countable choice

Countable choice


The axiom of countable choice (CCCC), also called AC ωAC_\omega or AC NAC_N, is a weak form of the axiom of choice; it says that the set of natural numbers is a projective object in Set. (Recall that the full axiom of choice states that every set is projective.) All the reasoning in this page is constructive.


More explicitly, let XX be any set and let p:XNp\colon X \to \mathbf{N} be a surjection. Then the axiom of countable choice states that pp has a section. If you phrase the axiom of choice in terms of entire relations, then countable choice states that any entire relation from N\mathbf{N} to any other set contains (in the 2-poset Rel) a functional entire relation.


Here we collect some consequences of the countable axiom of choice.



Unlike the full axiom of choice, countable choice is often considered to be a constructively acceptable principle. In particular, it does not imply the principle of excluded middle. It is a consequence of COSHEP. A stronger version of countable choice, also a consequence of COSHEPCOSHEP, is the axiom of dependent choice (DCDC). In general, DCDC is enough to justify results in analysis involving sequences.

AC 00AC_{00}

Sometimes in foundations it is useful to consider a weaker version of countable choice, called AC 00AC_{00}. This states that any entire relation from N\mathbf{N} to itself contains a functional entire relation. In terms of surjections, this states that any surjection p:XNp\colon X \to \mathbf{N} has a section if XX is a subset of N×N\mathbf{N} \times \mathbf{N} and pp is the restriction to XX of a product projection. AC 00AC_{00} is enough to prove that every Dedekind real number is a Cauchy real number (the converse is always true).

Weak countable choice

The axiom of weak countable choice (WCCWCC) states that a surjection p:XNp\colon X \to \mathbf{N} has a section if, whenever mnm \neq n, at least one of the preimages p *(m)p^*(m) and p *(n)p^*(n) is a singleton. WCCWCC follows (for different reasons) from either CCCC or excluded middle. On the other hand, WCCWCC is enough to prove that the fundamental theorem of algebra in the sense that every non-constant complex polynomial has a root; see Bridges et al (1998).

Very weak countable choice

An even weaker form of countable choice was proposed by Martin Escardo; it states that any surjection of the form A(N×B)NA \sqcup (\mathbf{N}\times B) \to \mathbf{N} has a section, where ANA\to \mathbf{N} is a decidable subset and BB is an arbitrary set with N×BN\mathbf{N}\times B \to \mathbf{N} the projection. This follows from WCC and also from the limited principle of omniscience; see the constructivenews discussion.


  • Douglas Bridges, Fred Richman, and Peter Schuster (1998). A weak countable choice principle. PDF AMS PDF

  • Martin Escardo et. al., Special case of countable choice, message and discussion to the constructivenews list, google groups

Revised on October 20, 2016 11:56:32 by Mike Shulman (