1. G. S. Boolos, The Unprovability of Consistency: An Essay in Modal Logic, Cambridge University Press, 1978.
2. C. W. Burrill, Measure, Integration and Probability, McGraw-Hill, 1972.
3. B. F. Chellas, Modal Logic: An Introduction, Cambridge University Press, 1980.
4. D. Costantini, Fondamenti del calcolo delle probabilit�, Feltrinelli, 1970.
5. D. Costantini, Introduzione alla probabilit�, Boringhieri, 1977.