TheoremBase

The Factorial as a Map on the Natural Numbers with Zero

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!.

Statement

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 n∈N0n\in\mathbb{N}_{0} let n!n! be the factorial of nn.

There is exactly one map F:N0→N0F:\mathbb{N}_{0}\to\mathbb{N}_{0} with F(n)=n!F(n)=n! for every n∈N0n\in\mathbb{N}_{0}.

The map FF of the clause map satisfies F(0)=1F(0)=1 and F(n+1)=(n+1)⋅F(n)F(n+1)=(n+1)\cdot F(n) for every n∈N0n\in\mathbb{N}_{0}.

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