The factorials of the natural numbers with zero are the values of exactly one map F from the natural numbers with zero to themselves, and F satisfies the factorial recursion. The recursion is stated so that inductions involving n! can run over a predicative formula with F as a parameter, instead of over a formula containing n!.
In the setting of The Natural Numbers and the Natural Numbers with Zero: Arithmetic, Order, Induction and Recursion, let maps and their values be as in Sets and Maps: Ordinary Notation §maps, and for let be the factorial of .
There is exactly one map with for every .
The map of the clause map satisfies and for every .
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