Theorem 19.9 2213

Description: A wff may be existentially quantified with a variable not free in it. Version of 19.3 2210 with an existential quantifier. Theorem 19.9 of [Margaris] p. 89. See 19.9v 1986 for a version requiring fewer axioms. (Contributed by FL, 24-Mar-2007.) (Revised by Mario Carneiro, 24-Sep-2016.) (Proof shortened by Wolf Lammen, 30-Dec-2017.) Revised to shorten other proofs. (Revised by Wolf Lammen, 14-Jul-2020.)

Hypothesis

Ref	Expression
19.9.1	⊢ Ⅎ𝑥𝜑

Assertion

Ref	Expression
19.9	⊢ (∃𝑥𝜑 ↔ 𝜑)

Proof of Theorem 19.9

Step	Hyp	Ref	Expression
1		19.9.1	. 2 ⊢ Ⅎ𝑥𝜑
2		19.9t 2212	. 2 ⊢ (Ⅎ𝑥𝜑 → (∃𝑥𝜑 ↔ 𝜑))
3	1, 2	ax-mp 5	1 ⊢ (∃𝑥𝜑 ↔ 𝜑)

Syntax hints:   ↔ wb 206  ∃wex 1781  Ⅎwnf 1785

This theorem was proved from axioms:  ax-mp 5  ax-1 6  ax-2 7  ax-3 8  ax-gen 1797  ax-4 1811  ax-5 1912  ax-6 1969  ax-7 2010  ax-12 2185

This theorem depends on definitions:  df-bi 207  df-ex 1782  df-nf 1786

This theorem is referenced by:  exlimd  2226  19.19  2237  19.36  2238  19.41  2243  19.44  2245  19.45  2246  19.9h  2293  eeor  2339  dfid3  5530  bnj1189  35185  bj-exexbiex  36945  bj-exalbial  36947  ax6e2ndeq  44915  e2ebind  44919  ax6e2ndeqVD  45264  e2ebindVD  45267  e2ebindALT  45284  ax6e2ndeqALT  45286