1. Chellas, B. F., Modal Logic: An Introduction, Cambridge University Press, Cambridge, 1980. Zbl 0431.03009 MR 81i:03019
2. Cresswell, M. J., “Necessity and contingency,” Studia Logica, vol. 47 (1988), pp. 145–149. Zbl 0666.03015 MR 90d:03032
3. Hiż, H., “A warning about translating axioms,” American Mathematics Monthly, vol. 65 (1958), pp. 613–614. Zbl 0082.01504 MR 21:3326
4. Mortensen, C., “A sequence of normal modal systems with non-contingency bases,” Logique et Analyse, vol. 19 (1976), pp. 341–344. Zbl 0347.02014 MR 58:16154
5. Montgomery, H., and R. Routley, “Contingency and non-contingency bases for normal modal logics,” Logique et Analyse, vol. 9 (1966), pp. 318–328. Zbl 0294.02008 MR 36:6270