On the characterizability of the frames for the ``unpreventability of the present and the past''
The language \({\mathcal L}_{MT}\) is obtained by adding the modal operator \(\diamond\), the past operator P and the future operator F to classical propositional logic. Formulas of \({\mathcal L}_{MT}\) are interpreted in frames that carry both a linear order and an accessibility relation. The author introduces the class \({\mathcal F}_ N\) of neutral frames guided by the intension, that \(\diamond \phi \leftrightarrow \square \phi\) should be true in all neutral frames for all formulas \(\phi\) not containing F. The objective of the paper is to show the non-characterizability in \({\mathcal L}_{MT}\) of certain subclasses of \({\mathcal F}_ N\).
scientific article; zbMATH DE number 4012577