1. V. M. Alekseev, V. M. Tikhomirov, and S. V. Fomin, Optimal Control [in Russian], Nauka, Moscow (1979).
2. V. G. Boltyansky, Mathematical Methods of Optimal Control [in Russian], Moscow, Nauka (1969).
3. D. A. Carlson, “An elementary proof of the maximum principle for optimal control problems governed by a Volterra integral equation,” J. Optim. Theory Appl., 54, No. 1, 43–61 (1987).
4. A. V. Dmitruk, “Maximum principle for a general optimal control problem with state and regular mixed constraints,” Comput. Math. Modeling, 4, No. 4, 364–377 (1993).
5. A. V. Dmitruk, “On the development of Pontryagin’s maximum principle in the works of A. Ya. Dubovitskii and A. A. Milyutin,” Control Cybern., 38, No. 4a, 923–958 (2009).