1. Abraham, Karl: Selected Papers. P. 423. Hogarth. London. 1949.
2. Boole, George: The Laws of Thought. Dover Publications, Inc. New York. 1854.
3. McCulloch, W. S., and Pitts, W.: A Logical Calculus of the Ideas Imminent in Nervous Activity. Bull. Math. Biophys., 5:115, 1943.
4. Rosenbloom, Paul C.: The Elements of Mathematical Logic. Dover Publications, Inc. New York. 1950.
5. Whitehead, A. N., and Russell, B.: Principia Mathematica. Cambridge University Press. Cambridge. 1925.