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Theorem axnul 4368
 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 4361. 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 4366). This proof, suggested by Jeff Hoffman, uses only ax-5 1567 and ax-gen 1556 from predicate calculus, which are valid in "free logic" i.e. logic holding in an empty domain (see Axiom A5 and Rule R2 of [LeBlanc] p. 277). Thus, our ax-sep 4361 implies the existence of at least one set. Note that Kunen's version of ax-sep 4361 (Axiom 3 of [Kunen] p. 11) does not imply the existence of a set because his is universally closed i.e. prefixed with universal quantifiers to eliminate all free variables. His existence is provided by a separate axiom stating (Axiom 0 of [Kunen] p. 10). See axnulALT 4367 for a proof directly from ax-rep 4351. This theorem should not be referenced by any proof. Instead, use ax-nul 4369 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
Distinct variable group:   ,

Proof of Theorem axnul
Dummy variable is distinct from all other variables.
StepHypRef Expression
1 ax-sep 4361 . 2
2 pm3.24 854 . . . . . 6
32intnan 882 . . . . 5
4 id 21 . . . . 5
53, 4mtbiri 296 . . . 4
65alimi 1569 . . 3
76eximi 1586 . 2
81, 7ax-mp 5 1
 Colors of variables: wff set class Syntax hints:   wn 3   wb 178   wa 360  wal 1550  wex 1551   wcel 1728 This theorem was proved from axioms:  ax-mp 5  ax-1 6  ax-2 7  ax-3 8  ax-gen 1556  ax-5 1567  ax-sep 4361 This theorem depends on definitions:  df-bi 179  df-an 362  df-ex 1552
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