On the history of variational methods of non-linear equations investigations and the contribution of Soviet scientists (1920s-1950s).
Variational methods for solving nonlinear equations (differential, integral, etc.) are perhaps the most common methods at the present time. However, the history of their origin and development, both in the USSR and in the whole world, has not been studied enough. The author attempts to fill this gap, limiting himself mainly to 1920's-1950's, starting with Hilbert’s works on justification of Dirichlet principle and with Poincaré’s work on the study of closed geodesics on convex surfaces. The article...