Theorem dfrex2

Description: Relationship between restricted universal and existential quantifiers. (Contributed by NM, 21-Jan-1997) (Proof shortened by Wolf Lammen, 26-Nov-2019)

	Ref	Expression
Assertion	dfrex2	$$⊢\exists x\in A\phi \leftrightarrow \neg \forall x\in A\neg \phi$$

Proof

Step	Hyp	Ref	Expression
1		ralnex	$$⊢\forall x\in A\neg \phi \leftrightarrow \neg \exists x\in A\phi$$
2	1	con2bii	$$⊢\exists x\in A\phi \leftrightarrow \neg \forall x\in A\neg \phi$$