Characterization, Definability and Separation via Saturated Models

TitleCharacterization, Definability and Separation via Saturated Models
Publication TypeJournal Article
Year of Publication2014
AuthorsAreces, C, Carreiro, F, Figueira, S
JournalTheoretical Computer Science
AbstractThree important results about the expressivity of a modal logic L are the Characterization Theorem (that identifies a modal logic L as a fragment of a better known logic), the Definability theorem (that provides conditions under which a class of L-models can be defined by a formula or a set of formulas of L), and the Separation Theorem (that provides conditions under which two disjoint classes of L-models can be separated by a class definable in L). In this article we provide general conditions under which these results can be established for a given choice of model class and modal language whose expressivity is below first order logic. Besides some basic constrains that most modal logics easily satisfy, the fundamental condition that we require is that the class of omega-saturated models in question has the Hennessy-Milner property with respect to the notion of observational equivalence under consideration. As the Characterization, Definability and Separation theorems are among the cornerstones in the model theory of L, this property can be seen as a test that identifies the adequate notion of observational equivalence for a particular modal logic.
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