Theorem 19.9t 1666

Description: A closed version of 19.9 1668. (Contributed by NM, 5-Aug-1993.) (Revised by Mario Carneiro, 24-Sep-2016.) (Proof shortended by Wolf Lammen, 30-Dec-2017.)

Assertion

Ref	Expression
19.9t	⊢ (Ⅎ𝑥𝜑 → (∃𝑥𝜑 ↔ 𝜑))

Proof of Theorem 19.9t

Step	Hyp	Ref	Expression
1		df-nf 1485	. . 3 ⊢ (Ⅎ𝑥𝜑 ↔ ∀𝑥(𝜑 → ∀𝑥𝜑))
2		19.9ht 1665	. . 3 ⊢ (∀𝑥(𝜑 → ∀𝑥𝜑) → (∃𝑥𝜑 → 𝜑))
3	1, 2	sylbi 121	. 2 ⊢ (Ⅎ𝑥𝜑 → (∃𝑥𝜑 → 𝜑))
4		19.8a 1614	. 2 ⊢ (𝜑 → ∃𝑥𝜑)
5	3, 4	impbid1 142	1 ⊢ (Ⅎ𝑥𝜑 → (∃𝑥𝜑 ↔ 𝜑))

Syntax hints:   → wi 4   ↔ wb 105  ∀wal 1371  Ⅎwnf 1484  ∃wex 1516

This theorem was proved from axioms:  ax-mp 5  ax-1 6  ax-2 7  ax-ia1 106  ax-ia2 107  ax-ia3 108  ax-gen 1473  ax-ie1 1517  ax-ie2 1518  ax-4 1534

This theorem depends on definitions:  df-bi 117  df-nf 1485

This theorem is referenced by:  19.9d  1685  19.23t  1701  spimt  1760  exdistrfor  1824  sbequi  1863  sbft  1872  vtoclegft  2849  copsexg  4296