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Axiom ax-inf 7355
Description: Axiom of Infinity. An axiom of Zermelo-Fraenkel set theory. This axiom is the gateway to "Cantor's paradise" (an expression coined by Hilbert). It asserts that given a starting set  x, an infinite set  y built from it exists. Although our version is apparently not given in the literature, it is similar to, but slightly shorter than, the Axiom of Infinity in [FreydScedrov] p. 283 (see inf1 7339 and inf2 7340). More standard versions, which essentially state that there exists a set containing all the natural numbers, are shown as zfinf2 7359 and omex 7360 and are based on the (nontrivial) proof of inf3 7352. This version has the advantage that when expanded to primitives, it has fewer symbols than the standard version ax-inf2 7358. Theorem inf0 7338 shows the reverse derivation of our axiom from a standard one. Theorem inf5 7362 shows a very short way to state this axiom.

The standard version of Infinity ax-inf2 7358 requires this axiom along with Regularity ax-reg 7322 for its derivation (as theorem axinf2 7357 below). In order to more easily identify the normal uses of Regularity, we will usually reference ax-inf2 7358 instead of this one. The derivation of this axiom from ax-inf2 7358 is shown by theorem axinf 7361.

Proofs should normally use the standard version ax-inf2 7358 instead of this axiom. (New usage is discouraged.) (Contributed by NM, 16-Aug-1993.)

Assertion
Ref Expression
ax-inf  |-  E. y
( x  e.  y  /\  A. z ( z  e.  y  ->  E. w ( z  e.  w  /\  w  e.  y ) ) )
Distinct variable group:    x, y, z, w

Detailed syntax breakdown of Axiom ax-inf
StepHypRef Expression
1 vx . . . 4  set  x
2 vy . . . 4  set  y
31, 2wel 1697 . . 3  wff  x  e.  y
4 vz . . . . . 6  set  z
54, 2wel 1697 . . . . 5  wff  z  e.  y
6 vw . . . . . . . 8  set  w
74, 6wel 1697 . . . . . . 7  wff  z  e.  w
86, 2wel 1697 . . . . . . 7  wff  w  e.  y
97, 8wa 358 . . . . . 6  wff  ( z  e.  w  /\  w  e.  y )
109, 6wex 1531 . . . . 5  wff  E. w
( z  e.  w  /\  w  e.  y
)
115, 10wi 4 . . . 4  wff  ( z  e.  y  ->  E. w
( z  e.  w  /\  w  e.  y
) )
1211, 4wal 1530 . . 3  wff  A. z
( z  e.  y  ->  E. w ( z  e.  w  /\  w  e.  y ) )
133, 12wa 358 . 2  wff  ( x  e.  y  /\  A. z ( z  e.  y  ->  E. w
( z  e.  w  /\  w  e.  y
) ) )
1413, 2wex 1531 1  wff  E. y
( x  e.  y  /\  A. z ( z  e.  y  ->  E. w ( z  e.  w  /\  w  e.  y ) ) )
Colors of variables: wff set class
This axiom is referenced by:  zfinf  7356
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