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Theorem alimex 1909
Description: A utility theorem. An interesting case is when the same formula is substituted for both 𝜑 and 𝜓, since then both implications express a type of non-freeness. See also eximal 1858. (Contributed by BJ, 12-May-2019.)
Assertion
Ref Expression
alimex ((𝜑 → ∀𝑥𝜓) ↔ (∃𝑥 ¬ 𝜓 → ¬ 𝜑))

Proof of Theorem alimex
StepHypRef Expression
1 alex 1904 . . 3 (∀𝑥𝜓 ↔ ¬ ∃𝑥 ¬ 𝜓)
21imbi2i 326 . 2 ((𝜑 → ∀𝑥𝜓) ↔ (𝜑 → ¬ ∃𝑥 ¬ 𝜓))
3 con2b 349 . 2 ((𝜑 → ¬ ∃𝑥 ¬ 𝜓) ↔ (∃𝑥 ¬ 𝜓 → ¬ 𝜑))
42, 3bitri 265 1 ((𝜑 → ∀𝑥𝜓) ↔ (∃𝑥 ¬ 𝜓 → ¬ 𝜑))
Colors of variables: wff setvar class
Syntax hints:  ¬ wn 3  wi 4  wb 197  wal 1632  wex 1855
This theorem was proved from axioms:  ax-mp 5  ax-1 6  ax-2 7  ax-3 8  ax-gen 1873  ax-4 1888
This theorem depends on definitions:  df-bi 198  df-ex 1856
This theorem is referenced by: (None)
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