# Axiomatizations of Peano Arithmetic: a truth-theoretic view

## Mateusz Łełyk, University of Warsaw

We employ the lens provided by formal truth theory to study axiomatizations of PA (Peano Arithmetic). More specifically, let EA (Elementary Arithmetic) be the fragment I∆0 + Exp of PA, and CT^{−}[EA] be the extension of EA by the commonly studied axioms of compositional truth CT^{−}. The truth theory delivers a natural preorder on the set of axiomatizations: an axiomatization A is greater or equal to an axiomatization B if and only if, over CT^{-}[EA], the assertion “All axioms from A are true” implies “All axioms from B are true”. Our focus is dominantly on two types of axiomatizations, namely: (1) schematic axiomatizations that are deductively equivalent to PA, and (2) axiomatizations that are proof-theoretically equivalent to the canonical axiomatization of PA.

The first part of the talk focuses on the axiomatizations of type (1). We adapt the argument by Visser and Pakhomov (“On a question of Krajewski’s”, JSL 84(1), 2019) to show that there is no weakest axiomatization of this form (even if the axiomatizations are ordered by relative interpretability). Secondly, we sketch an argument showing that such axiomatizations with the given ordering form a countably universal partial order. This part is based on our joint work with Ali Enayat, available at https://www.researchgate.net/publication/353496287_Axiomatizations_of_Peano_Arithmetic_A_truth-theoretic_view

In the second part of the talk we discuss axiomatizations of type (2). We narrow our attention to such axiomatizations A for which CT^{-}[EA] + “All axioms from A are true” is a conservative extension of PA. We explain why such theories have very diverse metamathematical properties (e.g. large speed-up). To illustrate our methods we show that, with the given ordering, such axiomatizations do not form a lattice. This is a work still in progress.