Fig. 9

a Response of forced Van der Pol–Duffing oscillator with \(\varepsilon = - \mu = 0.2, F = 0.53\), \(\omega = 1\), \(x\left( {t_{0} } \right) = 0.1\) and \(\dot{x}\left( {t_{0} } \right) = - 0.2\). b Response of forced Van der Pol–Duffing oscillator with \(\varepsilon = - \mu = 0.2, F = 0.53\), \(\omega = 1\), \(x\left( {t_{0} } \right) = 0.1\) and \(\dot{x}\left( {t_{0} } \right) = - 0.2\). c Response of forced Van der Pol–Duffing oscillator with \(\varepsilon = - \mu = 0.2, F = 0.53\), \(\omega = 1\), \(x\left( {t_{0} } \right) = 0.1\) and \(\dot{x}\left( {t_{0} } \right) = - 0.2\). d Response of forced Van der Pol–Duffing oscillator with \(\varepsilon = - \mu = 0.2, F = 0.53\), \(\omega = 1\), \(x\left( {t_{0} } \right) = 0.1\) and \(\dot{x}\left( {t_{0} } \right) = - 0.2\). e Response of forced Van der Pol–Duffing oscillator with \(\varepsilon = - \mu = 0.2, F = 0.53\), \(\omega = 1\), \(x\left( {t_{0} } \right) = 0.1\) and \(\dot{x}\left( {t_{0} } \right) = - 0.2\)