Primitive recursive set function

In mathematics, primitive recursive set functions or primitive recursive ordinal functions are analogs of primitive recursive functions, defined for sets or ordinals rather than natural numbers. They were introduced by Jensen & Karp (1971).

Definition

A primitive recursive function is one that can be obtained from the following basic functions by repeatedly applying the following rules of substitution and recursion:

The basic functions are:

• Projection: P_n,m(x₁,...,x^{n) = x_m}
• Zero: F(x)=0
• Adding an element to a set: F(x,y) = x∪{y}
• Testing membership: C(x,y,u,v) = x if u∈v, y otherwise.

The rules for generating new functions by substitution are

• F(x,y) = G(x,H(x),y)
• F(x,y) = G(H(x),y)

where x and y are finite sequences of variables.

The rule for generating new functions by recursion is

• F(z,x) = G(∪_u∈zF(u,x),z,x)

One can also add more basic functions to obtain a larger class of functions. For example, the ordinal function ω^α is not primitive recursive, because the constant function with value ω (or any other infinite set) is not primitive recursive, so one might want to add this constant function to the basic functions.

References

• Jensen, Ronald B.; Karp, Carol (1971), "Primitive recursive set functions", Axiomatic Set Theory, Proc. Sympos. Pure Math., vol. XIII, Part I, Providence, R.I.: Amer. Math. Soc., pp. 143–176, ISBN 9780821802458, MR 0281602{{citation}}: CS1 maint: extra punctuation (link)