|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The unpublished version given here says that given any set , there
exists a that is
a collection of unordered pairs, one pair for
each nonempty member of . One entry in the pair is the member of
, and the other
entry is some arbitrary member of that member of
. See the
rewritten version ac3 8889 for a more detailed
explanation. Theorem ac2 8888 shows an equivalent written compactly with
This version was specifically crafted to be short when expanded to
primitives. Kurt Maes' 5-quantifier version ackm 8892
is slightly shorter
when the biconditional of ax-ac 8886 is expanded into implication and
negation. In axac3 8891 we allow the constant CHOICE
to represent the
Axiom of Choice; this simplifies the representation of theorems like
gchac 9103 (the Generalized Continuum Hypothesis implies
the Axiom of
Standard textbook versions of AC are derived as ac8 8919,
ac5 8904, and
ac7 8900. The Axiom of Regularity ax-reg 8104 (among others) is used to
derive our version from the standard ones; this reverse derivation is
shown as theorem dfac2 8558. Equivalents to AC are the well-ordering
theorem weth 8922 and Zorn's lemma zorn 8934.
See ac4 8902 for comments about
stronger versions of AC.
In order to avoid uses of ax-reg 8104 for derivation of AC equivalents, we
provide ax-ac2 8890 (due to Kurt Maes), which is equivalent to
AC of textbooks. The derivation of ax-ac2 8890 from ax-ac 8886 is shown by
theorem axac2 8893, and the reverse derivation by axac 8894.
proofs should normally use ax-ac2 8890 instead.
(New usage is discouraged.) (Contributed by NM,