Y. Xu and A. C. Luo, “A Series of Symmetric Period-1 Motions to Chaos in a Two-degree-of-freedom van der Pol-Duffing Oscillator”, JVTSD, vol. 2, no. 2, pp. 119–153, Aug. 2026, doi: 10.5890/JVTSD.2018.06.003.