1. Bellman, R.: On monotone convergence to solutions ofu′ =g(u, t). Proc. Amer. Math. Soc.8, 1007–1009 (1957)
2. —— Quasi-linearization and upper and lower bounds for variational problems. Quart. Appl. Math.19, 349–350 (1962).
3. Collatz, L.: Functional analysis and applied mathematics. New York: Academic Press 1966.
4. Courant, R., Hilbert, D.: Methods of mathematical physics, vol. II. New York: Interscience 1962.
5. Noble, B.: Complementary variational principles for boundary value problems I: Basic principles. Math. Res. Cntr. Rept. No. 473 (1964).