1. V. M. Alekseev, V. M. Tikhomirov, and S. V. Fomin, Optimal Control, 2nd ed. (Fizmatlit, Moscow, 2005) [in Russian].
2. A. V. Arutyunov, G. G. Magaril-Il’yaev, and V. M. Tikhomirov, Pontryagin’s Maximum Principle: Proof and Applications (Factorial, Moscow, 2006) [in Russian].
3. G. A. Bliss, Lectures on the Calculus of Variations (Univ. Chicago Press, Chicago, 1946; Inostrannaya Literatura, Moscow, 1950).
4. L. M. Graves, “Some Mapping Theorems,” Duke Math. J. 17, 111–114 (1950).
5. F. John, “Extreme Problems with Inequalities as Subsidiary Conditions,” in Studies and Essays Presented to R. Courant on His 60th Birthday, Jan. 8, 1948 (Intersci., New York, 1948), pp. 187–204.