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VARIETIES OF POSITIVE MODAL ALGEBRAS AND STRUCTURAL COMPLETENESS
Published online by Cambridge University Press: 13 June 2019
Abstract
Positive modal algebras are the $$\left\langle { \wedge , \vee ,\diamondsuit ,\square,0,1} \right\rangle $$-subreducts of modal algebras. We prove that the variety of positive S4-algebras is not locally finite. On the other hand, the free one-generated positive S4-algebra is shown to be finite. Moreover, we describe the bottom part of the lattice of varieties of positive S4-algebras. Building on this, we characterize (passively, hereditarily) structurally complete varieties of positive K4-algebras.
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- Copyright © Association for Symbolic Logic 2019
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