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| Description: The friendship condition: any two (different) vertices in a friendship graph have a unique common neighbor. (Contributed by Alexander van der Vekens, 19-Dec-2017.) (Revised by AV, 29-Mar-2021.) | 
| Ref | Expression | 
|---|---|
| frcond1.v | ⊢ 𝑉 = (Vtx‘𝐺) | 
| frcond1.e | ⊢ 𝐸 = (Edg‘𝐺) | 
| Ref | Expression | 
|---|---|
| frcond2 | ⊢ (𝐺 ∈ FriendGraph → ((𝐴 ∈ 𝑉 ∧ 𝐶 ∈ 𝑉 ∧ 𝐴 ≠ 𝐶) → ∃!𝑏 ∈ 𝑉 ({𝐴, 𝑏} ∈ 𝐸 ∧ {𝑏, 𝐶} ∈ 𝐸))) | 
| Step | Hyp | Ref | Expression | 
|---|---|---|---|
| 1 | frcond1.v | . . 3 ⊢ 𝑉 = (Vtx‘𝐺) | |
| 2 | frcond1.e | . . 3 ⊢ 𝐸 = (Edg‘𝐺) | |
| 3 | 1, 2 | frcond1 30285 | . 2 ⊢ (𝐺 ∈ FriendGraph → ((𝐴 ∈ 𝑉 ∧ 𝐶 ∈ 𝑉 ∧ 𝐴 ≠ 𝐶) → ∃!𝑏 ∈ 𝑉 {{𝐴, 𝑏}, {𝑏, 𝐶}} ⊆ 𝐸)) | 
| 4 | prex 5437 | . . . . 5 ⊢ {𝐴, 𝑏} ∈ V | |
| 5 | prex 5437 | . . . . 5 ⊢ {𝑏, 𝐶} ∈ V | |
| 6 | 4, 5 | prss 4820 | . . . 4 ⊢ (({𝐴, 𝑏} ∈ 𝐸 ∧ {𝑏, 𝐶} ∈ 𝐸) ↔ {{𝐴, 𝑏}, {𝑏, 𝐶}} ⊆ 𝐸) | 
| 7 | 6 | bicomi 224 | . . 3 ⊢ ({{𝐴, 𝑏}, {𝑏, 𝐶}} ⊆ 𝐸 ↔ ({𝐴, 𝑏} ∈ 𝐸 ∧ {𝑏, 𝐶} ∈ 𝐸)) | 
| 8 | 7 | reubii 3389 | . 2 ⊢ (∃!𝑏 ∈ 𝑉 {{𝐴, 𝑏}, {𝑏, 𝐶}} ⊆ 𝐸 ↔ ∃!𝑏 ∈ 𝑉 ({𝐴, 𝑏} ∈ 𝐸 ∧ {𝑏, 𝐶} ∈ 𝐸)) | 
| 9 | 3, 8 | imbitrdi 251 | 1 ⊢ (𝐺 ∈ FriendGraph → ((𝐴 ∈ 𝑉 ∧ 𝐶 ∈ 𝑉 ∧ 𝐴 ≠ 𝐶) → ∃!𝑏 ∈ 𝑉 ({𝐴, 𝑏} ∈ 𝐸 ∧ {𝑏, 𝐶} ∈ 𝐸))) | 
| Colors of variables: wff setvar class | 
| Syntax hints: → wi 4 ∧ wa 395 ∧ w3a 1087 = wceq 1540 ∈ wcel 2108 ≠ wne 2940 ∃!wreu 3378 ⊆ wss 3951 {cpr 4628 ‘cfv 6561 Vtxcvtx 29013 Edgcedg 29064 FriendGraph cfrgr 30277 | 
| This theorem was proved from axioms: ax-mp 5 ax-1 6 ax-2 7 ax-3 8 ax-gen 1795 ax-4 1809 ax-5 1910 ax-6 1967 ax-7 2007 ax-8 2110 ax-9 2118 ax-10 2141 ax-11 2157 ax-12 2177 ax-ext 2708 ax-sep 5296 ax-nul 5306 ax-pr 5432 | 
| This theorem depends on definitions: df-bi 207 df-an 396 df-or 849 df-3an 1089 df-tru 1543 df-fal 1553 df-ex 1780 df-nf 1784 df-sb 2065 df-mo 2540 df-eu 2569 df-clab 2715 df-cleq 2729 df-clel 2816 df-nfc 2892 df-ne 2941 df-ral 3062 df-rex 3071 df-rmo 3380 df-reu 3381 df-rab 3437 df-v 3482 df-sbc 3789 df-csb 3900 df-dif 3954 df-un 3956 df-ss 3968 df-nul 4334 df-if 4526 df-sn 4627 df-pr 4629 df-op 4633 df-uni 4908 df-br 5144 df-iota 6514 df-fv 6569 df-frgr 30278 | 
| This theorem is referenced by: frgreu 30287 frgrncvvdeqlem9 30326 frgr2wwlkeu 30346 numclwwlk2lem1 30395 | 
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