In particular, within certain ranges of the system parameters (in the intervals of parametric instability) the lower position of the pendulum becomes unstable, as we already mentioned earlier. However, parametric resonance, as well as the modes of chaotic behaviour, occur at such driving frequencies (for the principal parametric
resonance ω ≈ 2ω0 ) that do not satisfy the conditions of applicability of the approach
used above. Next we show how to get instead of equation (6) the exact condition for
the boundaries of the subharmonic resonances.