Concept
Primitive recursive arithmetic (PRA) is a quantifier-free formalization of the natural numbers. It was first proposed by Norwegian mathematician , as a formalization of his finitistic conception of the foundations of arithmetic, and it is widely agreed that all reasoning of PRA is finitistic. Many also believe that all of finitism is captured by PRA, but others believe finitism can be extended to forms of recursion beyond primitive recursion, up to ε0, which is the proof-theoretic ordinal of Peano arithmetic. PRA's proof theoretic ordinal is ωω, where ω is the smallest transfinite ordinal. PRA is sometimes called Skolem arithmetic. The language of PRA can express arithmetic propositions involving natural numbers and any primitive recursive function, including the operations of addition, multiplication, and exponentiation. PRA cannot explicitly quantify over the domain of natural numbers. PRA is often taken as the basic metamathematical formal system for proof theory, in particular for consistency proofs such as Gentzen's consistency proof of first-order arithmetic. The language of PRA consists of: A countably infinite number of variables x, y, z,.... The propositional connectives; The equality symbol =, the constant symbol 0, and the successor symbol S (meaning add one); A symbol for each primitive recursive function. The logical axioms of PRA are the: Tautologies of the propositional calculus; Usual axiomatization of equality as an equivalence relation. The logical rules of PRA are modus ponens and variable substitution. The non-logical axioms are, firstly: where always denotes the negation of so that, for example, is a negated proposition. Further, recursive defining equations for every primitive recursive function may be adopted as axioms as desired. For instance, the most common characterization of the primitive recursive functions is as the 0 constant and successor function closed under projection, composition and primitive recursion.