Theorem dfexdc 1549

Description: Defining ∃𝑥𝜑 given decidability. It is common in classical logic to define ∃𝑥𝜑 as ¬ ∀𝑥¬ 𝜑 but in intuitionistic logic without a decidability condition, that is only an implication not an equivalence, as seen at exalim 1550. (Contributed by Jim Kingdon, 15-Mar-2018.)

Assertion

Ref	Expression
dfexdc	⊢ (DECID ∃𝑥𝜑 → (∃𝑥𝜑 ↔ ¬ ∀𝑥 ¬ 𝜑))

Proof of Theorem dfexdc

Step	Hyp	Ref	Expression
1		alnex 1547	. . 3 ⊢ (∀𝑥 ¬ 𝜑 ↔ ¬ ∃𝑥𝜑)
2	1	a1i 9	. 2 ⊢ (DECID ∃𝑥𝜑 → (∀𝑥 ¬ 𝜑 ↔ ¬ ∃𝑥𝜑))
3	2	con2biidc 886	1 ⊢ (DECID ∃𝑥𝜑 → (∃𝑥𝜑 ↔ ¬ ∀𝑥 ¬ 𝜑))

Syntax hints:  ¬ wn 3   → wi 4   ↔ wb 105  DECID wdc 841  ∀wal 1395  ∃wex 1540

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-in1 619  ax-in2 620  ax-io 716  ax-5 1495  ax-gen 1497  ax-ie2 1542

This theorem depends on definitions:  df-bi 117  df-dc 842  df-tru 1400  df-fal 1403

This theorem is referenced by:  dfrex2dc  2523