1. V. M. Alekseev, V. M. Tikhomirov, and S. V. Fomin, Optimal Control [in Russian], Nauka, Moscow (1979).
2. E. R. Avakov, G. G. Magaril-Il’yaev, and V. M. Tikhomirov, “Lagrange’s principle in extremal problems with constraints,” Russ. Math. Surv., 68, No. 3, 401–434 (2013).
3. N. N. Bogolyubov, “Sur quelques méthodes nouvelles dans le calcul des variations,” Ann. Math. Pure Appl., Ser. 4, 7, 249–271 (1930).
4. J. M. Borwein and J. Zhu, Techniques of Variational Analysis, CMS Books Math., Vol. 20, Springer, Berlin (2006).
5. F. H. Clarke, “A new approach to Lagrange multipliers,” Math. Oper. Res., 1, 165–174 (1976).