Metamath Proof Explorer


Theorem equs45f

Description: Two ways of expressing substitution when y is not free in ph . The implication "to the left" is equs4 and does not require the nonfreeness hypothesis. Theorem sbalex replaces the nonfreeness hypothesis with a disjoint variable condition and equs5 replaces it with a distinctor as antecedent. Usage of this theorem is discouraged because it depends on ax-13 . (Contributed by NM, 25-Apr-2008) (Revised by Mario Carneiro, 4-Oct-2016) (New usage is discouraged.)

Ref Expression
Hypothesis equs45f.1 𝑦 𝜑
Assertion equs45f ( ∃ 𝑥 ( 𝑥 = 𝑦𝜑 ) ↔ ∀ 𝑥 ( 𝑥 = 𝑦𝜑 ) )

Proof

Step Hyp Ref Expression
1 equs45f.1 𝑦 𝜑
2 1 nf5ri ( 𝜑 → ∀ 𝑦 𝜑 )
3 2 anim2i ( ( 𝑥 = 𝑦𝜑 ) → ( 𝑥 = 𝑦 ∧ ∀ 𝑦 𝜑 ) )
4 3 eximi ( ∃ 𝑥 ( 𝑥 = 𝑦𝜑 ) → ∃ 𝑥 ( 𝑥 = 𝑦 ∧ ∀ 𝑦 𝜑 ) )
5 equs5a ( ∃ 𝑥 ( 𝑥 = 𝑦 ∧ ∀ 𝑦 𝜑 ) → ∀ 𝑥 ( 𝑥 = 𝑦𝜑 ) )
6 4 5 syl ( ∃ 𝑥 ( 𝑥 = 𝑦𝜑 ) → ∀ 𝑥 ( 𝑥 = 𝑦𝜑 ) )
7 equs4 ( ∀ 𝑥 ( 𝑥 = 𝑦𝜑 ) → ∃ 𝑥 ( 𝑥 = 𝑦𝜑 ) )
8 6 7 impbii ( ∃ 𝑥 ( 𝑥 = 𝑦𝜑 ) ↔ ∀ 𝑥 ( 𝑥 = 𝑦𝜑 ) )