≺ (cf‘𝐴))) → (𝐵 ≺ (cf‘𝐴) → ∪ 𝐵 ≠ 𝐴))
Theorem	cofsmo 10034*	Any cofinal map implies the existence of a strictly monotone cofinal map with a domain no larger than the original. Proposition 11.7 of [TakeutiZaring] p. 101. (Contributed by Mario Carneiro, 20-Mar-2013.)
⊢ 𝐶 = {𝑦 ∈ 𝐵 ∣ ∀𝑤 ∈ 𝑦 (𝑓‘𝑤) ∈ (𝑓‘𝑦)}    &   ⊢ 𝐾 = ∩ {𝑥 ∈ 𝐵 ∣ 𝑧 ⊆ (𝑓‘𝑥)}    &   ⊢ 𝑂 = OrdIso( E , 𝐶)    ⇒   ⊢ ((Ord 𝐴 ∧ 𝐵 ∈ On) → (∃𝑓(𝑓:𝐵⟶𝐴 ∧ ∀𝑧 ∈ 𝐴 ∃𝑤 ∈ 𝐵 𝑧 ⊆ (𝑓‘𝑤)) → ∃𝑥 ∈ suc 𝐵∃𝑔(𝑔:𝑥⟶𝐴 ∧ Smo 𝑔 ∧ ∀𝑧 ∈ 𝐴 ∃𝑣 ∈ 𝑥 𝑧 ⊆ (𝑔‘𝑣))))
Theorem	cfsmolem 10035*	Lemma for cfsmo 10036. (Contributed by Mario Carneiro, 28-Feb-2013.)
⊢ 𝐹 = (𝑧 ∈ V ↦ ((𝑔‘dom 𝑧) ∪ ∪ 𝑡 ∈ dom 𝑧 suc (𝑧‘𝑡)))    &   ⊢ 𝐺 = (recs(𝐹) ↾ (cf‘𝐴))    ⇒   ⊢ (𝐴 ∈ On → ∃𝑓(𝑓:(cf‘𝐴)⟶𝐴 ∧ Smo 𝑓 ∧ ∀𝑧 ∈ 𝐴 ∃𝑤 ∈ (cf‘𝐴)𝑧 ⊆ (𝑓‘𝑤)))
Theorem	cfsmo 10036*	The map in cff1 10023 can be assumed to be a strictly monotone ordinal function without loss of generality. (Contributed by Mario Carneiro, 28-Feb-2013.)
⊢ (𝐴 ∈ On → ∃𝑓(𝑓:(cf‘𝐴)⟶𝐴 ∧ Smo 𝑓 ∧ ∀𝑧 ∈ 𝐴 ∃𝑤 ∈ (cf‘𝐴)𝑧 ⊆ (𝑓‘𝑤)))
Theorem	cfcoflem 10037*	Lemma for cfcof 10039, showing subset relation in one direction. (Contributed by Mario Carneiro, 9-Mar-2013.) (Revised by Mario Carneiro, 26-Dec-2014.)
⊢ ((𝐴 ∈ On ∧ 𝐵 ∈ On) → (∃𝑓(𝑓:𝐵⟶𝐴 ∧ Smo 𝑓 ∧ ∀𝑥 ∈ 𝐴 ∃𝑦 ∈ 𝐵 𝑥 ⊆ (𝑓‘𝑦)) → (cf‘𝐴) ⊆ (cf‘𝐵)))
Theorem	coftr 10038*	If there is a cofinal map from 𝐵 to 𝐴 and another from 𝐶 to 𝐴, then there is also a cofinal map from 𝐶 to 𝐵. Proposition 11.9 of [TakeutiZaring] p. 102. A limited form of transitivity for the "cof" relation. This is really a lemma for cfcof 10039. (Contributed by Mario Carneiro, 16-Mar-2013.)
