1. Stig Kanger, Provability in Logic, Almquist & Wiksell, Stockholm, 1957; ?A Note on Quantification and Modalities? and ?On the Characterization of Modalities?, both in Theoria 23 (1957).
2. Jaakko Hintikka, ?Modality and Quantification?, Theoria 27 (1961), and ?The Modes of Modality?, Acta Philosophica Fennica 16 (1963).
3. Saul Kripke, ?A Completeness Theorem in Modal Logic?, The Journal of Symbolic Logic 24 (1959), and ?Semantical Analysis of Modal Logic I?, Zeitschrift für mathematische Logik und Grundlagen der Mathematik 9 (1963).
4. Richard Montague, ?Logical Necessity, Physical Necessity, Ethics, and Quantifiers?, Inquiry 3 (1960).
5. J. Barkley Rosser, Logic for Mathematicians, McGraw-Hill, New York, 1953, Chapter IV, pp. 55?76.