User:Dfeuer/Principle of Recursive Definition/Peano Structure
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Theorem
Let $(\N, 0, s)$ be a Peano structure, where $\N$ is a set.
Let $S$ be a non-empty set or class.
Let $f: S \to S$ be a mapping.
Let $c \in S$.
Then there is a unique mapping $g: \N \to S$ such that:
- $g(0) = c$
- $g(s(n)) = f(g(n))$ for all $n \in \N$
Proof
Let $\le$ be the usual ordering of $\N$.
Lemma
For each $n \in \N$ there is a unique mapping $g_n: {\downarrow} n \to S$ such that:
- $$