The stability and bifurcation of a van der Pol-Duffing oscillator with the delay feedback are investigated, in which the strength of feedback control is a nonlinear function of delay. A geometrical method in conjunction with an analytical method is developed to identify the critical values for stability switches and Hopf bifurcations. The Hopf bifurcation curves and multi-stable regions are obtained as two parameters vary. Some weak resonant and non-resonant double Hopf bifurcation phenomena are observed due to the vanishing of the real parts of two pairs of characteristic roots on the margins of the "death island" regions simultaneously. By applying the center manifold theory, the normal forms near the double Hopf bifurcation points, as well as classifications of local dynamics are analyzed. Furthermore, some quasi-periodic and chaotic motions are verified in both theoretical and numerical ways. © 2007 Elsevier Inc. All rights reserved.
Ma, S., Lu, Q., & Feng, Z. (2008). Double Hopf bifurcation for van der Pol-Duffing oscillator with parametric delay feedback control. Journal of Mathematical Analysis and Applications, 338(2), 993–1007. https://doi.org/10.1016/j.jmaa.2007.05.072