Kevorkian, J., Two-Variable Expansion Procedure for the Approximate Solution of Certain Nonlinear Differential Equations, Lectures in Applied Mathematics, Vol. 7: Space Mathematics, Part 3, edited by J.B. Rosser, American Mathematical Society, Providence, R.I., 1966, pp. 206-275. 5 Van Woerkom, P.Th.L.M. and Lion, P.M., A Multi-Variable Approach to the Problem of Aerodynamically Perturbed Satellite Trajectories, paper presented at the XXII International Astronautical Congress, Brussels, Belgium, Sept. 20-25, 1971. Van Woerkom, P.Th.L.M., A Multi-Variable Approach to Perturbed Aerospace Vehicle Motion, Ph.D. Thesis, Dept. of Aerospace and Mechanical Sciences, Princeton University, 1972. Bruno, C., Secular Perturbations of an Earth Satellite, Seminar, Dept. of Aerospace and Mechanical Sciences, Princeton University, 1970. Van Woerkom, P.Th.L.M., Generalized Models for the Study of Interaction between Spacecraft and Atmosphere, Paper 74-063, XXV International Astronautical Congress, Amsterdam, The Netherlands, Sept. 30-Oct. 5, 1974. Geyling, F.T. and Westerman, H.R., Introduction to Orbital Mechanics, Addison-Wesley, Reading, Mass., 1971, pp. 180-184.