  Gruner put forward a single particle model of charge-density wave, which is a
typical nonlinear differential equation, and also a mathematical model of
pendulum. This Letter analyzes the solution of equation by the rotated vector
fields theory, providing the relation between the applied field E and the
periodic solution, and the conclusion that the critical value of E for the
periodic solution is fixed in the over-damped situation. With these
conclusions, it derives the formulae of nonlinear conductivity, narrow-band
noise, which are consistent with the empirical ones given by Fleming.