⊢ 𝐻 = (𝑡 ∈ 𝐶 ↦ ∩ {𝑛 ∈ 𝐵 ∣ (𝑔‘𝑡) ⊆ (𝑓‘𝑛)})    ⇒   ⊢ (∃𝑓(𝑓:𝐵⟶𝐴 ∧ Smo 𝑓 ∧ ∀𝑥 ∈ 𝐴 ∃𝑦 ∈ 𝐵 𝑥 ⊆ (𝑓‘𝑦)) → (∃𝑔(𝑔:𝐶⟶𝐴 ∧ ∀𝑧 ∈ 𝐴 ∃𝑤 ∈ 𝐶 𝑧 ⊆ (𝑔‘𝑤)) → ∃ℎ(ℎ:𝐶⟶𝐵 ∧ ∀𝑠 ∈ 𝐵 ∃𝑤 ∈ 𝐶 𝑠 ⊆ (ℎ‘𝑤))))
Theorem	cfcof 10039*	If there is a cofinal map from 𝐴 to 𝐵, then they have the same cofinality. This was used as Definition 11.1 of [TakeutiZaring] p. 100, who defines an equivalence relation cof (𝐴, 𝐵) and defines our cf(𝐵) as the minimum 𝐵 such that cof (𝐴, 𝐵). (Contributed by Mario Carneiro, 20-Mar-2013.)
⊢ ((𝐴 ∈ On ∧ 𝐵 ∈ On) → (∃𝑓(𝑓:𝐵⟶𝐴 ∧ Smo 𝑓 ∧ ∀𝑧 ∈ 𝐴 ∃𝑤 ∈ 𝐵 𝑧 ⊆ (𝑓‘𝑤)) → (cf‘𝐴) = (cf‘𝐵)))
Theorem	cfidm 10040	The cofinality function is idempotent. (Contributed by Mario Carneiro, 7-Mar-2013.) (Revised by Mario Carneiro, 15-Sep-2013.)
⊢ (cf‘(cf‘𝐴)) = (cf‘𝐴)
Theorem	alephsing 10041	The cofinality of a limit aleph is the same as the cofinality of its argument, so if (ℵ‘𝐴) < 𝐴, then (ℵ‘𝐴) is singular. Conversely, if (ℵ‘𝐴) is regular (i.e. weakly inaccessible), then (ℵ‘𝐴) = 𝐴, so 𝐴 has to be rather large (see alephfp 9873). Proposition 11.13 of [TakeutiZaring] p. 103. (Contributed by Mario Carneiro, 9-Mar-2013.)
⊢ (Lim 𝐴 → (cf‘(ℵ‘𝐴)) = (cf‘𝐴))
2.6.16  Eight inequivalent definitions of finite set
Theorem	sornom 10042*	The range of a single-step monotone function from ω into a partially ordered set is a chain. (Contributed by Stefan O'Rear, 3-Nov-2014.)
⊢ ((𝐹 Fn ω ∧ ∀𝑎 ∈ ω ((𝐹‘𝑎)𝑅(𝐹‘suc 𝑎) ∨ (𝐹‘𝑎) = (𝐹‘suc 𝑎)) ∧ 𝑅 Po ran 𝐹) → 𝑅 Or ran 𝐹)
Syntax	cfin1a 10043	Extend class notation to include the class of Ia-finite sets.
class FinIa
Syntax	cfin2 10044	Extend class notation to include the class of II-finite sets.
class FinII
Syntax	cfin4 10045	Extend class notation to include the class of IV-finite sets.
class FinIV
Syntax	cfin3 10046	Extend class notation to include the class of III-finite sets.
class FinIII
Syntax	cfin5 10047	Extend class notation to include the class of V-finite sets.
class FinV
Syntax	cfin6 10048	Extend class notation to include the class of VI-finite sets.
class FinVI
Syntax	cfin7 10049	Extend class notation to include the class of VII-finite sets.
class FinVII
Definition	df-fin1a 10050*	A set is Ia-finite iff it is not the union of two I-infinite sets. Equivalent to definition Ia of [Levy58] p. 2. A I-infinite Ia-finite set is also known as an amorphous set. This is the second of Levy's eight definitions of finite set. Levy's I-finite is equivalent to our df-fin 8746 and not repeated here. These eight definitions are equivalent with Choice but strictly decreasing in strength in models where Choice fails; conversely, they provide a series of increasingly stronger notions of infiniteness. (Contributed by Stefan O'Rear, 12-Nov-2014.)
