1. Cut-free tableau calculi for some propositional normal modal logics;Amerbauer;Stud. Log.,1996
2. A. Avron, I. Lev, Canonical propositional Gentzen-type systems, in: R. Goré, A. Leitsch, T. Nipkow (Eds.), International Joint Conferences on Automated Reasoning, IJCAR 01, Lecture Notes in Computer Science, vol. 2083, Springer, 2001, pp. 529–544.
3. A new method for bounding the complexity of modal logics;Basin,1997
4. Modal Logic;Blackburn,2001
5. B. Chellas, Modal Logic, Cambridge, 1980.