Theorem mo0sn 46049

Description: Two ways of expressing "at most one" element in a class. (Contributed by Zhi Wang, 19-Sep-2024.)

Assertion

Ref	Expression
mo0sn	⊢ (∃*𝑥 𝑥 ∈ 𝐴 ↔ (𝐴 = ∅ ∨ ∃𝑦 𝐴 = {𝑦}))

Distinct variable groups:   𝑥,𝐴   𝑦,𝐴

Proof of Theorem mo0sn

Dummy variable 𝑧 is distinct from all other variables.

Step	Hyp	Ref	Expression
1		nfv 1918	. . 3 ⊢ Ⅎ𝑧 𝑥 ∈ 𝐴
2		nfv 1918	. . 3 ⊢ Ⅎ𝑥 𝑧 ∈ 𝐴
3		eleq1w 2821	. . 3 ⊢ (𝑥 = 𝑧 → (𝑥 ∈ 𝐴 ↔ 𝑧 ∈ 𝐴))
4	1, 2, 3	cbvmow 2603	. 2 ⊢ (∃*𝑥 𝑥 ∈ 𝐴 ↔ ∃*𝑧 𝑧 ∈ 𝐴)
5		neq0 4276	. . . . . . . 8 ⊢ (¬ 𝐴 = ∅ ↔ ∃𝑧 𝑧 ∈ 𝐴)
6	5	anbi1i 623	. . . . . . 7 ⊢ ((¬ 𝐴 = ∅ ∧ ∃*𝑧 𝑧 ∈ 𝐴) ↔ (∃𝑧 𝑧 ∈ 𝐴 ∧ ∃*𝑧 𝑧 ∈ 𝐴))
7		df-eu 2569	. . . . . . 7 ⊢ (∃!𝑧 𝑧 ∈ 𝐴 ↔ (∃𝑧 𝑧 ∈ 𝐴 ∧ ∃*𝑧 𝑧 ∈ 𝐴))
8		eu6 2574	. . . . . . 7 ⊢ (∃!𝑧 𝑧 ∈ 𝐴 ↔ ∃𝑦∀𝑧(𝑧 ∈ 𝐴 ↔ 𝑧 = 𝑦))
9	6, 7, 8	3bitr2i 298	. . . . . 6 ⊢ ((¬ 𝐴 = ∅ ∧ ∃*𝑧 𝑧 ∈ 𝐴) ↔ ∃𝑦∀𝑧(𝑧 ∈ 𝐴 ↔ 𝑧 = 𝑦))
10		dfcleq 2731	. . . . . . . 8 ⊢ (𝐴 = {𝑦} ↔ ∀𝑧(𝑧 ∈ 𝐴 ↔ 𝑧 ∈ {𝑦}))
11		velsn 4574	. . . . . . . . . 10 ⊢ (𝑧 ∈ {𝑦} ↔ 𝑧 = 𝑦)
12	11	bibi2i 337	. . . . . . . . 9 ⊢ ((𝑧 ∈ 𝐴 ↔ 𝑧 ∈ {𝑦}) ↔ (𝑧 ∈ 𝐴 ↔ 𝑧 = 𝑦))
13	12	albii 1823	. . . . . . . 8 ⊢ (∀𝑧(𝑧 ∈ 𝐴 ↔ 𝑧 ∈ {𝑦}) ↔ ∀𝑧(𝑧 ∈ 𝐴 ↔ 𝑧 = 𝑦))
14	10, 13	sylbbr 235	. . . . . . 7 ⊢ (∀𝑧(𝑧 ∈ 𝐴 ↔ 𝑧 = 𝑦) → 𝐴 = {𝑦})
15	14	eximi 1838	. . . . . 6 ⊢ (∃𝑦∀𝑧(𝑧 ∈ 𝐴 ↔ 𝑧 = 𝑦) → ∃𝑦 𝐴 = {𝑦})
16	9, 15	sylbi 216	. . . . 5 ⊢ ((¬ 𝐴 = ∅ ∧ ∃*𝑧 𝑧 ∈ 𝐴) → ∃𝑦 𝐴 = {𝑦})
17	16	expcom 413	. . . 4 ⊢ (∃*𝑧 𝑧 ∈ 𝐴 → (¬ 𝐴 = ∅ → ∃𝑦 𝐴 = {𝑦}))
18	17	orrd 859	. . 3 ⊢ (∃*𝑧 𝑧 ∈ 𝐴 → (𝐴 = ∅ ∨ ∃𝑦 𝐴 = {𝑦}))
19		mo0 46047	. . . 4 ⊢ (𝐴 = ∅ → ∃*𝑧 𝑧 ∈ 𝐴)
20		mosn 46046	. . . . 5 ⊢ (𝐴 = {𝑦} → ∃*𝑧 𝑧 ∈ 𝐴)
21	20	exlimiv 1934	. . . 4 ⊢ (∃𝑦 𝐴 = {𝑦} → ∃*𝑧 𝑧 ∈ 𝐴)
22	19, 21	jaoi 853	. . 3 ⊢ ((𝐴 = ∅ ∨ ∃𝑦 𝐴 = {𝑦}) → ∃*𝑧 𝑧 ∈ 𝐴)
23	18, 22	impbii 208	. 2 ⊢ (∃*𝑧 𝑧 ∈ 𝐴 ↔ (𝐴 = ∅ ∨ ∃𝑦 𝐴 = {𝑦}))
24	4, 23	bitri 274	1 ⊢ (∃*𝑥 𝑥 ∈ 𝐴 ↔ (𝐴 = ∅ ∨ ∃𝑦 𝐴 = {𝑦}))

Syntax hints:  ¬ wn 3   ↔ wb 205   ∧ wa 395   ∨ wo 843  ∀wal 1537   = wceq 1539  ∃wex 1783   ∈ wcel 2108  ∃*wmo 2538  ∃!weu 2568  ∅c0 4253  {csn 4558

This theorem was proved from axioms:  ax-mp 5  ax-1 6  ax-2 7  ax-3 8  ax-gen 1799  ax-4 1813  ax-5 1914  ax-6 1972  ax-7 2012  ax-8 2110  ax-9 2118  ax-10 2139  ax-11 2156  ax-12 2173  ax-ext 2709  ax-sep 5218

This theorem depends on definitions:  df-bi 206  df-an 396  df-or 844  df-3an 1087  df-tru 1542  df-fal 1552  df-ex 1784  df-nf 1788  df-sb 2069  df-mo 2540  df-eu 2569  df-clab 2716  df-cleq 2730  df-clel 2817  df-nfc 2888  df-ral 3068  df-rex 3069  df-reu 3070  df-rmo 3071  df-v 3424  df-sbc 3712  df-dif 3886  df-nul 4254  df-sn 4559

This theorem is referenced by:  mosssn2  46050  mofmo  46062  mofeu  46063  f1mo  46068  setc2othin  46225