⊢ FinIa = {𝑥 ∣ ∀𝑦 ∈ 𝒫 𝑥(𝑦 ∈ Fin ∨ (𝑥 ∖ 𝑦) ∈ Fin)}
Definition	df-fin2 10051*	A set is II-finite (Tarski finite) iff every nonempty chain of subsets contains a maximum element. Definition II of [Levy58] p. 2. (Contributed by Stefan O'Rear, 12-Nov-2014.)
⊢ FinII = {𝑥 ∣ ∀𝑦 ∈ 𝒫 𝒫 𝑥((𝑦 ≠ ∅ ∧ [⊊] Or 𝑦) → ∪ 𝑦 ∈ 𝑦)}
Definition	df-fin4 10052*	A set is IV-finite (Dedekind finite) iff it has no equinumerous proper subset. Definition IV of [Levy58] p. 3. (Contributed by Stefan O'Rear, 12-Nov-2014.)
⊢ FinIV = {𝑥 ∣ ¬ ∃𝑦(𝑦 ⊊ 𝑥 ∧ 𝑦 ≈ 𝑥)}
Definition	df-fin3 10053	A set is III-finite (weakly Dedekind finite) iff its power set is Dedekind finite. Definition III of [Levy58] p. 2. (Contributed by Stefan O'Rear, 12-Nov-2014.)
⊢ FinIII = {𝑥 ∣ 𝒫 𝑥 ∈ FinIV}
Definition	df-fin5 10054	A set is V-finite iff it behaves finitely under ⊔. Definition V of [Levy58] p. 3. (Contributed by Stefan O'Rear, 12-Nov-2014.)
⊢ FinV = {𝑥 ∣ (𝑥 = ∅ ∨ 𝑥 ≺ (𝑥 ⊔ 𝑥))}
Definition	df-fin6 10055	A set is VI-finite iff it behaves finitely under ×. Definition VI of [Levy58] p. 4. (Contributed by Stefan O'Rear, 12-Nov-2014.)
⊢ FinVI = {𝑥 ∣ (𝑥 ≺ 2o ∨ 𝑥 ≺ (𝑥 × 𝑥))}
Definition	df-fin7 10056*	A set is VII-finite iff it cannot be infinitely well-ordered. Equivalent to definition VII of [Levy58] p. 4. (Contributed by Stefan O'Rear, 12-Nov-2014.)
⊢ FinVII = {𝑥 ∣ ¬ ∃𝑦 ∈ (On ∖ ω)𝑥 ≈ 𝑦}
Theorem	isfin1a 10057*	Definition of a Ia-finite set. (Contributed by Stefan O'Rear, 16-May-2015.)
⊢ (𝐴 ∈ 𝑉 → (𝐴 ∈ FinIa ↔ ∀𝑦 ∈ 𝒫 𝐴(𝑦 ∈ Fin ∨ (𝐴 ∖ 𝑦) ∈ Fin)))
Theorem	fin1ai 10058	Property of a Ia-finite set. (Contributed by Stefan O'Rear, 16-May-2015.)
⊢ ((𝐴 ∈ FinIa ∧ 𝑋 ⊆ 𝐴) → (𝑋 ∈ Fin ∨ (𝐴 ∖ 𝑋) ∈ Fin))
Theorem	isfin2 10059*	Definition of a II-finite set. (Contributed by Stefan O'Rear, 16-May-2015.)
⊢ (𝐴 ∈ 𝑉 → (𝐴 ∈ FinII ↔ ∀𝑦 ∈ 𝒫 𝒫 𝐴((𝑦 ≠ ∅ ∧ [⊊] Or 𝑦) → ∪ 𝑦 ∈ 𝑦)))
Theorem	fin2i 10060	Property of a II-finite set. (Contributed by Stefan O'Rear, 16-May-2015.)
