ABSTRACT
This chapter discusses various properties of the Fermi liquid in thermodynamic equilibrium. Nonequilibrium states are described by distribution functions that depend not only on the momenta p but also on the coordinates and time. The distinctive feature about the transport equation of a Fermi liquid in comparison with a Fermi gas is that the quasiparticle energy in a Fermi liquid is a functional of the distribution function. Zero sound is an essentially nonequilibrium phenomenon. The first observation of zero sound was reported in 1963 by B. E. Keen and co-workers who were measuring sound attenuation in 3He. Treating the coordinate and momentum as classical variables amounts to making a quasiclassical assumption for the motion of the quasiparticles. The quasiclassical assumption also allows to define a distribution function that is simultaneously a function of the coordinates and momenta of the quasiparticles. In the equilibrium state, the Fermi surface is a sphere.
