1. S. Goldman,Frequency Analysis, Modulation and Noise(McGraw-Hill Book Company, Inc., New York, 1948).
2. H. Cramer,Mathematical Methods of Statistics(Princeton University Press, Princeton, New Jersey, 1946).
3. Theory of Servomechanisms, MIT Radiation Laboratory Series, Volume 25.
4. S. O. Rice, Bell System Tech. J. (January, 1945).