Theorem pm54.43 9923

Description: Theorem *54.43 of [WhiteheadRussell] p. 360. "From this proposition it will follow, when arithmetical addition has been defined, that 1+1=2." See http://en.wikipedia.org/wiki/Principia_Mathematica#Quotations. This theorem states that two sets of cardinality 1 are disjoint iff their union has cardinality 2.

Whitehead and Russell define 1 as the collection of all sets with cardinality 1 (i.e. all singletons; see card1 9890), so that their 𝐴 ∈ 1 means, in our notation, 𝐴 ∈ {𝑥 ∣ (card‘𝑥) = 1_o} which is the same as 𝐴 ≈ 1_o by pm54.43lem 9922. We do not have several of their earlier lemmas available (which would otherwise be unused by our different approach to arithmetic), so our proof is longer. (It is also longer because we must show every detail.)

Theorem dju1p1e2 10094 shows the derivation of 1+1=2 for cardinal numbers from this theorem. (Contributed by NM, 4-Apr-2007.)

Assertion

Ref	Expression
pm54.43	⊢ ((𝐴 ≈ 1o ∧ 𝐵 ≈ 1o) → ((𝐴 ∩ 𝐵) = ∅ ↔ (𝐴 ∪ 𝐵) ≈ 2o))

Proof of Theorem pm54.43

Dummy variables 𝑥 𝑦 are mutually distinct and distinct from all other variables.

