1. P. Bernays andA. A. Fraenkel,Axiomatic Set Theory. North Holland Publishing Co., Amsterdam. 1958.
2. M. J. Cresswell, “Functions of Propositions”Journal of Symbolic Logic, 31, 1966, pp. 545–560.
3. D. Hilbert and W. Ackermann,Principles of Mathematical Logic (English translation ofGrundzüge der Theoretischen Logik, Berlin, 1928). Chelsea. 1951.
4. D. Hilbert and P. Bernays,Grundlagen der Mathematik, vol. II, Berlin. 1939 (repr. Ann Arbor, Mich. 1944).
5. H. Lewandowski andR. Suszko, “A Note concerning the Theory of Descriptions”Studia Logica 22. 1968. pp. 51–55.