⊢ (((𝐴 ∈ FinII ∧ 𝐵 ⊆ 𝒫 𝐴) ∧ (𝐵 ≠ ∅ ∧ [⊊] Or 𝐵)) → ∪ 𝐵 ∈ 𝐵)
Theorem	isfin3 10061	Definition of a III-finite set. (Contributed by Stefan O'Rear, 16-May-2015.)
⊢ (𝐴 ∈ FinIII ↔ 𝒫 𝐴 ∈ FinIV)
Theorem	isfin4 10062*	Definition of a IV-finite set. (Contributed by Stefan O'Rear, 16-May-2015.)
⊢ (𝐴 ∈ 𝑉 → (𝐴 ∈ FinIV ↔ ¬ ∃𝑦(𝑦 ⊊ 𝐴 ∧ 𝑦 ≈ 𝐴)))
Theorem	fin4i 10063	Infer that a set is IV-infinite. (Contributed by Stefan O'Rear, 16-May-2015.)
⊢ ((𝑋 ⊊ 𝐴 ∧ 𝑋 ≈ 𝐴) → ¬ 𝐴 ∈ FinIV)
Theorem	isfin5 10064	Definition of a V-finite set. (Contributed by Stefan O'Rear, 16-May-2015.)
⊢ (𝐴 ∈ FinV ↔ (𝐴 = ∅ ∨ 𝐴 ≺ (𝐴 ⊔ 𝐴)))
Theorem	isfin6 10065	Definition of a VI-finite set. (Contributed by Stefan O'Rear, 16-May-2015.)
⊢ (𝐴 ∈ FinVI ↔ (𝐴 ≺ 2o ∨ 𝐴 ≺ (𝐴 × 𝐴)))
Theorem	isfin7 10066*	Definition of a VII-finite set. (Contributed by Stefan O'Rear, 16-May-2015.)
⊢ (𝐴 ∈ 𝑉 → (𝐴 ∈ FinVII ↔ ¬ ∃𝑦 ∈ (On ∖ ω)𝐴 ≈ 𝑦))
Theorem	sdom2en01 10067	A set with less than two elements has 0 or 1. (Contributed by Stefan O'Rear, 30-Oct-2014.)
⊢ (𝐴 ≺ 2o ↔ (𝐴 = ∅ ∨ 𝐴 ≈ 1o))
Theorem	infpssrlem1 10068	Lemma for infpssr 10073. (Contributed by Stefan O'Rear, 30-Oct-2014.)
⊢ (𝜑 → 𝐵 ⊆ 𝐴)    &   ⊢ (𝜑 → 𝐹:𝐵–1-1-onto→𝐴)    &   ⊢ (𝜑 → 𝐶 ∈ (𝐴 ∖ 𝐵))    &   ⊢ 𝐺 = (rec(◡𝐹, 𝐶) ↾ ω)    ⇒   ⊢ (𝜑 → (𝐺‘∅) = 𝐶)
Theorem	infpssrlem2 10069	Lemma for infpssr 10073. (Contributed by Stefan O'Rear, 30-Oct-2014.)
⊢ (𝜑 → 𝐵 ⊆ 𝐴)    &   ⊢ (𝜑 → 𝐹:𝐵–1-1-onto→𝐴)    &   ⊢ (𝜑 → 𝐶 ∈ (𝐴 ∖ 𝐵))    &   ⊢ 𝐺 = (rec(◡𝐹, 𝐶) ↾ ω)    ⇒   ⊢ (𝑀 ∈ ω → (𝐺‘suc 𝑀) = (◡𝐹‘(𝐺‘𝑀)))
Theorem	infpssrlem3 10070	Lemma for infpssr 10073. (Contributed by Stefan O'Rear, 30-Oct-2014.)
⊢ (𝜑 → 𝐵 ⊆ 𝐴)    &   ⊢ (𝜑 → 𝐹:𝐵–1-1-onto→𝐴)    &   ⊢ (𝜑 → 𝐶 ∈ (𝐴 ∖ 𝐵))    &   ⊢ 𝐺 = (rec(◡𝐹, 𝐶) ↾ ω)    ⇒   ⊢ (𝜑 → 𝐺:ω⟶𝐴)
Theorem	infpssrlem4 10071	Lemma for infpssr 10073. (Contributed by Stefan O'Rear, 30-Oct-2014.)
