An abstract algebraic logic approach to tetravalent modal logics

This paper contains a joint study of two sentential logics that combine a m any-valued character, namely tetravalence, with a modal character: one of t hem is normal and the other one quasi-normal. The method is to study their algebraic counterparts and their abstract models with the tools of Abstract Algebraic Logic, and particularly with those of BROWN and SUSZKO's theory of abstract logics as recently developed by FONT and JANSANA in their "A Ge neral Algebraic Semantics for Sentential Logics". The logics studied here a rise from the algebraic and lattice-theoretical properties we review of Tet ravalent Modal Algebras. a class uf algebras studied mainly by LOUREIRO, an d also by FIGALLO, LANDINI and ZILIANI, at the suggestion of the late ANTON IO MONTEIRO.