According to equation (7), both high harmonics have equal amplitudes (m/2)A. However, we see from the plots in Fig. 2 that these amplitudes are slightly different. Therefore we can try to improve the approximate solution for ϕ(t), equation (7), as well as the theoretical values for the lower boundaries of subharmonic resonances, equation (6), by assuming for the possible solution a similar spectrum but with unequal amplitudes, An−1 and An+1 , of the two high harmonics (for n > 2, the case of n = 2 will be considered separately):