⊢ (𝜑 → 𝐵 ⊆ 𝐴)    &   ⊢ (𝜑 → 𝐹:𝐵–1-1-onto→𝐴)    &   ⊢ (𝜑 → 𝐶 ∈ (𝐴 ∖ 𝐵))    &   ⊢ 𝐺 = (rec(◡𝐹, 𝐶) ↾ ω)    ⇒   ⊢ ((𝜑 ∧ 𝑀 ∈ ω ∧ 𝑁 ∈ 𝑀) → (𝐺‘𝑀) ≠ (𝐺‘𝑁))
Theorem	infpssrlem5 10072	Lemma for infpssr 10073. (Contributed by Stefan O'Rear, 30-Oct-2014.)
⊢ (𝜑 → 𝐵 ⊆ 𝐴)    &   ⊢ (𝜑 → 𝐹:𝐵–1-1-onto→𝐴)    &   ⊢ (𝜑 → 𝐶 ∈ (𝐴 ∖ 𝐵))    &   ⊢ 𝐺 = (rec(◡𝐹, 𝐶) ↾ ω)    ⇒   ⊢ (𝜑 → (𝐴 ∈ 𝑉 → ω ≼ 𝐴))
Theorem	infpssr 10073	Dedekind infinity implies existence of a denumerable subset: take a single point witnessing the proper subset relation and iterate the embedding. (Contributed by Stefan O'Rear, 30-Oct-2014.) (Revised by Mario Carneiro, 16-May-2015.)
⊢ ((𝑋 ⊊ 𝐴 ∧ 𝑋 ≈ 𝐴) → ω ≼ 𝐴)
Theorem	fin4en1 10074	Dedekind finite is a cardinal property. (Contributed by Stefan O'Rear, 30-Oct-2014.) (Revised by Mario Carneiro, 16-May-2015.)
⊢ (𝐴 ≈ 𝐵 → (𝐴 ∈ FinIV → 𝐵 ∈ FinIV))
Theorem	ssfin4 10075	Dedekind finite sets have Dedekind finite subsets. (Contributed by Stefan O'Rear, 30-Oct-2014.) (Revised by Mario Carneiro, 6-May-2015.) (Revised by Mario Carneiro, 16-May-2015.)
⊢ ((𝐴 ∈ FinIV ∧ 𝐵 ⊆ 𝐴) → 𝐵 ∈ FinIV)
Theorem	domfin4 10076	A set dominated by a Dedekind finite set is Dedekind finite. (Contributed by Mario Carneiro, 16-May-2015.)
⊢ ((𝐴 ∈ FinIV ∧ 𝐵 ≼ 𝐴) → 𝐵 ∈ FinIV)
Theorem	ominf4 10077	ω is Dedekind infinite. (Contributed by Stefan O'Rear, 30-Oct-2014.) (Proof shortened by Mario Carneiro, 16-May-2015.)
⊢ ¬ ω ∈ FinIV
Theorem	infpssALT 10078*	Alternate proof of infpss 9982, shorter but requiring Replacement (ax-rep 5210). (Contributed by Stefan O'Rear, 30-Oct-2014.) (Revised by Mario Carneiro, 16-May-2015.) (Proof modification is discouraged.) (New usage is discouraged.)
⊢ (ω ≼ 𝐴 → ∃𝑥(𝑥 ⊊ 𝐴 ∧ 𝑥 ≈ 𝐴))
Theorem	isfin4-2 10079	Alternate definition of IV-finite sets: they lack a denumerable subset. (Contributed by Stefan O'Rear, 30-Oct-2014.) (Revised by Mario Carneiro, 17-May-2015.)
