Hydrodynamics of superfluid systems and the method of quasiaverages

The thermodynamics and hydrodynamics of macroscopic superfluid systems with broken phase invariance are constructed on the basis of Bogolyubov's concept of quasiaverages and the reduced-description method. The case of broken Galilean invariance is considered. A method for finding the low-frequency behavior of the Green's functions for arbitrary quasilocal operators is developed. An extension of the approach to systems in which translational invariance is also broken is discussed