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Theorem alexdc 1555
Description: Theorem 19.6 of [Margaris] p. 89, given a decidability condition. The forward direction holds for all propositions, as seen at alexim 1581. (Contributed by Jim Kingdon, 2-Jun-2018.)
Assertion
Ref Expression
alexdc (∀𝑥DECID 𝜑 → (∀𝑥𝜑 ↔ ¬ ∃𝑥 ¬ 𝜑))

Proof of Theorem alexdc
StepHypRef Expression
1 nfa1 1479 . . 3 𝑥𝑥DECID 𝜑
2 notnotbdc 804 . . . 4 (DECID 𝜑 → (𝜑 ↔ ¬ ¬ 𝜑))
32sps 1475 . . 3 (∀𝑥DECID 𝜑 → (𝜑 ↔ ¬ ¬ 𝜑))
41, 3albid 1551 . 2 (∀𝑥DECID 𝜑 → (∀𝑥𝜑 ↔ ∀𝑥 ¬ ¬ 𝜑))
5 alnex 1433 . 2 (∀𝑥 ¬ ¬ 𝜑 ↔ ¬ ∃𝑥 ¬ 𝜑)
64, 5syl6bb 194 1 (∀𝑥DECID 𝜑 → (∀𝑥𝜑 ↔ ¬ ∃𝑥 ¬ 𝜑))
Colors of variables: wff set class
Syntax hints:  ¬ wn 3  wi 4  wb 103  DECID wdc 780  wal 1287  wex 1426
This theorem was proved from axioms:  ax-1 5  ax-2 6  ax-mp 7  ax-ia1 104  ax-ia2 105  ax-ia3 106  ax-in1 579  ax-in2 580  ax-io 665  ax-5 1381  ax-gen 1383  ax-ie2 1428  ax-4 1445  ax-ial 1472
This theorem depends on definitions:  df-bi 115  df-dc 781  df-tru 1292  df-fal 1295  df-nf 1395
This theorem is referenced by: (None)
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