Intuitively, a recursive function may be defined as an integer-valued function of one or more integer variables which may be computed by a definite algorithm. In order to produce a rigorous definition, one may proceed long at least two approaches; one may define the notion of algorithm rigorously in order to complete the intuitive definition given above or one may proceed inductively, first declaring certain functions to be recursive and then specifying definite procedures by which one may construct any other recursive function starting from the initial set. In this entry, we shall concentrate on the latter approach, only making a few brief remarks regarding the former approach towards the end.

1. The constant function c : ℤ₊ → ℤ₊ defined by c(x) = 1 for all x ∈ ℤ₊ is a recursive function.
2. The addition function + : ℤ₊² → ℤ ₊ and the multiplication function × : ℤ₊² → ℤ ₊ are recursive function.
3. The projection functions I_mⁿ: ℤ ₊ⁿ → ℤ ₊ with 1 ≤ m ≤ n defined as I_mⁿ(x ₁,…,x_n) = x_m are recursive functions.
4. (Closure under composition) If f : ℤ₊ⁿ → ℤ ₊ is a recursive function and g_i: ℤ₊^m → ℤ ₊ with i = 1,…n are recursive functions, then h: ℤ₊ⁿ → ℤ ₊, defined by h(x₁,…,x_n) = f(g₁(x₁,…,x_m),…,g_n(x₁,…,x_m)) is a recursive function.
5. (Closure under primitive recursion)If f : ℤ₊ⁿ → ℤ ₊ and g: ℤ₊ⁿ⁺² → ℤ ₊ are recursive function, then h: ℤ₊ⁿ⁺¹ → ℤ ₊, defined by the recursion
   

   with the initial condition

   

   is a recursive function.

6. (Closure under minimization) If f : ℤ₊ⁿ⁺¹ → ℤ ₊ is a recursive function then g: ℤ₊ⁿ → ℤ ₊ is a recursive function, where g is defined to equal y if there exists a y ∈ ℤ₊ such that
   • f(0,x₁,…,x_n),f(1,x₁,…,x_n),…,f(y,x₁,…,x_n) are all defined,
   • f(z,x₁,…,x_n) = 0 when 1 ≤ z < y, and
   • f(y,x₁,…,x_n) = 0, otherwise g(x₁,…,x_n) is undefined.