Theorem isbth 6944

Description: Schroeder-Bernstein Theorem. Theorem 18 of [Suppes] p. 95. This theorem states that if set A is smaller (has lower cardinality) than B and vice-versa, then A and B are equinumerous (have the same cardinality). The interesting thing is that this can be proved without invoking the Axiom of Choice, as we do here, but the proof as you can see is quite difficult. (The theorem can be proved more easily if we allow AC.) The main proof consists of lemmas sbthlem1 6934 through sbthlemi10 6943; this final piece mainly changes bound variables to eliminate the hypotheses of sbthlemi10 6943. We follow closely the proof in Suppes, which you should consult to understand our proof at a higher level. Note that Suppes' proof, which is credited to J. M. Whitaker, does not require the Axiom of Infinity. The proof does require the law of the excluded middle which cannot be avoided as shown at exmidsbthr 14055. (Contributed by NM, 8-Jun-1998.)

Assertion

Ref	Expression
isbth	|- ( (EXMID /\ ( A ~<_ B /\ B ~<_ A ) ) -> A ~~ B )

Proof of Theorem isbth

Dummy variables x y z w f g are mutually distinct and distinct from all other variables.

Step	Hyp	Ref	Expression
1		simprl 526	. 2 |- ( (EXMID /\ ( A ~<_ B /\ B ~<_ A ) ) -> A ~<_ B )
2		simprr 527	. 2 |- ( (EXMID /\ ( A ~<_ B /\ B ~<_ A ) ) -> B ~<_ A )
3		reldom 6723	. . . . 5 |- Rel ~<_
4	3	brrelex1i 4654	. . . 4 |- ( B ~<_ A -> B e. _V )
5	2, 4	syl 14	. . 3 |- ( (EXMID /\ ( A ~<_ B /\ B ~<_ A ) ) -> B e. _V )
6		breq2 3993	. . . . . 6 |- ( w = B -> ( A ~<_ w <-> A ~<_ B ) )
7		breq1 3992	. . . . . 6 |- ( w = B -> ( w ~<_ A <-> B ~<_ A ) )
8	6, 7	anbi12d 470	. . . . 5 |- ( w = B -> ( ( A ~<_ w /\ w ~<_ A ) <-> ( A ~<_ B /\ B ~<_ A ) ) )
9		breq2 3993	. . . . 5 |- ( w = B -> ( A ~~ w <-> A ~~ B ) )
10	8, 9	imbi12d 233	. . . 4 |- ( w = B -> ( ( ( A ~<_ w /\ w ~<_ A ) -> A ~~ w ) <-> ( ( A ~<_ B /\ B ~<_ A ) -> A ~~ B ) ) )
11	10	adantl 275	. . 3 |- ( ( (EXMID /\ ( A ~<_ B /\ B ~<_ A ) ) /\ w = B ) -> ( ( ( A ~<_ w /\ w ~<_ A ) -> A ~~ w ) <-> ( ( A ~<_ B /\ B ~<_ A ) -> A ~~ B ) ) )
12	3	brrelex1i 4654	. . . . 5 |- ( A ~<_ B -> A e. _V )
13	1, 12	syl 14	. . . 4 |- ( (EXMID /\ ( A ~<_ B /\ B ~<_ A ) ) -> A e. _V )
14		breq1 3992	. . . . . . 7 |- ( z = A -> ( z ~<_ w <-> A ~<_ w ) )
15		breq2 3993	. . . . . . 7 |- ( z = A -> ( w ~<_ z <-> w ~<_ A ) )
16	14, 15	anbi12d 470	. . . . . 6 |- ( z = A -> ( ( z ~<_ w /\ w ~<_ z ) <-> ( A ~<_ w /\ w ~<_ A ) ) )
17		breq1 3992	. . . . . 6 |- ( z = A -> ( z ~~ w <-> A ~~ w ) )
18	16, 17	imbi12d 233	. . . . 5 |- ( z = A -> ( ( ( z ~<_ w /\ w ~<_ z ) -> z ~~ w ) <-> ( ( A ~<_ w /\ w ~<_ A ) -> A ~~ w ) ) )
19	18	adantl 275	. . . 4 |- ( ( (EXMID /\ ( A ~<_ B /\ B ~<_ A ) ) /\ z = A ) -> ( ( ( z ~<_ w /\ w ~<_ z ) -> z ~~ w ) <-> ( ( A ~<_ w /\ w ~<_ A ) -> A ~~ w ) ) )
20		vex 2733	. . . . . . 7 |- z e. _V
21		sseq1 3170	. . . . . . . . 9 |- ( y = x -> ( y C_ z <-> x C_ z ) )
22		imaeq2 4949	. . . . . . . . . . . 12 |- ( y = x -> ( f " y ) = ( f " x ) )
23	22	difeq2d 3245	. . . . . . . . . . 11 |- ( y = x -> ( w \ ( f " y ) ) = ( w \ ( f " x ) ) )
24	23	imaeq2d 4953	. . . . . . . . . 10 |- ( y = x -> ( g " ( w \ ( f " y ) ) ) = ( g " ( w \ ( f " x ) ) ) )
25		difeq2 3239	. . . . . . . . . 10 |- ( y = x -> ( z \ y ) = ( z \ x ) )
26	24, 25	sseq12d 3178	. . . . . . . . 9 |- ( y = x -> ( ( g " ( w \ ( f " y ) ) ) C_ ( z \ y ) <-> ( g " ( w \ ( f " x ) ) ) C_ ( z \ x ) ) )
27	21, 26	anbi12d 470	. . . . . . . 8 |- ( y = x -> ( ( y C_ z /\ ( g " ( w \ ( f " y ) ) ) C_ ( z \ y ) ) <-> ( x C_ z /\ ( g " ( w \ ( f " x ) ) ) C_ ( z \ x ) ) ) )
28	27	cbvabv 2295	. . . . . . 7 |- { y | ( y C_ z /\ ( g " ( w \ ( f " y ) ) ) C_ ( z \ y ) ) } = { x | ( x C_ z /\ ( g " ( w \ ( f " x ) ) ) C_ ( z \ x ) ) }
29		eqid 2170	. . . . . . 7 |- ( ( f |` U. { y | ( y C_ z /\ ( g " ( w \ ( f " y ) ) ) C_ ( z \ y ) ) } ) u. ( `' g |` ( z \ U. { y | ( y C_ z /\ ( g " ( w \ ( f " y ) ) ) C_ ( z \ y ) ) } ) ) ) = ( ( f |` U. { y | ( y C_ z /\ ( g " ( w \ ( f " y ) ) ) C_ ( z \ y ) ) } ) u. ( `' g |` ( z \ U. { y | ( y C_ z /\ ( g " ( w \ ( f " y ) ) ) C_ ( z \ y ) ) } ) ) )
30		vex 2733	. . . . . . 7 |- w e. _V
31	20, 28, 29, 30	sbthlemi10 6943	. . . . . 6 |- ( (EXMID /\ ( z ~<_ w /\ w ~<_ z ) ) -> z ~~ w )
32	31	ex 114	. . . . 5 |- (EXMID -> ( ( z ~<_ w /\ w ~<_ z ) -> z ~~ w ) )
33	32	adantr 274	. . . 4 |- ( (EXMID /\ ( A ~<_ B /\ B ~<_ A ) ) -> ( ( z ~<_ w /\ w ~<_ z ) -> z ~~ w ) )
34	13, 19, 33	vtocld 2782	. . 3 |- ( (EXMID /\ ( A ~<_ B /\ B ~<_ A ) ) -> ( ( A ~<_ w /\ w ~<_ A ) -> A ~~ w ) )
35	5, 11, 34	vtocld 2782	. 2 |- ( (EXMID /\ ( A ~<_ B /\ B ~<_ A ) ) -> ( ( A ~<_ B /\ B ~<_ A ) -> A ~~ B ) )
36	1, 2, 35	mp2and 431	1 |- ( (EXMID /\ ( A ~<_ B /\ B ~<_ A ) ) -> A ~~ B )

