The present compilation includes four articles on various topics, each of which contains an exposition of the present state of art in the most interesting and timely problems of solid-state theory. The authors are physicists who have contributed significantly to this field of science. … It goes without saying that the selection of material to illustrate the achievements in solid-state physics must inevitably be subjective. For the compilation does not hold works on superconductivity, a problem to which a separate book could be devoted. But we still believe that the articles should be of interest to readers who wish to know what is new in solid-state physics.
