  The distribution function for a system of interacting electrons in metals is
multivalent in a certain region of wave vectors. One solution among many is
isotropic. For other solutions the distribution of electrons over the wave
vectors is anisotropic. In the simplest case, the anisotropy arises as a result
of the repulsion between electrons in states with the wave vectors $\bf k$ and
$-\hh\bf k$.
