1. See, for example, S. Lefschetz,Differential Equations: Geometric Theory(Interscience Publishers, Inc., New York, 1957), pp. 230–238.
2. See, for example, A. A. Andronow and C. E. Chaiken,Theory of Oscillations(Princeton University Press, Princeton, New Jersey, 1949), pp. 217–224.
3. Reference 3, p. 227.