1. Boolos, G., The Unprovability of Consistency, Cambridge University Press, Cambridge, 1979.
2. Boolos, G., The Logic of Provability, Cambridge University Press, Cambridge, 1993.
3. Feferman, S., "The arithmetization of metamathematics in a general setting", Fundamentum Mathematicae, vol. 49 (1960), pp. 35–92.
4. Gabbay, D. M., "Craig's interpolation theorem for modal logics", pp. 111–27 in Conference in Mathematical Logic–-London '70, edited by W. Hodges, Springer, Berlin, 1972.
5. Hughes, G. E., and M. J. Cresswell, A New Introduction to Modal Logic, Routledge, London, 1996.