Theorem bj-biexal1 36215

Description: A general FOL biconditional that generalizes 19.9ht 2308 among others. For this and the following theorems, see also 19.35 1872, 19.21 2195, 19.23 2199. When 𝜑 is substituted for 𝜓, both sides express a form of nonfreeness. (Contributed by BJ, 20-Oct-2019.)

Assertion

Ref	Expression
bj-biexal1	⊢ (∀𝑥(𝜑 → ∀𝑥𝜓) ↔ (∃𝑥𝜑 → ∀𝑥𝜓))

Proof of Theorem bj-biexal1

Step	Hyp	Ref	Expression
1		nfa1 2140	. 2 ⊢ Ⅎ𝑥∀𝑥𝜓
2	1	19.23 2199	1 ⊢ (∀𝑥(𝜑 → ∀𝑥𝜓) ↔ (∃𝑥𝜑 → ∀𝑥𝜓))

Syntax hints:   → wi 4   ↔ wb 205  ∀wal 1531  ∃wex 1773

This theorem was proved from axioms:  ax-mp 5  ax-1 6  ax-2 7  ax-3 8  ax-gen 1789  ax-4 1803  ax-5 1905  ax-6 1963  ax-7 2003  ax-10 2129  ax-12 2166

This theorem depends on definitions:  df-bi 206  df-or 846  df-ex 1774  df-nf 1778

This theorem is referenced by:  bj-biexal3  36217