1. O. D. Kellog:Foundations of Potential Theory (Berlin, 1967), p. 184.
2. E. Weber:Electromagnetic Fields, Theory and Applications, Vol. 1 (New York, N. Y., 1960), p. 414.
3. B. W. Montague:Fourth-order coupling resonances excited by space-charge forces in a synchrotron, CERN/ISR/68-38 (1968).