Axiom of Choice.
Let be a collection of nonempty sets. Then we can choose a member from each set in that collection. In other words, there exists a function defined on with the property that, for each set in the collection, is a member of
The function is then called a choice function.
For example, if is the collection , then we can define quite easily: just let be the smallest member of
Then the function that assigns 0 to the set 2 to and 4 to is a choice function on X.
Comments
If the set is finite, then there is a finite number of choice functions over this set. It was also discussed at https://math.stackexchange.com/questions/2469459/count-the-number-of-choice-functions .
so how many choice function can we defined on the set X