1. H. Goldstein,Classical Mechanics(Addison-Wesley Publishing Company, Inc., Reading, Massachusetts, 1951), Chap. 9.
2. H. C. Corben and P. Stehle,Classical Mechanics(John Wiley & Sons, Inc., New York, 1960), p. 318.
3. During the looping motion θ̇ will reverse sign over part of the loop.
4. A. A. Andronow and C. E. Chaikin,Theory of Oscillations(Princeton University Press, Princeton, New Jersey, 1949), Chap. II.
5. N. Minorsky,Introduction to Non-Linear Mechanics(Edwards Brothers, Inc., Ann Arbor, Michigan, 1947), Chap. 2.