Bifurcations and chaotic thresholds for the spring-pendulum oscillator with irrational and fractional nonlinear restoring forcesRuilan Tian, Qiliang Wu, Yeping Xiong, Xinwei Yang and Wenjie FengEur. Phys. J. Plus, 129 5 (2014) 85DOI: https://doi.org/10.1140/epjp/i2014-14085-3