Axiom of countable choice
The axiom of countable choice or axiom of denumerable choice, denoted ACω, is an axiom of set theory that states that every countable collection of non-empty sets must have a choice function.
denotes the set of natural numbers) such that
ACω is particularly useful for the development of mathematical analysis, where many results depend on having a choice function for a countable collection of sets of real numbers.
For instance, in order to prove that every accumulation point
is the limit of some sequence of elements of
, one needs (a weak form of) the axiom of countable choice.
When formulated for accumulation points of arbitrary metric spaces, the statement becomes equivalent to ACω.
The ability to perform analysis using countable choice has led to the inclusion of ACω as an axiom in some forms of constructive mathematics, despite its assertion that a choice function exists without constructing it.
[1] As an example of an application of ACω, here is a proof (from ZF + ACω) that every infinite set is Dedekind-infinite:[2] Let
Application of ACω yields a sequence
One can then concatenate these tuples into a single sequence
, possibly with repeating elements.
Suppressing repetitions produces a sequence
elements selected previously.
(and leaves all other elements of
fixed) is a one-to-one map from
[2] The axiom of countable choice (ACω) is strictly weaker than the axiom of dependent choice (DC),[3] which in turn is weaker than the axiom of choice (AC).
DC, and therefore also ACω, hold in the Solovay model, constructed in 1970 by Robert M. Solovay as a model of set theory without the full axiom of choice, in which all sets of real numbers are measurable.
[4] Urysohn's lemma (UL) and the Tietze extension theorem (TET) are independent of ZF+ACω: there exist models of ZF+ACω in which UL and TET are true, and models in which they are false.
Both UL and TET are implied by DC.
[5] Paul Cohen showed that ACω is not provable in Zermelo–Fraenkel set theory (ZF) without the axiom of choice.
[6] However, some countably infinite sets of non-empty sets can be proven to have a choice function in ZF without any form of the axiom of choice.
is the set of hereditarily finite sets, i.e. the first set in the Von Neumann universe of non-finite rank.
The choice function is (trivially) the least element in the well-ordering.
Another example is the set of proper and bounded open intervals of real numbers with rational endpoints.
These statements are not equivalent: Cohen's First Model supplies an example where countable unions of countable sets are countable, but where ACω does not hold.
[7] There are many equivalent forms to the axiom of countable choice, in the sense that any one of them can be proven in ZF assuming any other of them.
They include the following:[8][9] This article incorporates material from axiom of countable choice on PlanetMath, which is licensed under the Creative Commons Attribution/Share-Alike License.