⊢ (𝐴 ∈ 𝑉 → (𝐴 ∈ FinIV ↔ ¬ ω ≼ 𝐴))
Theorem	isfin4p1 10080	Alternate definition of IV-finite sets: they are strictly dominated by their successors. (Thus, the proper subset referred to in isfin4 10062 can be assumed to be only a singleton smaller than the original.) (Contributed by Mario Carneiro, 18-May-2015.)
⊢ (𝐴 ∈ FinIV ↔ 𝐴 ≺ (𝐴 ⊔ 1o))
Theorem	fin23lem7 10081*	Lemma for isfin2-2 10084. The componentwise complement of a nonempty collection of sets is nonempty. (Contributed by Stefan O'Rear, 31-Oct-2014.) (Revised by Mario Carneiro, 16-May-2015.)
⊢ ((𝐴 ∈ 𝑉 ∧ 𝐵 ⊆ 𝒫 𝐴 ∧ 𝐵 ≠ ∅) → {𝑥 ∈ 𝒫 𝐴 ∣ (𝐴 ∖ 𝑥) ∈ 𝐵} ≠ ∅)
Theorem	fin23lem11 10082*	Lemma for isfin2-2 10084. (Contributed by Stefan O'Rear, 31-Oct-2014.) (Revised by Mario Carneiro, 16-May-2015.)
⊢ (𝑧 = (𝐴 ∖ 𝑥) → (𝜓 ↔ 𝜒))    &   ⊢ (𝑤 = (𝐴 ∖ 𝑣) → (𝜑 ↔ 𝜃))    &   ⊢ ((𝑥 ⊆ 𝐴 ∧ 𝑣 ⊆ 𝐴) → (𝜒 ↔ 𝜃))    ⇒   ⊢ (𝐵 ⊆ 𝒫 𝐴 → (∃𝑥 ∈ {𝑐 ∈ 𝒫 𝐴 ∣ (𝐴 ∖ 𝑐) ∈ 𝐵}∀𝑤 ∈ {𝑐 ∈ 𝒫 𝐴 ∣ (𝐴 ∖ 𝑐) ∈ 𝐵} ¬ 𝜑 → ∃𝑧 ∈ 𝐵 ∀𝑣 ∈ 𝐵 ¬ 𝜓))
Theorem	fin2i2 10083	A II-finite set contains minimal elements for every nonempty chain. (Contributed by Mario Carneiro, 16-May-2015.)
⊢ (((𝐴 ∈ FinII ∧ 𝐵 ⊆ 𝒫 𝐴) ∧ (𝐵 ≠ ∅ ∧ [⊊] Or 𝐵)) → ∩ 𝐵 ∈ 𝐵)
Theorem	isfin2-2 10084*	FinII expressed in terms of minimal elements. (Contributed by Stefan O'Rear, 2-Nov-2014.) (Proof shortened by Mario Carneiro, 16-May-2015.)
⊢ (𝐴 ∈ 𝑉 → (𝐴 ∈ FinII ↔ ∀𝑦 ∈ 𝒫 𝒫 𝐴((𝑦 ≠ ∅ ∧ [⊊] Or 𝑦) → ∩ 𝑦 ∈ 𝑦)))
Theorem	ssfin2 10085	A subset of a II-finite set is II-finite. (Contributed by Stefan O'Rear, 2-Nov-2014.) (Revised by Mario Carneiro, 16-May-2015.)
⊢ ((𝐴 ∈ FinII ∧ 𝐵 ⊆ 𝐴) → 𝐵 ∈ FinII)
Theorem	enfin2i 10086	II-finiteness is a cardinal property. (Contributed by Mario Carneiro, 18-May-2015.)
⊢ (𝐴 ≈ 𝐵 → (𝐴 ∈ FinII → 𝐵 ∈ FinII))
Theorem	fin23lem24 10087	Lemma for fin23 10154. In a class of ordinals, each element is fully identified by those of its predecessors which also belong to the class. (Contributed by Stefan O'Rear, 1-Nov-2014.)
