Most properties are undecidable even in $\mathop{\mathsf{NExt}} \mathsf{Grz}_t$

Abstract

We investigate decidability of properties in the lattice $\mathop{\mathsf{NExt}} \mathsf{Grz}_t$ of extensions of the Grzegorczyk tense logic $\mathsf{Grz}_t$ and the lattice $\mathop{\mathsf{NExt}} \mathsf{S4}_t$ of reflexive and transitive tense logics, with applications to the lattice $\mathop{\mathsf{Ext}} \mathsf{biIPC}$ of bi-superintuitionistic logics. We prove that a broad class of properties is undecidable in $\mathop{\mathsf{NExt}} \mathsf{Grz}_t$, including tabularity, Kripke completeness, the finite model property, and decidability, which also yields their undecidability in $\mathop{\mathsf{NExt}} \mathsf{S4}_t$. We also construct infinitely many tabular extensions of $\mathsf{Grz}_t$ (and thus of $\mathsf{S4}_t$) whose coincidence problems are undecidable, while presenting one tabular extension of $\mathsf{Grz}_t$ and infinitely many ones of $\mathsf{S4}_t$ with a decidable coincidence problem. As a consequence, we obtain that the finite model property and tabularity are undecidable in $\mathop{\mathsf{Ext}} \mathsf{biIPC}$, and that there are infinitely many tabular extensions of $\mathsf{biIPC}$ whose coincidence problems are undecidable. These results clarify some similarities and differences between $\mathop{\mathsf{NExt}} \mathsf{Grz}_t$ and $\mathop{\mathsf{NExt}} \mathsf{Grz}$, $\mathop{\mathsf{NExt}} \mathsf{S4}_t$ and $\mathop{\mathsf{NExt}} \mathsf{S4}$, as well as $\mathop{\mathsf{Ext}} \mathsf{biIPC}$ and $\mathop{\mathsf{Ext}} \mathsf{IPC}$. The proofs adapt Chagrov's method of reducing from an undecidable problem for Minsky machines. We isolate and explicitly formulate the method of good valuations, a recurring technique underlying several proofs in the literature that use large frames, making it available for further applications.