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Theorem 19.38b 1937
 Description: Under a non-freeness hypothesis, the implication 19.38 1934 can be strengthened to an equivalence. See also 19.38a 1935. (Contributed by BJ, 3-Nov-2021.) (Proof shortened by Wolf Lammen, 9-Jul-2022.)
Assertion
Ref Expression
19.38b (Ⅎ𝑥𝜓 → ((∃𝑥𝜑 → ∀𝑥𝜓) ↔ ∀𝑥(𝜑𝜓)))

Proof of Theorem 19.38b
StepHypRef Expression
1 19.38 1934 . 2 ((∃𝑥𝜑 → ∀𝑥𝜓) → ∀𝑥(𝜑𝜓))
2 exim 1929 . . 3 (∀𝑥(𝜑𝜓) → (∃𝑥𝜑 → ∃𝑥𝜓))
3 id 22 . . . 4 (Ⅎ𝑥𝜓 → Ⅎ𝑥𝜓)
43nfrd 1887 . . 3 (Ⅎ𝑥𝜓 → (∃𝑥𝜓 → ∀𝑥𝜓))
52, 4syl9r 78 . 2 (Ⅎ𝑥𝜓 → (∀𝑥(𝜑𝜓) → (∃𝑥𝜑 → ∀𝑥𝜓)))
61, 5impbid2 218 1 (Ⅎ𝑥𝜓 → ((∃𝑥𝜑 → ∀𝑥𝜓) ↔ ∀𝑥(𝜑𝜓)))
 Colors of variables: wff setvar class Syntax hints:   → wi 4   ↔ wb 198  ∀wal 1651  ∃wex 1875  Ⅎwnf 1879 This theorem was proved from axioms:  ax-mp 5  ax-1 6  ax-2 7  ax-3 8  ax-gen 1891  ax-4 1905 This theorem depends on definitions:  df-bi 199  df-ex 1876  df-nf 1880 This theorem is referenced by:  19.23t  2244
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