Step	Hyp	Ref	Expression
1		1oex 8412	. . . . . . 7 ⊢ 1o ∈ V
2	1	ensn1 8965	. . . . . 6 ⊢ {1o} ≈ 1o
3	2	ensymi 8948	. . . . 5 ⊢ 1o ≈ {1o}
4		entr 8950	. . . . 5 ⊢ ((𝐵 ≈ 1o ∧ 1o ≈ {1o}) → 𝐵 ≈ {1o})
5	3, 4	mpan2 697	. . . 4 ⊢ (𝐵 ≈ 1o → 𝐵 ≈ {1o})
6		1on 8414	. . . . . . . 8 ⊢ 1o ∈ On
7	6	onirri 6431	. . . . . . 7 ⊢ ¬ 1o ∈ 1o
8		disjsn 4650	. . . . . . 7 ⊢ ((1o ∩ {1o}) = ∅ ↔ ¬ 1o ∈ 1o)
9	7, 8	mpbir 232	. . . . . 6 ⊢ (1o ∩ {1o}) = ∅
10		unen 8989	. . . . . 6 ⊢ (((𝐴 ≈ 1o ∧ 𝐵 ≈ {1o}) ∧ ((𝐴 ∩ 𝐵) = ∅ ∧ (1o ∩ {1o}) = ∅)) → (𝐴 ∪ 𝐵) ≈ (1o ∪ {1o}))
11	9, 10	mpanr2 710	. . . . 5 ⊢ (((𝐴 ≈ 1o ∧ 𝐵 ≈ {1o}) ∧ (𝐴 ∩ 𝐵) = ∅) → (𝐴 ∪ 𝐵) ≈ (1o ∪ {1o}))
12	11	ex 413	. . . 4 ⊢ ((𝐴 ≈ 1o ∧ 𝐵 ≈ {1o}) → ((𝐴 ∩ 𝐵) = ∅ → (𝐴 ∪ 𝐵) ≈ (1o ∪ {1o})))
13	5, 12	sylan2 599	. . 3 ⊢ ((𝐴 ≈ 1o ∧ 𝐵 ≈ 1o) → ((𝐴 ∩ 𝐵) = ∅ → (𝐴 ∪ 𝐵) ≈ (1o ∪ {1o})))
14		df-2o 8403	. . . . 5 ⊢ 2o = suc 1o
15		df-suc 6323	. . . . 5 ⊢ suc 1o = (1o ∪ {1o})
16	14, 15	eqtri 2763	. . . 4 ⊢ 2o = (1o ∪ {1o})
17	16	breq2i 5087	. . 3 ⊢ ((𝐴 ∪ 𝐵) ≈ 2o ↔ (𝐴 ∪ 𝐵) ≈ (1o ∪ {1o}))
18	13, 17	imbitrrdi 253	. 2 ⊢ ((𝐴 ≈ 1o ∧ 𝐵 ≈ 1o) → ((𝐴 ∩ 𝐵) = ∅ → (𝐴 ∪ 𝐵) ≈ 2o))
19		en1 8968	. . 3 ⊢ (𝐴 ≈ 1o ↔ ∃𝑥 𝐴 = {𝑥})
20		en1 8968	. . 3 ⊢ (𝐵 ≈ 1o ↔ ∃𝑦 𝐵 = {𝑦})
21		sneq 4572	. . . . . . . . . . . . . . 15 ⊢ (𝑥 = 𝑦 → {𝑥} = {𝑦})
22	21	uneq2d 4105	. . . . . . . . . . . . . 14 ⊢ (𝑥 = 𝑦 → ({𝑥} ∪ {𝑥}) = ({𝑥} ∪ {𝑦}))
23		unidm 4094	. . . . . . . . . . . . . 14 ⊢ ({𝑥} ∪ {𝑥}) = {𝑥}
24	22, 23	eqtr3di 2790	. . . . . . . . . . . . 13 ⊢ (𝑥 = 𝑦 → ({𝑥} ∪ {𝑦}) = {𝑥})
25		vex 3436	. . . . . . . . . . . . . . 15 ⊢ 𝑥 ∈ V
26	25	ensn1 8965	. . . . . . . . . . . . . 14 ⊢ {𝑥} ≈ 1o
27		1sdom2 9155	. . . . . . . . . . . . . 14 ⊢ 1o ≺ 2o
28		ensdomtr 9048	. . . . . . . . . . . . . 14 ⊢ (({𝑥} ≈ 1o ∧ 1o ≺ 2o) → {𝑥} ≺ 2o)
29	26, 27, 28	mp2an 698	. . . . . . . . . . . . 13 ⊢ {𝑥} ≺ 2o
30	24, 29	eqbrtrdi 5118	. . . . . . . . . . . 12 ⊢ (𝑥 = 𝑦 → ({𝑥} ∪ {𝑦}) ≺ 2o)
31		sdomnen 8925	. . . . . . . . . . . 12 ⊢ (({𝑥} ∪ {𝑦}) ≺ 2o → ¬ ({𝑥} ∪ {𝑦}) ≈ 2o)
32	30, 31	syl 17	. . . . . . . . . . 11 ⊢ (𝑥 = 𝑦 → ¬ ({𝑥} ∪ {𝑦}) ≈ 2o)
33	32	necon2ai 2964	. . . . . . . . . 10 ⊢ (({𝑥} ∪ {𝑦}) ≈ 2o → 𝑥 ≠ 𝑦)
34		disjsn2 4651	. . . . . . . . . 10 ⊢ (𝑥 ≠ 𝑦 → ({𝑥} ∩ {𝑦}) = ∅)
35	33, 34	syl 17	. . . . . . . . 9 ⊢ (({𝑥} ∪ {𝑦}) ≈ 2o → ({𝑥} ∩ {𝑦}) = ∅)
36	35	a1i 11	. . . . . . . 8 ⊢ ((𝐴 = {𝑥} ∧ 𝐵 = {𝑦}) → (({𝑥} ∪ {𝑦}) ≈ 2o → ({𝑥} ∩ {𝑦}) = ∅))
37		uneq12 4100	. . . . . . . . 9 ⊢ ((𝐴 = {𝑥} ∧ 𝐵 = {𝑦}) → (𝐴 ∪ 𝐵) = ({𝑥} ∪ {𝑦}))
38	37	breq1d 5089	. . . . . . . 8 ⊢ ((𝐴 = {𝑥} ∧ 𝐵 = {𝑦}) → ((𝐴 ∪ 𝐵) ≈ 2o ↔ ({𝑥} ∪ {𝑦}) ≈ 2o))
39		ineq12 4151	. . . . . . . . 9 ⊢ ((𝐴 = {𝑥} ∧ 𝐵 = {𝑦}) → (𝐴 ∩ 𝐵) = ({𝑥} ∩ {𝑦}))
40	39	eqeq1d 2742	. . . . . . . 8 ⊢ ((𝐴 = {𝑥} ∧ 𝐵 = {𝑦}) → ((𝐴 ∩ 𝐵) = ∅ ↔ ({𝑥} ∩ {𝑦}) = ∅))
41	36, 38, 40	3imtr4d 295	. . . . . . 7 ⊢ ((𝐴 = {𝑥} ∧ 𝐵 = {𝑦}) → ((𝐴 ∪ 𝐵) ≈ 2o → (𝐴 ∩ 𝐵) = ∅))
42	41	ex 413	. . . . . 6 ⊢ (𝐴 = {𝑥} → (𝐵 = {𝑦} → ((𝐴 ∪ 𝐵) ≈ 2o → (𝐴 ∩ 𝐵) = ∅)))
43	42	exlimdv 1940	. . . . 5 ⊢ (𝐴 = {𝑥} → (∃𝑦 𝐵 = {𝑦} → ((𝐴 ∪ 𝐵) ≈ 2o → (𝐴 ∩ 𝐵) = ∅)))
44	43	exlimiv 1937	. . . 4 ⊢ (∃𝑥 𝐴 = {𝑥} → (∃𝑦 𝐵 = {𝑦} → ((𝐴 ∪ 𝐵) ≈ 2o → (𝐴 ∩ 𝐵) = ∅)))
45	44	imp 407	. . 3 ⊢ ((∃𝑥 𝐴 = {𝑥} ∧ ∃𝑦 𝐵 = {𝑦}) → ((𝐴 ∪ 𝐵) ≈ 2o → (𝐴 ∩ 𝐵) = ∅))
46	19, 20, 45	syl2anb 604	. 2 ⊢ ((𝐴 ≈ 1o ∧ 𝐵 ≈ 1o) → ((𝐴 ∪ 𝐵) ≈ 2o → (𝐴 ∩ 𝐵) = ∅))
47	18, 46	impbid 213	1 ⊢ ((𝐴 ≈ 1o ∧ 𝐵 ≈ 1o) → ((𝐴 ∩ 𝐵) = ∅ ↔ (𝐴 ∪ 𝐵) ≈ 2o))

