Categoricity for an inferential $\omega$-logic and in $L_{\omega_1,\omega}$

Abstract

This paper provides two extensions of first order logic by `$\omega$-rules'. In each case we characterize the countable structures whose theory in the logic is categorical (has a unique model). In the one-sorted inferential $\omega$-logic, both Robinson's system $Q$ and Peano Arithmetic become categorical. In the two-sorted generalized $\omega$-logic we show each complete $L_{\omega_1,\omega}$ sentence defines the same class of structures as a first-order theory with the appropriate $G-\omega$-rule. The results depend on proving that the inferential rules for the logics are categorical, i.e. they uniquely determine certain truth-conditions for the logical connectives and quantifiers.