⊢ (((Ord 𝐴 ∧ 𝐵 ⊆ 𝐴) ∧ (𝐶 ∈ 𝐵 ∧ 𝐷 ∈ 𝐵)) → ((𝐶 ∩ 𝐵) = (𝐷 ∩ 𝐵) ↔ 𝐶 = 𝐷))
Theorem	fincssdom 10088	In a chain of finite sets, dominance and subset coincide. (Contributed by Stefan O'Rear, 8-Nov-2014.)
⊢ ((𝐴 ∈ Fin ∧ 𝐵 ∈ Fin ∧ (𝐴 ⊆ 𝐵 ∨ 𝐵 ⊆ 𝐴)) → (𝐴 ≼ 𝐵 ↔ 𝐴 ⊆ 𝐵))
Theorem	fin23lem25 10089	Lemma for fin23 10154. In a chain of finite sets, equinumerosity is equivalent to equality. (Contributed by Stefan O'Rear, 1-Nov-2014.)
⊢ ((𝐴 ∈ Fin ∧ 𝐵 ∈ Fin ∧ (𝐴 ⊆ 𝐵 ∨ 𝐵 ⊆ 𝐴)) → (𝐴 ≈ 𝐵 ↔ 𝐴 = 𝐵))
Theorem	fin23lem26 10090*	Lemma for fin23lem22 10092. (Contributed by Stefan O'Rear, 1-Nov-2014.)
⊢ (((𝑆 ⊆ ω ∧ ¬ 𝑆 ∈ Fin) ∧ 𝑖 ∈ ω) → ∃𝑗 ∈ 𝑆 (𝑗 ∩ 𝑆) ≈ 𝑖)
Theorem	fin23lem23 10091*	Lemma for fin23lem22 10092. (Contributed by Stefan O'Rear, 1-Nov-2014.)
⊢ (((𝑆 ⊆ ω ∧ ¬ 𝑆 ∈ Fin) ∧ 𝑖 ∈ ω) → ∃!𝑗 ∈ 𝑆 (𝑗 ∩ 𝑆) ≈ 𝑖)
Theorem	fin23lem22 10092*	Lemma for fin23 10154 but could be used elsewhere if we find a good name for it. Explicit construction of a bijection (actually an isomorphism, see fin23lem27 10093) between an infinite subset of ω and ω itself. (Contributed by Stefan O'Rear, 1-Nov-2014.)
⊢ 𝐶 = (𝑖 ∈ ω ↦ (℩𝑗 ∈ 𝑆 (𝑗 ∩ 𝑆) ≈ 𝑖))    ⇒   ⊢ ((𝑆 ⊆ ω ∧ ¬ 𝑆 ∈ Fin) → 𝐶:ω–1-1-onto→𝑆)
Theorem	fin23lem27 10093*	The mapping constructed in fin23lem22 10092 is in fact an isomorphism. (Contributed by Stefan O'Rear, 2-Nov-2014.)
⊢ 𝐶 = (𝑖 ∈ ω ↦ (℩𝑗 ∈ 𝑆 (𝑗 ∩ 𝑆) ≈ 𝑖))    ⇒   ⊢ ((𝑆 ⊆ ω ∧ ¬ 𝑆 ∈ Fin) → 𝐶 Isom E , E (ω, 𝑆))
Theorem	isfin3ds 10094*	Property of a III-finite set (descending sequence version). (Contributed by Mario Carneiro, 16-May-2015.)
⊢ 𝐹 = {𝑔 ∣ ∀𝑎 ∈ (𝒫 𝑔 ↑m ω)(∀𝑏 ∈ ω (𝑎‘suc 𝑏) ⊆ (𝑎‘𝑏) → ∩ ran 𝑎 ∈ ran 𝑎)}    ⇒   ⊢ (𝐴 ∈ 𝑉 → (𝐴 ∈ 𝐹 ↔ ∀𝑓 ∈ (𝒫 𝐴 ↑m ω)(∀𝑥 ∈ ω (𝑓‘suc 𝑥) ⊆ (𝑓‘𝑥) → ∩ ran 𝑓 ∈ ran 𝑓)))
Theorem	ssfin3ds 10095*	A subset of a III-finite set is III-finite. (Contributed by Stefan O'Rear, 4-Nov-2014.)
