Diagrammatic Methods in the Theory of Superconductivity and Ferromagnetism
Diagrammatic methods are applied to strongly interacting electron systems. Several problems are studied in the atomic representation. It is assumed that electron-electron interaction in an atom is stronger that the interaction between electrons belonging to different atoms. The theory of high-temperature superconductivity is considered in the Hubbard model. Then the theory of ferromagnetism in transition metals is also presented.