Axiom ax-ac 8740

Description: Axiom of Choice. The Axiom of Choice (AC) is usually considered an extension of ZF set theory rather than a proper part of it. It is sometimes considered philosophically controversial because it asserts the existence of a set without telling us what the set is. ZF set theory that includes AC is called ZFC.

The unpublished version given here says that given any set x, there exists a y that is a collection of unordered pairs, one pair for each nonempty member of x. One entry in the pair is the member of x, and the other entry is some arbitrary member of that member of x. See the rewritten version ac3 8743 for a more detailed explanation. Theorem ac2 8742 shows an equivalent written compactly with restricted quantifiers.

This version was specifically crafted to be short when expanded to primitives. Kurt Maes' 5-quantifier version ackm 8746 is slightly shorter when the biconditional of ax-ac 8740 is expanded into implication and negation. In axac3 8745 we allow the constant CHOICE to represent the Axiom of Choice; this simplifies the representation of theorems like gchac 8960 (the Generalized Continuum Hypothesis implies the Axiom of Choice).

Standard textbook versions of AC are derived as ac8 8773, ac5 8758, and ac7 8754. The Axiom of Regularity ax-reg 7919 (among others) is used to derive our version from the standard ones; this reverse derivation is shown as theorem dfac2 8412. Equivalents to AC are the well-ordering theorem weth 8776 and Zorn's lemma zorn 8788. See ac4 8756 for comments about stronger versions of AC.

In order to avoid uses of ax-reg 7919 for derivation of AC equivalents, we provide ax-ac2 8744 (due to Kurt Maes), which is equivalent to the standard AC of textbooks. The derivation of ax-ac2 8744 from ax-ac 8740 is shown by theorem axac2 8747, and the reverse derivation by axac 8748. Therefore, new proofs should normally use ax-ac2 8744 instead. (New usage is discouraged.) (Contributed by NM, 18-Jul-1996.)

Assertion

Ref	Expression
ax-ac	|- E. y A. z A. w ( ( z e. w /\ w e. x ) -> E. v A. u ( E. t ( ( u e. w /\ w e. t ) /\ ( u e. t /\ t e. y ) ) <-> u = v ) )

Distinct variable group:   x, y, z, w, v, u, t

Detailed syntax breakdown of Axiom ax-ac

Step	Hyp	Ref	Expression
1		vz	. . . . . . 7 setvar z
2		vw	. . . . . . 7 setvar w
3	1, 2	wel 1759	. . . . . 6 wff z e. w
4		vx	. . . . . . 7 setvar x
5	2, 4	wel 1759	. . . . . 6 wff w e. x
6	3, 5	wa 369	. . . . 5 wff ( z e. w /\ w e. x )
7		vu	. . . . . . . . . . . 12 setvar u
8	7, 2	wel 1759	. . . . . . . . . . 11 wff u e. w
9		vt	. . . . . . . . . . . 12 setvar t
10	2, 9	wel 1759	. . . . . . . . . . 11 wff w e. t
11	8, 10	wa 369	. . . . . . . . . 10 wff ( u e. w /\ w e. t )
12	7, 9	wel 1759	. . . . . . . . . . 11 wff u e. t
13		vy	. . . . . . . . . . . 12 setvar y
14	9, 13	wel 1759	. . . . . . . . . . 11 wff t e. y
15	12, 14	wa 369	. . . . . . . . . 10 wff ( u e. t /\ t e. y )
16	11, 15	wa 369	. . . . . . . . 9 wff ( ( u e. w /\ w e. t ) /\ ( u e. t /\ t e. y ) )
17	16, 9	wex 1587	. . . . . . . 8 wff E. t ( ( u e. w /\ w e. t ) /\ ( u e. t /\ t e. y ) )
18		vv	. . . . . . . . 9 setvar v
19	7, 18	weq 1696	. . . . . . . 8 wff u = v
20	17, 19	wb 184	. . . . . . 7 wff ( E. t ( ( u e. w /\ w e. t ) /\ ( u e. t /\ t e. y ) ) <-> u = v )
21	20, 7	wal 1368	. . . . . 6 wff A. u ( E. t ( ( u e. w /\ w e. t ) /\ ( u e. t /\ t e. y ) ) <-> u = v )
22	21, 18	wex 1587	. . . . 5 wff E. v A. u ( E. t ( ( u e. w /\ w e. t ) /\ ( u e. t /\ t e. y ) ) <-> u = v )
23	6, 22	wi 4	. . . 4 wff ( ( z e. w /\ w e. x ) -> E. v A. u ( E. t ( ( u e. w /\ w e. t ) /\ ( u e. t /\ t e. y ) ) <-> u = v ) )
24	23, 2	wal 1368	. . 3 wff A. w ( ( z e. w /\ w e. x ) -> E. v A. u ( E. t ( ( u e. w /\ w e. t ) /\ ( u e. t /\ t e. y ) ) <-> u = v ) )
25	24, 1	wal 1368	. 2 wff A. z A. w ( ( z e. w /\ w e. x ) -> E. v A. u ( E. t ( ( u e. w /\ w e. t ) /\ ( u e. t /\ t e. y ) ) <-> u = v ) )
26	25, 13	wex 1587	1 wff E. y A. z A. w ( ( z e. w /\ w e. x ) -> E. v A. u ( E. t ( ( u e. w /\ w e. t ) /\ ( u e. t /\ t e. y ) ) <-> u = v ) )

This axiom is referenced by:  zfac  8741  ac2  8742