# Primitive recursive function

Primitive recursive functions are a class of functions which form an important building block on the way to a full formalization of computability. They are defined using recursion and composition as central operations. The primitive recursive functions are a subset of the recursive functions (which are exactly those functions which we call "computable" -- see Equivalence of models of computation).

### Definition

Primitive recursive functions take natural numbers as arguments and produce a natural number. A function which takes n arguments is called n-ary. The basic primitive recursive functions are given by these axioms:

1. The constant function 0 is primitive recursive.
2. The "successor function" S, which takes one argument and returns the succeeding number as given by the Peano postulates, is primitive recursive.
3. The "projection functions" Pin, which take n arguments and return their ith argument, are primitive recursive.

More complex primitive recursive functions can be obtained by applying the operators given by these axioms:

1. Composition: Given f a k-ary primitive recursive function and k l-ary primitive recursive functions g0,...,gk-1, the composition of f with g0,...,gk-1, i.e. the function h(x0,...,xl-1) = f(g0(x0,...,xl-1),...,gk-1(x0,...,xl-1)), is primitive recursive.
2. Primitive recursion: Given f a k-ary primitive recursive function and g a (k+2)-ary primitive recursive function, the (k+1)-ary function defined as the primitive recursion of f and g, i.e. the function h where h(0,x0,...,xk-1) = f(x0,...,xk-1) and h(S(n),x0,...,xk-1) = g(h(n,x0,...,xk-1),n,x0,...,xk-1), is primitive recursive.

(Note that the projection functions allow us to get around the apparent rigidity in terms of the arity of the functions above, as via composition we can pass any subset of the arguments.)

A function is primitive recursive if it is one of the basic functions above, or can be obtained from one of the basic functions by applying the operations a finite number of times.

### Example primitive recursive definitions

Intuitively we would like to define addition recursively as:
In order to fit this into a strict primitive recursive definition, we define:
Note that P01 is simply the identity function; its inclusion is required by the definition of the primitive recursion operator above; it plays the role of h. The composition of S and P03, which is primitive recursive, plays the role of g.
Subtraction
We can define limited subtraction, i.e. subtraction that bottoms out at 0 (since we have no concept of negative numbers yet). First we must define the "predecessor" function, which acts as the opposite of the successor function.
Intuitively we would like to define predecessor as:
pred(0)=0
pred(n+1)=n
To fit this in to a formal primitive recursive definition, we write:
pred(0)=0
pred(S(n))=P12(pred(n),n)
Now we can define subtraction in a very similar way to how we defined addition.
sub(0,x)=P01(x)
sub(n+1,x)=pred(P03(sub(n,x),n,x))
(Note that for the sake of simplicity, the order of the arguments has been switched from the "standard" definition to fit the requirements of primitive recursion, i.e. sub(a,b) corresponds to b-a. This could easily be rectified using composition with suitable projections.)

Many other familiar functions can be shown to be primitive recursive; some examples include conditionals, exponentiation, primality testing, and course-of-values induction.

### Primitive recursive functions are a proper subset of the recursive functions

Primitive recursive functions tend to correspond very closely with our intuition of what a computable function must be. Certainly the initial set of functions are intuitively computable (in their very simplicity), and the two operations by which one can create new primitive recursive functions are also very straightforward. However the set of primitive recursive functions does not include every possible computable function --- this can be seen with a variant of Cantor's diagonalization argument. This argument provides a computable function which is not primitive recursive. A sketch of the proof is as follows:

The primitive recursive functions can be computably numbered. This numbering is unique on the definitions of functions, though not unique on the actual functions themselves (as every function can have an infinite number of definitions --- consider simply composing by the identity operator). The numbering is computable in the sense that one can build a a Turing machine that, given an index x and a value y, computes the value of the xth primitive recursive function on input y (though an appeal to the Church-Turing thesis is likely sufficient to make the case).

Now consider a matrix where the rows are the primitive recursive functions of one argument under this numbering, and the columns are the natural numbers. Then each element (i, j) correponds to the ith unary primitive recursive function being calculated on the number j. We can write this as fi(j).

Now consider the function g(x)=S(fx(x)). g lies on the diagonal of this matrix and simply adds one to the value it finds. This function is computable (by the above), but clearly no primitive recursive function exists which computes it as it differs from each possible primitive recursive function by at least one value. Thus there must be computable functions which are not primitive recursive.

Note that this argument can be applied to any class of functions that can be enumerated in this way. In other words, any explicit list of the computable functions cannot be complete.

One can also explicitly exhibit a simple 1-ary computable function which is recursively defined for any natural number, but which is not primitive recursive, see Ackermann function.

### Relation to the recursive functions

By extending the definition of primitive recursive functions to allow for partial functions and by adding the concept of an unbounded search operator (see Recursive function), we arrive at a full formal model of computability -- the recursive or computable functions.