Syntax hints:   -> wi 4   /\ wa 103   <-> wb 104   = wceq 1348   e. wcel 2141   {cab 2156   _Vcvv 2730   \ cdif 3118   u. cun 3119   C_ wss 3121   U.cuni 3796   class class class wbr 3989  EXMIDwem 4180   `'ccnv 4610   |` cres 4613   "cima 4614   ~~ cen 6716   ~<_ cdom 6717

This theorem was proved from axioms:  ax-mp 5  ax-1 6  ax-2 7  ax-ia1 105  ax-ia2 106  ax-ia3 107  ax-in1 609  ax-in2 610  ax-io 704  ax-5 1440  ax-7 1441  ax-gen 1442  ax-ie1 1486  ax-ie2 1487  ax-8 1497  ax-10 1498  ax-11 1499  ax-i12 1500  ax-bndl 1502  ax-4 1503  ax-17 1519  ax-i9 1523  ax-ial 1527  ax-i5r 1528  ax-13 2143  ax-14 2144  ax-ext 2152  ax-sep 4107  ax-nul 4115  ax-pow 4160  ax-pr 4194  ax-un 4418

This theorem depends on definitions:  df-bi 116  df-stab 826  df-dc 830  df-3an 975  df-tru 1351  df-nf 1454  df-sb 1756  df-eu 2022  df-mo 2023  df-clab 2157  df-cleq 2163  df-clel 2166  df-nfc 2301  df-ral 2453  df-rex 2454  df-rab 2457  df-v 2732  df-dif 3123  df-un 3125  df-in 3127  df-ss 3134  df-nul 3415  df-pw 3568  df-sn 3589  df-pr 3590  df-op 3592  df-uni 3797  df-br 3990  df-opab 4051  df-exmid 4181  df-id 4278  df-xp 4617  df-rel 4618  df-cnv 4619  df-co 4620  df-dm 4621  df-rn 4622  df-res 4623  df-ima 4624  df-fun 5200  df-fn 5201  df-f 5202  df-f1 5203  df-fo 5204  df-f1o 5205  df-en 6719  df-dom 6720

This theorem is referenced by:  exmidsbth  14056