Cai, Yuting, and Chunrui Zhang. “Hopf-Pitchfork Bifurcation of Coupled Van Der Pol Oscillator With Delay”. Nonlinear Analysis: Modelling and Control, vol. 22, no. 5, Sept. 2017, pp. 598-13, https://doi.org/10.15388/NA.2017.5.2.