Syntax hints:  ¬ wn 3   → wi 4   ↔ wb 207   ∧ wa 396   = wceq 1547  ∃wex 1786   ∈ wcel 2119   ≠ wne 2935   ∪ cun 3888   ∩ cin 3889  ∅c0 4268  {csn 4562   class class class wbr 5079  suc csuc 6319  1_oc1o 8395  2_oc2o 8396   ≈ cen 8887   ≺ csdm 8889

This theorem was proved from axioms:  ax-mp 5  ax-1 6  ax-2 7  ax-3 8  ax-gen 1802  ax-4 1816  ax-5 1917  ax-6 1974  ax-7 2015  ax-8 2121  ax-9 2129  ax-10 2152  ax-11 2168  ax-12 2189  ax-ext 2712  ax-sep 5225  ax-nul 5235  ax-pow 5301  ax-pr 5369  ax-un 7685

This theorem depends on definitions:  df-bi 208  df-an 397  df-or 854  df-3or 1093  df-3an 1094  df-tru 1550  df-fal 1560  df-ex 1787  df-nf 1791  df-sb 2074  df-mo 2543  df-eu 2573  df-clab 2719  df-cleq 2732  df-clel 2815  df-nfc 2889  df-ne 2936  df-ral 3055  df-rex 3065  df-reu 3346  df-rab 3393  df-v 3434  df-dif 3893  df-un 3895  df-in 3897  df-ss 3907  df-pss 3910  df-nul 4269  df-if 4462  df-pw 4538  df-sn 4563  df-pr 4565  df-op 4569  df-uni 4846  df-br 5080  df-opab 5142  df-tr 5187  df-id 5520  df-eprel 5525  df-po 5533  df-so 5534  df-fr 5578  df-we 5580  df-xp 5631  df-rel 5632  df-cnv 5633  df-co 5634  df-dm 5635  df-rn 5636  df-res 5637  df-ima 5638  df-ord 6320  df-on 6321  df-suc 6323  df-iota 6448  df-fun 6494  df-fn 6495  df-f 6496  df-f1 6497  df-fo 6498  df-f1o 6499  df-fv 6500  df-1o 8402  df-2o 8403  df-er 8640  df-en 8891  df-dom 8892  df-sdom 8893

This theorem is referenced by:  dju1p1e2  10094