1. O. Heaviside,Electromagnetic Theory(D. Van Nostrand Company, Inc., New York, 1925), Vol. II, pp. 107–119.
2. E. T. Whittaker, and G. N. Watson,Modern Analysis(Cambridge University Press, London, 1927), pp. 134–136.
3. L. A. Pipes,Applied Mathematics for Engineers and Physicists(McGraw-Hill Book Company, Inc., New York, 1946), Chapter XXI.