1. 3 Singular Extremals
2. Marec, J. P., Optimal SpaceTrajectories, Elsevier Scientific, Amsterdam, 1979, pp.95-101.
3. Optimality of intermediate-thrust arcs of rocket trajectories
4. Bell, D. J., and Jacobson, D. H., Singular Optimal Control Problems, Academic Press, New York, 1975, pp.61-151.