1. R. Bellman, Introduction to the Mathematical Theory of Control Processes, Vol. II, Academic Press, Inc., New York, 1971.
2. R. Bellman, ‘Functional Equations in the Theory of Dynamic Programming. XV. Layered Functionals and Partial Differential Equations’, Journal of Mathematical Analysis and Applications 28 (1969), 183.
3. R. Bellman, Methods of Nonlinear Analysis, Vol. II, Academic Press, New York, 1973.