Fagin's Theorem for Semiring Turing Machines

Abstract: In recent years, quantitative complexity over semirings has been intensively investigated. An important problem in this context is to connect computational complexity with logical expressiveness. In this paper we improve on the model of \emph{Semiring Turing Machines} (distinct from so called weighted Turing machines) introduced by Eiter & Kiesel (Semiring Reasoning Frameworks in AI and Their Computational Complexity, \emph{J. Artif. Intell. Res.}, 2023). Our central result is a Fagin-style theorem for a new quantitative complexity class using a suitable weighted logical formalism. We show that the quantitative complexity class that we call \NPnewinf{$\mathcal{R}$}, where $\mathcal{R}$ is a commutative semiring, can be captured using a version of weighted existential second-order logic that allows for predicates interpreted as semiring-annotated relations. This result provides a precise logical characterization of the power series that form the class \NPnewinf{$\mathcal{R}$}. We also give the exact relation between Eiter & Kiesel's version of NP, called \NPoldinf{$\mathcal{R}$}, and the class \NPnewinf{$\mathcal{R}$}. Incidentally, we are able to recapture all the complexity results by Eiter & Kiesel (2023) in our new model, connecting a quantitative version of NP to various counting complexity classes.