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COUNTING TO INFINITY: GRADED MODAL LOGIC WITH AN INFINITY DIAMOND

Published online by Cambridge University Press:  08 July 2022

IGNACIO BELLAS ACOSTA*
Affiliation:
INDEPENDENT SCHOLAR
YDE VENEMA
Affiliation:
INSTITUTE FOR LOGIC, LANGUAGE AND COMPUTATION UNIVERSITEIT VAN AMSTERDAM P.O. BOX 94242, 1090 GE AMSTERDAM THE NETHERLANDS E-mail: y.venema@uva.nl

Abstract

We extend the languages of both basic and graded modal logic with the infinity diamond, a modality that expresses the existence of infinitely many successors having a certain property. In both cases we define a natural notion of bisimilarity for the resulting formalisms, that we dub $\mathtt {ML}^{\infty }$ and $\mathtt {GML}^{\infty }$, respectively. We then characterise these logics as the bisimulation-invariant fragments of the naturally corresponding predicate logic, viz., the extension of first-order logic with the infinity quantifier. Furthermore, for both $\mathtt {ML}^{\infty }$ and $\mathtt {GML}^{\infty }$ we provide a sound and complete axiomatisation for the set of formulas that are valid in every Kripke frame, we prove a small model property with respect to a widened class of weighted models, and we establish decidability of the satisfiability problem.

Type
Research Article
Copyright
© The Author(s), 2022. Published by Cambridge University Press on behalf of The Association for Symbolic Logic

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