User:Dfeuer/Principle of Recursive Definition/Peano Numbers

Theorem
Let $\N$ be a class satisfying Peano's axioms.

Let $A$ be a set or class.

Let $F: A \to A$ be a mapping.

Let $a \in A$.

Then there is a unique mapping $G: \N \to A$ such that:


 * $G(0) = a$


 * $\forall n \in \N: G(n^+) = F(G(n))$

Finite Sections
For each $n \in \N$, there is a unique mapping $g_n: \N_n \to A$ such that: