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Theorem fin23 8790
Description: 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 nonempty 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 8719) this sequence contains its intersection, call it ; since by induction every subset in the sequence is nonempty, the intersection must be nonempty.

Suppose that an element of has nonempty intersection with . Thus, said element has a nonempty 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 nonempty set .

The preceding 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 8847 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.)

Assertion
Ref Expression
fin23

Proof of Theorem fin23
Dummy variables are mutually distinct and distinct from all other variables.
StepHypRef Expression
1 isf33lem 8767 . 2
21fin23lem40 8752 1
Colors of variables: wff setvar class
Syntax hints:  ->wi 4  e.wcel 1818   cfin2 8680   cfin3 8682
This theorem is referenced by:  fin1a2s  8815  finngch  9054  fin2so  30040
This theorem was proved from axioms:  ax-mp 5  ax-1 6  ax-2 7  ax-3 8  ax-gen 1618  ax-4 1631  ax-5 1704  ax-6 1747  ax-7 1790  ax-8 1820  ax-9 1822  ax-10 1837  ax-11 1842  ax-12 1854  ax-13 1999  ax-ext 2435  ax-rep 4563  ax-sep 4573  ax-nul 4581  ax-pow 4630  ax-pr 4691  ax-un 6592
This theorem depends on definitions:  df-bi 185  df-or 370  df-an 371  df-3or 974  df-3an 975  df-tru 1398  df-ex 1613  df-nf 1617  df-sb 1740  df-eu 2286  df-mo 2287  df-clab 2443  df-cleq 2449  df-clel 2452  df-nfc 2607  df-ne 2654  df-ral 2812  df-rex 2813  df-reu 2814  df-rmo 2815  df-rab 2816  df-v 3111  df-sbc 3328  df-csb 3435  df-dif 3478  df-un 3480  df-in 3482  df-ss 3489  df-pss 3491  df-nul 3785  df-if 3942  df-pw 4014  df-sn 4030  df-pr 4032  df-tp 4034  df-op 4036  df-uni 4250  df-int 4287  df-iun 4332  df-br 4453  df-opab 4511  df-mpt 4512  df-tr 4546  df-eprel 4796  df-id 4800  df-po 4805  df-so 4806  df-fr 4843  df-se 4844  df-we 4845  df-ord 4886  df-on 4887  df-lim 4888  df-suc 4889  df-xp 5010  df-rel 5011  df-cnv 5012  df-co 5013  df-dm 5014  df-rn 5015  df-res 5016  df-ima 5017  df-iota 5556  df-fun 5595  df-fn 5596  df-f 5597  df-f1 5598  df-fo 5599  df-f1o 5600  df-fv 5601  df-isom 5602  df-riota 6257  df-ov 6299  df-oprab 6300  df-mpt2 6301  df-rpss 6580  df-om 6701  df-1st 6800  df-2nd 6801  df-recs 7061  df-rdg 7095  df-seqom 7132  df-1o 7149  df-oadd 7153  df-er 7330  df-map 7441  df-en 7537  df-dom 7538  df-sdom 7539  df-fin 7540  df-wdom 8006  df-card 8341  df-fin2 8687  df-fin4 8688  df-fin3 8689
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