1. An Introduction to Mathematical Logic, I;Church,1956
2. Sur la logique discoursive de Jaśkowski;da Costa;Bulletin Acad. Polonaise des Sciences Math. Astr. et Phys.,1968
3. Remarks on Jaśkowski's discussive logic;da Costa;Reports on Math. Logic,1975
4. Principles of Mathematical Logic;Hilbert,1950
5. An introduction to Modal Logic;Hughes,1968