A Dedekind-style axiomatization and the corresponding universal property of an ordinal number system

Abstract

In this paper, we give an axiomatization of the ordinal number system, in the style of Dedekind's axiomatization of the natural number system. The latter is based on a structure \((N,0,s)\) consisting of a set \(N\), a distinguished element \(0\in N\) and a function \(s\colon N\to N\). The structure in our axiomatization is a triple \((O,L,s)\), where \(O\) is a class, \(L\) is a function defined on all \(s\)-closed `subsets' of \(O\), and \(s\) is a class function \(s\colon O\to O\). In fact, we develop the theory relative to a Grothendieck-style universe (minus the power-set axiom), as a way of bringing the natural and the ordinal cases under one framework. We also establish a universal property for the ordinal number system, analogous to the well-known universal property for the natural number system.