+ \frac{e}{c}\dot{\bf u} \times {\bf B}
+ m\frac{\d\dot{\bf u}}{\d t}
, \qquad \tilde {\bf B} = {\bf B} - \frac{mc}{e}\,
\curl\dot{\bf u} .
\ee          %42

\par
The first expression in (42) consists of two parts --- the electrical force
$\dx \,e \bf E'$ where $\dx{\bf E}' = {\bf E} + \frac{1}{c} {\bf \dot u \times
B}\,$ is the field in the co-moving frame, and the inertial force
$\dx\, m \frac
{\d {\bf \dot u}}{\d t}\,$.
The second relation in (42) can also be written in the form
$\dx \, \tilde {\bf B} = \curl ({\bf A} - \frac {c}{e}m \dot {\bf u})\, $
where $\, \bf A \,$ is the vector potential in an agreement with the well-known
rule of replacement of the particle momentum in a magnetic field
$\dx {\cal P} \rightarrow {\cal P} - \frac{e}{c}\bf A\,$ where now
${\cal P} = m \bf {\dot u} \,$.

 \par
    We shall use also another form of the kinetic equation.
The reason is that
further on we need to integrate some physical quantities over the Brillouin
zone, take integrals by parts as well as differentiate with respect to the
coordinates and the time. However, the Brillouin zone boundaries are functions
not 