  The hierarchy of equations for reduced density matrices that describes a
thermodynamically equilibrium quantum system obtained earlier by the author is
investigated in the momentum representation. In the paper it is shown that the
use of the momentum representation opens up new opportunities in studies of
macroscopic quantum systems both nonsuperfluid and superfluid. It is found that
the distribution over momenta in a quantum fluid is not a Bose or Fermi
distribution even in the limit of practically noninteracting particles. The
distribution looks like a Maxwellian one although, strictly speaking, it is not
Maxwellian. The momentum distribution in a quantum crystal depends upon the
interaction potential and the crystalline structure. The momentum distribution
in a superfluid contains a delta function. The momentum distribution for the
condensate in a superfluid crystal consists of delta peaks that are arranged
periodically in momentum space. The periodical structure remains if the
condensate crystal is not superfluid.
