From Numbers to Container Strings

Abstract

In this paper we examine two ways of coding sequences in arithmetical theories. We investigate under what conditions they work. To be more precise, we study the creation of objects of a data-type that we call ur-strings, roughly sequences where the components are ordered but where we do not have an explicitly given projection function. First, we have a brief look at the beta-function which was already carefully studied by Emil Je\v{r}\'abek. We study in detail our two target constructions. These constructions both employ theories of strings. The first is based on Smullyan coding and the second on the representation of binary strings in the special linear monoid of the non-negative part of discretely ordered commutative rings as introduced by Markov. We use the Markov coding to obtain an alternative proof that ${\sf PA}^{-}$ is sequential.