1. R. Fagin, J. Halpern, Y. Moses, M. Vardi, Reasoning about knowledge, MIT Press, 1995.
2. J. Hintikka, Knowledge and belief, Cornell University Press, Ithaca, N.Y., 1962.
3. S. Kripke, A semantical analysis of modal logic I: normal modal prepositional calculi, Zeitschrift für Mathematische Logik und Grundlagen Mathematik 9 (1963), 67–96.
4. J. Hintikka, Impossible possible worlds vindicated, J. of Phil. Logic 4 (1975), 475–484.
5. H.J. Levesque, A logic of implicit and explicit belief, Proceedings of the Conference of the American Association for Artificial Intelligence, AAAI-84 (1984), 198–202.