1. J. Bernoulli, Variational Principles of Mechanics (Fizmatlit, Moscow, 1959), p. 11 [in Russian].
2. J. Bernoulli, Variational Principles of Mechanics (Fizmatlit, Moscow, 1959), pp. 12–17 [in Russian].
3. P. Appel, Theoretical Mechanics (Fizmatgiz, Moscow, 1960), Vol. 1 [in Russian].
4. M. A. Lavrent’ev and L. A. Lyusternik, Fundamentals of Calculus of Variations (Gostekhizdat, Moscow, 1935), Vol. 2 [in Russian].
5. V. I. Smirnov, A Treatise of Higher Mathematics (Nauka, Moscow, 1974; Addison-Wesley, Reading, Mass., 1964), Vol. 4, Part 1.