Theorem axnul 4208

Description: The Null Set Axiom of ZF set theory: there exists a set with no elements. Axiom of Empty Set of [Enderton] p. 18. In some textbooks, this is presented as a separate axiom; here we show it can be derived from Separation ax-sep 4201. This version of the Null Set Axiom tells us that at least one empty set exists, but does not tell us that it is unique - we need the Axiom of Extensionality to do that (see zfnuleu 4207).

This theorem should not be referenced by any proof. Instead, use ax-nul 4209 below so that the uses of the Null Set Axiom can be more easily identified. (Contributed by Jeff Hoffman, 3-Feb-2008.) (Revised by NM, 4-Feb-2008.) (New usage is discouraged.) (Proof modification is discouraged.)

Assertion

Ref	Expression
axnul	|- E. x A. y -. y e. x

Distinct variable group:   x, y

Proof of Theorem axnul

Dummy variable z is distinct from all other variables.

Step	Hyp	Ref	Expression
1		ax-sep 4201	. 2 |- E. x A. y ( y e. x <-> ( y e. z /\ ( y e. y /\ -. y e. y ) ) )
2		pm3.24 698	. . . . . 6 |- -. ( y e. y /\ -. y e. y )
3	2	intnan 934	. . . . 5 |- -. ( y e. z /\ ( y e. y /\ -. y e. y ) )
4		id 19	. . . . 5 |- ( ( y e. x <-> ( y e. z /\ ( y e. y /\ -. y e. y ) ) ) -> ( y e. x <-> ( y e. z /\ ( y e. y /\ -. y e. y ) ) ) )
5	3, 4	mtbiri 679	. . . 4 |- ( ( y e. x <-> ( y e. z /\ ( y e. y /\ -. y e. y ) ) ) -> -. y e. x )
6	5	alimi 1501	. . 3 |- ( A. y ( y e. x <-> ( y e. z /\ ( y e. y /\ -. y e. y ) ) ) -> A. y -. y e. x )
7	6	eximi 1646	. 2 |- ( E. x A. y ( y e. x <-> ( y e. z /\ ( y e. y /\ -. y e. y ) ) ) -> E. x A. y -. y e. x )
8	1, 7	ax-mp 5	1 |- E. x A. y -. y e. x

Syntax hints:   -. wn 3   /\ wa 104   <-> wb 105   A.wal 1393   E.wex 1538   e. wcel 2200

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 617  ax-in2 618  ax-5 1493  ax-gen 1495  ax-ie1 1539  ax-ie2 1540  ax-4 1556  ax-ial 1580  ax-sep 4201

This theorem depends on definitions:  df-bi 117

This theorem is referenced by: (None)