1. L. S. Pontryagin, V. G. Boltyanskii, R. V. Gamkrelidze, and E. F. Mishchenko, The Mathematical Theory of Optimal Processes (Fizmatgiz, Moscow, 1961; Pergamon, Oxford, 1964).
2. G. G. Magaril-Il’yaev, “The Pontryagin maximum principle: Statement and proof,” Dokl. Akad. Nauk SSSR 442 (1), 20–23 (2012) [Dokl. Math. 85 (1), 14–17 (2012)].
3. I. P. Natanson, Theory of Functions of a Real Variable (Ungar, New York, 1955; Nauka, Moscow, 1974).
4. V. M. Alekseev, V. M. Tikhomirov, and S. V. Fomin, Optimal Control (Nauka, Moscow, 1979; Consultants Bureau, New York, 1987).