1. LAWDEN, D. F., Optimal Trajectories for Space Navigation, Academic Press, New York, New York, 1967.
2. KELLEY, H. J., Gradient Theory of Optimal Flight Paths, ARS Journal, Vol. 30, pp. 947–954, 1960.
3. KELLEY, H. J., Method of Gradients, Optimization Techniques, Edited by G. Leitmann, Academic Press, New York, New York, pp. 205–254, 1962.
4. KELLEY, H. J., KOPP, R. E., and MOYER, H. G., A Trajectory Optimization Technique Based upon the Theory of the Second Variation, Progress in Astronautics and Aeronautics: Celestial Mechanics and Astrodynamics, Edited by V. G. Szebehely, Academic Press, New York, New York, Vol. 14, p. 559, 1964.
5. KELLEY, H. J., KOPP, R. E., and MOYER, H. G., Singular Extremals, Optimization Techniques, Edited by G. Leitmann, Academic Press, New York, New York, pp. 63–101, 1967.