where k = (ω0 /ω)2 . As we already indicated above, negative values of the parameter k (negative ω2 values) can be treated as referring to the inverted pendulum. Then the boundaries of subharmonic resonances can be expressed both for the hanging down
and inverted pendulum by the same formula: mmin = p2(1/n2 − k). The limit of this
expression at n → ∞ gives the mentioned earlier approximate condition of stability of the inverted pendulum: mmin = √−2k (where k < 0).
Being based on a decomposition of motion on slow oscillations and rapid vibrations with the driving frequency, equation (6) is approximate and valid if the
‡ At horizontal forcing of the pivot the hanging down vertical equilibrium position destabilizes and two symmetric lateral dynamically stabilized equilibrium positions appear if the driving amplitude and frequency satisfy the same condition that corresponds to the dynamic stabilization of the inverted pendulum at vertical forcing.