1. Andréka, H., Givant, S., Németi, I.: Decision Problems for Equational Theories of Relation Algebras. Memoirs of the American Mathematical Society. American Mathematical Society, Providence (1997)
2. Andréka, H., Givant, S., Németi, I.: The lattice of varieties of representable relation algebras. J. Symb. Logic 59, 631–661 (1994)
3. Andréka, H., Jónsson, B., Németi, I.: Free algebras in discriminator varieties. Algebra Universalis 28, 401–447 (1991)
4. Blackburn, P., de Rijke, M., Venema, Y.: Modal Logic. Cambridge Tracts in Theoretical Computer Science. Cambridge University Press, Cambridge (2002)
5. Blok, W.: The lattice of modal logics: an algebraic investigation. J. Symb. Logic 45, 221–236 (1980)