Theorem nf2 1691

Description: An alternate definition of df-nf 1484, which does not involve nested quantifiers on the same variable. (Contributed by Mario Carneiro, 24-Sep-2016.)

Assertion

Ref	Expression
nf2	⊢ (Ⅎ𝑥𝜑 ↔ (∃𝑥𝜑 → ∀𝑥𝜑))

Proof of Theorem nf2

Step	Hyp	Ref	Expression
1		df-nf 1484	. 2 ⊢ (Ⅎ𝑥𝜑 ↔ ∀𝑥(𝜑 → ∀𝑥𝜑))
2		nfa1 1564	. . . 4 ⊢ Ⅎ𝑥∀𝑥𝜑
3	2	nfri 1542	. . 3 ⊢ (∀𝑥𝜑 → ∀𝑥∀𝑥𝜑)
4	3	19.23h 1521	. 2 ⊢ (∀𝑥(𝜑 → ∀𝑥𝜑) ↔ (∃𝑥𝜑 → ∀𝑥𝜑))
5	1, 4	bitri 184	1 ⊢ (Ⅎ𝑥𝜑 ↔ (∃𝑥𝜑 → ∀𝑥𝜑))

Syntax hints:   → wi 4   ↔ wb 105  ∀wal 1371  Ⅎwnf 1483  ∃wex 1515

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 1472  ax-ie2 1517  ax-4 1533  ax-ial 1557

This theorem depends on definitions:  df-bi 117  df-nf 1484

This theorem is referenced by:  nf3  1692  nf4dc  1693  nf4r  1694  nfd2  2050  eusv2i  4502