Theorem 19.9h 1574

Description: A wff may be existentially quantified with a variable not free in it. Theorem 19.9 of [Margaris] p. 89. (Contributed by FL, 24-Mar-2007.)

Hypothesis

Ref	Expression
19.9h.1	⊢ (𝜑 → ∀𝑥𝜑)

Assertion

Ref	Expression
19.9h	⊢ (∃𝑥𝜑 ↔ 𝜑)

Proof of Theorem 19.9h

Step	Hyp	Ref	Expression
1		19.9ht 1572	. . 3 ⊢ (∀𝑥(𝜑 → ∀𝑥𝜑) → (∃𝑥𝜑 → 𝜑))
2		19.9h.1	. . 3 ⊢ (𝜑 → ∀𝑥𝜑)
3	1, 2	mpg 1380	. 2 ⊢ (∃𝑥𝜑 → 𝜑)
4		19.8a 1522	. 2 ⊢ (𝜑 → ∃𝑥𝜑)
5	3, 4	impbii 124	1 ⊢ (∃𝑥𝜑 ↔ 𝜑)

Syntax hints:   → wi 4   ↔ wb 103  ∀wal 1282  ∃wex 1421

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-gen 1378  ax-ie1 1422  ax-ie2 1423  ax-4 1440

This theorem depends on definitions:  df-bi 115

This theorem is referenced by:  19.9  1575  excomim  1593  exdistrfor  1721  sbcof2  1731  ax11ev  1749  19.9v  1792  exists1  2037