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Theorem onfin 8701
 Description: An ordinal number is finite iff it is a natural number. Proposition 10.32 of [TakeutiZaring] p. 92. (Contributed by NM, 26-Jul-2004.)
Assertion
Ref Expression
onfin (𝐴 ∈ On → (𝐴 ∈ Fin ↔ 𝐴 ∈ ω))

Proof of Theorem onfin
Dummy variable 𝑥 is distinct from all other variables.
StepHypRef Expression
1 isfi 8523 . 2 (𝐴 ∈ Fin ↔ ∃𝑥 ∈ ω 𝐴𝑥)
2 onomeneq 8700 . . . . 5 ((𝐴 ∈ On ∧ 𝑥 ∈ ω) → (𝐴𝑥𝐴 = 𝑥))
3 eleq1a 2911 . . . . . 6 (𝑥 ∈ ω → (𝐴 = 𝑥𝐴 ∈ ω))
43adantl 485 . . . . 5 ((𝐴 ∈ On ∧ 𝑥 ∈ ω) → (𝐴 = 𝑥𝐴 ∈ ω))
52, 4sylbid 243 . . . 4 ((𝐴 ∈ On ∧ 𝑥 ∈ ω) → (𝐴𝑥𝐴 ∈ ω))
65rexlimdva 3277 . . 3 (𝐴 ∈ On → (∃𝑥 ∈ ω 𝐴𝑥𝐴 ∈ ω))
7 enrefg 8531 . . . 4 (𝐴 ∈ ω → 𝐴𝐴)
8 breq2 5056 . . . . 5 (𝑥 = 𝐴 → (𝐴𝑥𝐴𝐴))
98rspcev 3609 . . . 4 ((𝐴 ∈ ω ∧ 𝐴𝐴) → ∃𝑥 ∈ ω 𝐴𝑥)
107, 9mpdan 686 . . 3 (𝐴 ∈ ω → ∃𝑥 ∈ ω 𝐴𝑥)
116, 10impbid1 228 . 2 (𝐴 ∈ On → (∃𝑥 ∈ ω 𝐴𝑥𝐴 ∈ ω))
121, 11syl5bb 286 1 (𝐴 ∈ On → (𝐴 ∈ Fin ↔ 𝐴 ∈ ω))
 Colors of variables: wff setvar class Syntax hints:   → wi 4   ↔ wb 209   ∧ wa 399   = wceq 1538   ∈ wcel 2115  ∃wrex 3134   class class class wbr 5052  Oncon0 6178  ωcom 7570   ≈ cen 8496  Fincfn 8499 This theorem was proved from axioms:  ax-mp 5  ax-1 6  ax-2 7  ax-3 8  ax-gen 1797  ax-4 1811  ax-5 1912  ax-6 1971  ax-7 2016  ax-8 2117  ax-9 2125  ax-10 2146  ax-11 2162  ax-12 2179  ax-ext 2796  ax-sep 5189  ax-nul 5196  ax-pow 5253  ax-pr 5317  ax-un 7451 This theorem depends on definitions:  df-bi 210  df-an 400  df-or 845  df-3or 1085  df-3an 1086  df-tru 1541  df-ex 1782  df-nf 1786  df-sb 2071  df-mo 2624  df-eu 2655  df-clab 2803  df-cleq 2817  df-clel 2896  df-nfc 2964  df-ne 3015  df-ral 3138  df-rex 3139  df-rab 3142  df-v 3482  df-sbc 3759  df-dif 3922  df-un 3924  df-in 3926  df-ss 3936  df-pss 3938  df-nul 4276  df-if 4450  df-pw 4523  df-sn 4550  df-pr 4552  df-tp 4554  df-op 4556  df-uni 4825  df-br 5053  df-opab 5115  df-tr 5159  df-id 5447  df-eprel 5452  df-po 5461  df-so 5462  df-fr 5501  df-we 5503  df-xp 5548  df-rel 5549  df-cnv 5550  df-co 5551  df-dm 5552  df-rn 5553  df-res 5554  df-ima 5555  df-ord 6181  df-on 6182  df-lim 6183  df-suc 6184  df-iota 6302  df-fun 6345  df-fn 6346  df-f 6347  df-f1 6348  df-fo 6349  df-f1o 6350  df-fv 6351  df-om 7571  df-er 8279  df-en 8500  df-dom 8501  df-sdom 8502  df-fin 8503 This theorem is referenced by:  onfin2  8702  fin17  9808  isfin7-2  9810
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