The method of two-time Green's functions in the molecular hydrodynamics of a superfluid

For a system of interacting bosons at temperature below the critical temperature the single-particle functions as well as the Green's functions for the fluctuations of the density and the temperature are constructed on the basis of the hierarchy of equations for the irreducible Green's functions. In the limiting case of low frequencies and small wave vectors a comparison is made with the results obtained on the basis of the two-fluid hydrodynamics of a superfluid Bose liquid. It is shown that the macroscopic parameters of the two-fluid hydrodynamics (the susceptibilities and the transport coefficients) are completely determined in the framework of the microscopic approach.