  Hard self-oscillation excitation in systems with infinitely many equilibrium
points forming a line of equilibria in the phase space is analyzed. The
demonstrated bifurcation phenomena correspond to a self-oscillation excitation
mechanism being equivalent to the excitation scenario via the subcritical
Andronov-Hopf bifurcation observed in classical self-oscillators with isolated
equilibrium points. Hysteresis and bistability accompanying the discussed
processes are shown and explained. The study is carried out on an example of a
nonlinear memristor-based self-oscillator. First, a simpler model including
Chua's memristor with a piecewise-smooth characteristic is explored by means of
numerical modelling and analytical solution. Then the memristor characteristic
is changed to a function being smooth everywhere in order to demonstrate that
the observed effects are not associated with the smoothness of the memristor
model. Finally, the action of the memristor forgetting effect is taken into
consideration.