⊢ 𝐹 = {𝑔 ∣ ∀𝑎 ∈ (𝒫 𝑔 ↑m ω)(∀𝑏 ∈ ω (𝑎‘suc 𝑏) ⊆ (𝑎‘𝑏) → ∩ ran 𝑎 ∈ ran 𝑎)}    ⇒   ⊢ ((𝐴 ∈ 𝐹 ∧ 𝐵 ⊆ 𝐴) → 𝐵 ∈ 𝐹)
Theorem	fin23lem12 10096*	The beginning of the proof that every II-finite set (every chain of subsets has a maximal element) is III-finite (has no denumerable collection of subsets). This first section is dedicated to the construction of 𝑈 and its intersection. First, the value of 𝑈 at a successor. (Contributed by Stefan O'Rear, 1-Nov-2014.)
⊢ 𝑈 = seqω((𝑖 ∈ ω, 𝑢 ∈ V ↦ if(((𝑡‘𝑖) ∩ 𝑢) = ∅, 𝑢, ((𝑡‘𝑖) ∩ 𝑢))), ∪ ran 𝑡)    ⇒   ⊢ (𝐴 ∈ ω → (𝑈‘suc 𝐴) = if(((𝑡‘𝐴) ∩ (𝑈‘𝐴)) = ∅, (𝑈‘𝐴), ((𝑡‘𝐴) ∩ (𝑈‘𝐴))))
Theorem	fin23lem13 10097*	Lemma for fin23 10154. Each step of 𝑈 is a decrease. (Contributed by Stefan O'Rear, 1-Nov-2014.)
⊢ 𝑈 = seqω((𝑖 ∈ ω, 𝑢 ∈ V ↦ if(((𝑡‘𝑖) ∩ 𝑢) = ∅, 𝑢, ((𝑡‘𝑖) ∩ 𝑢))), ∪ ran 𝑡)    ⇒   ⊢ (𝐴 ∈ ω → (𝑈‘suc 𝐴) ⊆ (𝑈‘𝐴))
Theorem	fin23lem14 10098*	Lemma for fin23 10154. 𝑈 will never evolve to an empty set if it did not start with one. (Contributed by Stefan O'Rear, 1-Nov-2014.)
⊢ 𝑈 = seqω((𝑖 ∈ ω, 𝑢 ∈ V ↦ if(((𝑡‘𝑖) ∩ 𝑢) = ∅, 𝑢, ((𝑡‘𝑖) ∩ 𝑢))), ∪ ran 𝑡)    ⇒   ⊢ ((𝐴 ∈ ω ∧ ∪ ran 𝑡 ≠ ∅) → (𝑈‘𝐴) ≠ ∅)
Theorem	fin23lem15 10099*	Lemma for fin23 10154. 𝑈 is a monotone function. (Contributed by Stefan O'Rear, 1-Nov-2014.)
⊢ 𝑈 = seqω((𝑖 ∈ ω, 𝑢 ∈ V ↦ if(((𝑡‘𝑖) ∩ 𝑢) = ∅, 𝑢, ((𝑡‘𝑖) ∩ 𝑢))), ∪ ran 𝑡)    ⇒   ⊢ (((𝐴 ∈ ω ∧ 𝐵 ∈ ω) ∧ 𝐵 ⊆ 𝐴) → (𝑈‘𝐴) ⊆ (𝑈‘𝐵))
Theorem	fin23lem16 10100*	Lemma for fin23 10154. 𝑈 ranges over the original set; in particular ran 𝑈 is a set, although we do not assume here that 𝑈 is. (Contributed by Stefan O'Rear, 1-Nov-2014.)
⊢ 𝑈 = seqω((𝑖 ∈ ω, 𝑢 ∈ V ↦ if(((𝑡‘𝑖) ∩ 𝑢) = ∅, 𝑢, ((𝑡‘𝑖) ∩ 𝑢))), ∪ ran 𝑡)    ⇒   ⊢ ∪ ran 𝑈 = ∪ ran 𝑡