The method of two-time Green's functions in the magnetism theory of metals with strong correlations

We apply the Bogoliubov–Tyablikov method of two-time Green's functions to a multielectron theory of metals in the framework of the models with strong correlations, which are the Hubbard model and the Vonsovskii $s$–$d(f)$ exchange model. We discuss the atomic representation in the problem of describing electron states, the metal–insulator transition, the problem of noncoherent states, and the Kondo effect. We analyze the ferromagnetism criterion in systems of strongly correlated electrons, which differs drastically from the Stoner condition.