Metamath Proof Explorer


Theorem en1eqsn

Description: A set with one element is a singleton. (Contributed by FL, 18-Aug-2008) Avoid ax-pow , ax-un . (Revised by BTernaryTau, 4-Jan-2025)

Ref Expression
Assertion en1eqsn ( ( 𝐴𝐵𝐵 ≈ 1o ) → 𝐵 = { 𝐴 } )

Proof

Step Hyp Ref Expression
1 en1 ( 𝐵 ≈ 1o ↔ ∃ 𝑥 𝐵 = { 𝑥 } )
2 eleq2 ( 𝐵 = { 𝑥 } → ( 𝐴𝐵𝐴 ∈ { 𝑥 } ) )
3 elsni ( 𝐴 ∈ { 𝑥 } → 𝐴 = 𝑥 )
4 3 sneqd ( 𝐴 ∈ { 𝑥 } → { 𝐴 } = { 𝑥 } )
5 2 4 syl6bi ( 𝐵 = { 𝑥 } → ( 𝐴𝐵 → { 𝐴 } = { 𝑥 } ) )
6 5 imp ( ( 𝐵 = { 𝑥 } ∧ 𝐴𝐵 ) → { 𝐴 } = { 𝑥 } )
7 eqtr3 ( ( 𝐵 = { 𝑥 } ∧ { 𝐴 } = { 𝑥 } ) → 𝐵 = { 𝐴 } )
8 6 7 syldan ( ( 𝐵 = { 𝑥 } ∧ 𝐴𝐵 ) → 𝐵 = { 𝐴 } )
9 8 ex ( 𝐵 = { 𝑥 } → ( 𝐴𝐵𝐵 = { 𝐴 } ) )
10 9 exlimiv ( ∃ 𝑥 𝐵 = { 𝑥 } → ( 𝐴𝐵𝐵 = { 𝐴 } ) )
11 1 10 sylbi ( 𝐵 ≈ 1o → ( 𝐴𝐵𝐵 = { 𝐴 } ) )
12 11 impcom ( ( 𝐴𝐵𝐵 ≈ 1o ) → 𝐵 = { 𝐴 } )