Theorem equsex 1721

Description: A useful equivalence related to substitution. (Contributed by NM, 5-Aug-1993.) (Revised by NM, 3-Feb-2015.)

Hypotheses

Ref	Expression
equsex.1	⊢ (𝜓 → ∀𝑥𝜓)
equsex.2	⊢ (𝑥 = 𝑦 → (𝜑 ↔ 𝜓))

Assertion

Ref	Expression
equsex	⊢ (∃𝑥(𝑥 = 𝑦 ∧ 𝜑) ↔ 𝜓)

Proof of Theorem equsex

Step	Hyp	Ref	Expression
1		equsex.1	. . 3 ⊢ (𝜓 → ∀𝑥𝜓)
2		equsex.2	. . . 4 ⊢ (𝑥 = 𝑦 → (𝜑 ↔ 𝜓))
3	2	biimpa 294	. . 3 ⊢ ((𝑥 = 𝑦 ∧ 𝜑) → 𝜓)
4	1, 3	exlimih 1586	. 2 ⊢ (∃𝑥(𝑥 = 𝑦 ∧ 𝜑) → 𝜓)
5		a9e 1689	. . 3 ⊢ ∃𝑥 𝑥 = 𝑦
6		idd 21	. . . . 5 ⊢ (𝜓 → (𝑥 = 𝑦 → 𝑥 = 𝑦))
7	2	biimprcd 159	. . . . 5 ⊢ (𝜓 → (𝑥 = 𝑦 → 𝜑))
8	6, 7	jcad 305	. . . 4 ⊢ (𝜓 → (𝑥 = 𝑦 → (𝑥 = 𝑦 ∧ 𝜑)))
9	1, 8	eximdh 1604	. . 3 ⊢ (𝜓 → (∃𝑥 𝑥 = 𝑦 → ∃𝑥(𝑥 = 𝑦 ∧ 𝜑)))
10	5, 9	mpi 15	. 2 ⊢ (𝜓 → ∃𝑥(𝑥 = 𝑦 ∧ 𝜑))
11	4, 10	impbii 125	1 ⊢ (∃𝑥(𝑥 = 𝑦 ∧ 𝜑) ↔ 𝜓)

Syntax hints:   → wi 4   ∧ wa 103   ↔ wb 104  ∀wal 1346   = wceq 1348  ∃wex 1485

This theorem was proved from axioms:  ax-mp 5  ax-1 6  ax-2 7  ax-ia1 105  ax-ia2 106  ax-ia3 107  ax-5 1440  ax-gen 1442  ax-ie1 1486  ax-ie2 1487  ax-4 1503  ax-i9 1523  ax-ial 1527

This theorem depends on definitions:  df-bi 116

This theorem is referenced by:  cbvexv1  1745  cbvexh  1748  sb56  1878  sb10f  1988  cleljust  2147