Geometrical (in particular, topological) methods in nonlinear analysis
were originally invented by Banach, Birkhoff, Kellogg, Schauder, Leray,
and others in existence proofs. Since about the fifties, these methods
turned out to be essentially the sole approach to a variety of new
problems: the investigation of iteration processes and other procedures
in numerical analysis, in bifur- cation problems and branching of
solutions, estimates on the number of solutions and criteria for the
existence of nonzero solutions, the analysis of the structure of the
solution set, etc. These methods have been widely applied to the theory
of forced vibrations and auto-oscillations, to various problems in the
theory of elasticity and fluid. mechanics, to control theory,
theoretical physics, and various parts of mathematics. At present,
nonlinear analysis along with its geometrical, topological, analytical,
variational, and other methods is developing tremendously thanks to
research work in many countries. Totally new ideas have been advanced,
difficult problems have been solved, and new applications have been
indicated. To enumerate the publications of the last few years one would
need dozens of pages. On the other hand, many problems of non- linear
analysis are still far from a solution (problems arising from the
internal development of mathematics and, in particular, problems arising
in the process of interpreting new problems in the natural sciences). We
hope that the English edition of our book will contribute to the further
propagation of the ideas of nonlinear analysis.