# Definition:Primitive Recursion

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## Definition

### Primitive Recursion on Several Variables

Let $f: \N^k \to \N$ and $g: \N^{k+2} \to \N$ be functions.

Let $\left({n_1, n_2, \ldots, n_k}\right) \in \N^k$.

Then the function $h: \N^{k+1} \to \N$ is obtained from $f$ and $g$ by primitive recursion if:

$\forall n \in \N: h \left({n_1, n_2, \ldots, n_k, n}\right) = \begin{cases} f \left({n_1, n_2, \ldots, n_k}\right) & : n = 0 \\ g \left({n_1, n_2, \ldots, n_k, n-1, h \left({n_1, n_2, \ldots, n_k, n-1}\right)}\right) & : n > 0 \end{cases}$

### Primitive Recursion on One Variable

Let $a \in \N$ be a natural number.

Let $g: \N^2 \to \N$ be a function.

Then the function $h: \N \to \N$ is obtained from the constant $a$ and $g$ by primitive recursion if:

$\forall n \in \N: h \left({n}\right) = \begin{cases} a & : n = 0 \\ g \left({n-1, h \left({n-1}\right)}\right) & : n > 0 \end{cases}$

### Primitive Recursion on Partial Functions

Let $f: \N^k \to \N$ and $g: \N^{k+2} \to \N$ be partial functions.

Let $\left({n_1, n_2, \ldots, n_k}\right) \in \N^k$.

Then the partial function $h: \N^{k+1} \to \N$ is obtained from $f$ and $g$ by primitive recursion if:

$\forall n \in \N: h \left({n_1, n_2, \ldots, n_k, n}\right) \approx \begin{cases} f \left({n_1, n_2, \ldots, n_k}\right) & : n = 0 \\ g \left({n_1, n_2, \ldots, n_k, n-1, h \left({n_1, n_2, \ldots, n_k, n-1}\right)}\right) & : n > 0 \end{cases}$

where $\approx$ is as defined in Partial Function Equality.

Note that $h \left({n_1, n_2, \ldots, n_k, n}\right)$ is defined only when:

• $h \left({n_1, n_2, \ldots, n_k, n - 1}\right)$ is defined
• $g \left({n_1, n_2, \ldots, n_k, n-1, h \left({n_1, n_2, \ldots, n_k, n-1}\right)}\right)$ is defined.