# On the Notions of Rudimentarity, Primitive Recursivity and Representability of Functions and Relations

## TL;DR

This paper clarifies the relationships between rudimentary, primitive recursive, and representable functions and relations, providing new proofs and insights to resolve common misunderstandings in mathematical logic.

## Contribution

It offers a simple proof that some primitive recursive relations are not rudimentary and establishes the equivalence of weak and strong representability in arithmetical theories.

## Key findings

- Primitive recursive relations are not all rudimentary.
- Weak and strong representability are equivalent in strong arithmetical theories.
- Provides clearer understanding of function and relation notions in logic.

## Abstract

It is quite well-known from Kurt Godel's (1931) ground-breaking result on the Incompleteness Theorem that rudimentary relations (i.e., those definable by bounded formulae) are primitive recursive, and that primitive recursive functions are representable in sufficiently strong arithmetical theories. It is also known, though perhaps not as well-known as the former one, that some primitive recursive relations are not rudimentary. We present a simple and elementary proof of this fact in the first part of the paper. In the second part, we review some possible notions of representability of functions studied in the literature, and give a new proof of the equivalence of the weak representability with the (strong) representability of functions in sufficiently strong arithmetical theories. Our results shed some new light on the notions of rudimentary, primitive recursive, and representable functions and relations, and clarify, hopefully, some misunderstandings and confusing errors in the literature.