Principal congruences of tetravalent modal algebras.
Isabel Loureiro · Notre Dame Journal of Formal Logic · 1985
We show that tetravalent modal algebras form a discriminator variety.Consequently, we obtain a characterization for congruences and mainly for principal congruences. / Discriminator variety T and congruencesWe begin with the four-element algebra S 4 (there is no connection with the modal system S 4 of Lewis and Langford) of type (2,2,1,1,0).Its operations are the two lattice operations Λ, V on 1 a/ \b 0 with the two unary operations:x