**March 16**

Mateusz Łełyk,
University of Warsaw

**Nonequivalent axiomatizations of PA and the Tarski Boundary: Part II**

This is a continuation of the talk from Feb 16th. This time we shall study different theories of the form ${\rm CT}^-[\delta]$ which are conservative extensions of a ${\rm PA}$. In particular, we prove the following theorem.

**Theorem 2** There exists a family $\{\delta_f\}_{f\in\omega^*}$ such that for all $f,g\in\omega^*$

1) ${\rm CT}^-[\delta_f]$ is conservative over ${\rm PA}$;

2) if $f\subsetneq g$, then ${\rm CT}^-[\delta_g]$ properly extends ${\rm CT}^-[\delta_f]$;

3) if $f\perp g$ then ${\rm CT}^-[\delta_g]\cup {\rm CT}^-[\delta_f]$ is nonconservative over ${\rm PA}$ (but consistent).