the linearized differential equation of the system, equation (9), by the same method that is commonly used for determination of conditions which lead to the loss of stability of the non-inverted pendulum through excitation of ordinary parametric resonance (the ranges of parametric instability; see, for example, [10]). We can apply the linearized equation to this problem because at the boundary of dynamic stability the amplitude of oscillations is infinitely small. The periodic solution to equation (9), which corresponds to the boundary of instability, can be represented as a superposition of the fundamental harmonic whose frequency ω/2 equals half the driving frequency, and the third harmonic with the frequency 3ω/2: