Signum Function is Primitive Recursive
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Theorem
Let $\operatorname{sgn}: \N \to \N$ be defined as the signum function.
Then $\operatorname{sgn}$ is primitive recursive.
Signum Complement
Let $\overline {\operatorname{sgn}}: \N \to \N$ by defined as the signum-bar function.
Then $\overline {\operatorname{sgn}}$ is primitive recursive.
Proof
By Signum Function on Natural Numbers as Characteristic Function, $\map {\operatorname{sgn} } n = \chi_{\N^*}$, where $\N^* = \N \setminus \set 0$.
By Set of Non-Zero Natural Numbers is Primitive Recursive, $\N^*$ is primitive recursive.
Thus $\operatorname{sgn}$ is primitive recursive by definition of Primitive Recursive Set.
$\blacksquare$