Theorem 19.9v 1981

Description: Version of 19.9 2203 with a disjoint variable condition, requiring fewer axioms. Any formula can be existentially quantified using a variable which it does not contain. See also 19.3v 1979. (Contributed by NM, 28-May-1995.) Remove dependency on ax-7 2005. (Revised by Wolf Lammen, 4-Dec-2017.)

Assertion

Ref	Expression
19.9v	⊢ (∃𝑥𝜑 ↔ 𝜑)

Distinct variable group:   𝜑,𝑥

Proof of Theorem 19.9v

Step	Hyp	Ref	Expression
1		ax5e 1910	. 2 ⊢ (∃𝑥𝜑 → 𝜑)
2		19.8v 1980	. 2 ⊢ (𝜑 → ∃𝑥𝜑)
3	1, 2	impbii 209	1 ⊢ (∃𝑥𝜑 ↔ 𝜑)

Syntax hints:   ↔ wb 206  ∃wex 1776

This theorem was proved from axioms:  ax-mp 5  ax-1 6  ax-2 7  ax-3 8  ax-gen 1792  ax-4 1806  ax-5 1908  ax-6 1965

This theorem depends on definitions:  df-bi 207  df-ex 1777

This theorem is referenced by:  19.36v  1985  19.44v  1990  19.45v  1991  zfcndpow  10654  volfiniune  34211  bnj937  34764  bnj594  34905  bnj907  34960  bnj1128  34983  bnj1145  34986  coss0  38461  prter2  38863  relopabVD  44899  rfcnnnub  44974