Definition:Choice Function

Definition
Let $\mathbb S$ be a set of sets such that:
 * $\forall S \in \mathbb S: S \ne \varnothing$

that is, none of the sets in $\mathbb S$ may be empty.

A choice function on $\mathbb S$ is a mapping $f: \mathbb S \to \bigcup \mathbb S$ satisfying:
 * $\forall S \in \mathbb S: f \left({S}\right) \in S$.

That is, for any set in $\mathbb S$, a choice function selects an element from that set.

The domain of $f$ is $\mathbb S$.

Use of Axiom of Choice
The Axiom of Choice (abbreviated AoC or AC) is the following statement:
 * All $\mathbb S$ as above have a choice function.

It can be shown that the AoC it does not follow from the other usual axioms of set theory, and that it is relative consistent to these axioms (i.e., that AoC does not make the axiom system inconsistent, provided it was consistent without AoC).

Note that for any given set $S \in \mathbb S$, one can select an element from it (without using AoC). AoC guarantees that there is a choice function, i.e., a function that "simultaneously" picks elements of all $S \in \mathbb S$.

AoC is needed to prove statements such as "all countable unions of finite sets are countable" (for many specific such unions this can be shown without AoC), and AoC is equivalent to many other mathematical statements such as "every vector space has a basis".

In some situations, AoC is not needed to get a choice function:

A Choice Function Exists for Set of Well-Ordered Sets
If every member of $\mathbb S$ is a well-ordered, then we can define a choice function $f$ by:
 * $\forall S \in \mathbb S: f \left({S}\right) = \inf \left({S}\right)$

Note that this only applies if we are given a well order for each $S \in \mathbb S$, more formally, if there is a function that maps $S \in \mathbb S$ to a well-order of $S$. If we just know that each $S \in \mathbb S$ is well-orderable, we generally do need AoC to get a choice function (to apply the proof above, we have to pick a well-order for each $S\in \mathbb S$, which requires AoC. This is related to the fact that generally we need AoC to show that, for example, the countable union of countable sets is countable.)

A Choice Function Exists for Well-Orderable Union of Sets
If the union $\bigcup \mathbb S$ is well-orderable, we can create a choice function for $\bigcup \mathbb S$.

Also see

 * Well-Ordering Theorem


 * Well-Ordering Theorem is Equivalent to Axiom of Choice