The density matrix method in the quantum theory of cooperative phenomena

The book contains a sequential presentation of the theory of density matrices, which are used in quantum mechanics for a statistical description of both equilibrium and non-equilibrium systems. A variational method is proposed, which makes it possible from the principle of a minimum of the free energy of a multipartic system to obtain equations for a density matrix describing the equilibrium state of such a system. With the help of this method, microscopic theories of such cooperative phenomena are constructed, as ferromagnetism, superconductivity and superfluidity. The theory is developed for non-equilibrium density matrices. An equation for a quantum harmonic oscillator interacting with a thermostat was obtained.
The book may be of interest to high school students, graduate students, teachers and researchers