Azbelev, N. V.     On a new approach to some problems of classical calculum of variations / N. V. Azbelev, E. Bravy, S. Gusarenko // Nonlinear dynamics and systems theory an international journal of research and surveys. - 2002. - Vol. 2, № 2. - С. 117-124 Доп.точки доступа: Bravy, E.; Gusarenko, S. A.