The other difference in the system behaviour occurs to the left side of the
resonance peak where multiple solutions of the nonlinear system appear (in this case resonant and non-resonant
solutions) in the region of the resonance. One can observe that starting from A ¼ 0:26 m the curves in
Fig. 8 show a discontinuity signaling jumps between the resonant and non-resonant vibration amplitude
AOUT. Note in all cases a series of simulations were performed to calculate the system response, with X0
decreasing as in Fig. 3a and b. For most of curves the same initial conditions were used for large X0, namely
½xin; vin ¼ ½0:15; 0:1. However, if the system escaped from the potential well initial conditions of
½xin; vin ¼ ½0:15; 0:1 and [0,0.1] were used to avoid this effect. For A ¼ 0:36 m this was not possible in the
vicinity of the resonance peak where the system escaped from the potential well for any initial conditions.
Moreover, just before this escape (for A ¼ 0:36 m Ac) we observe a further increase in the vibration amplitude
AOUT. Examining the related bifurcations diagrams, a period doubling phenomenon occurs in this region,
which may be classified as a precursor of chaotic vibrations. Indeed alternative criteria to the Melnikov
approach (Eq. (24)) are based on the period doubling cascade [27,28]. For larger amplitudes the unstable
vibration region, where escape from the potential is possible, increases. On the other hand, at A ¼ Ac the border
between the basins of attraction belonging to different solutions disappears. To avoid these difficulties for
further analysis the synchronized solution for X0 ¼ 0:8 at A ¼ 0:31 m (Fig. 8) was used, and then the excitation
amplitude was increased slightly to A ¼ 0:41 m, crossing the critical amplitude of Ac ffi 0:35 m. Fig. 9a and b
shows the phase portraits (by lines) and Poincare maps (by points) for these two cases.