Oscillations of an Elastic Pendulum
This Demonstration considers the motion of an elastic pendulum. In the case that the spring frequency is double the pendulum frequency, the motions exhibit a resonance. There is nonlinear coupling between the spring motion and the lateral pendulum motion. Two subsystems can exchange energy, which depends on the initial conditions.
 A.Vitt and G. Gorelik, "Oscillations of an Elastic Pendulum as an Example of the Oscillations of Two Parametrically Coupled Linear Systems," Zhurnal Tekhnicheskoy Fiziki, 3(2–3), 1933 pp. 294–307.
 R. Carretero-Gonzalez, H.-N. Nuñez-Yepez and A. L. Salas-Brito, "Regular and Chaotic Behavior in an Extensible Pendulum," European Journal of Physics, 15(3), 1994 pp. 139–148. doi:10.1088/0143-0807/15/3/009.
 M. C. de Sousa, F. A. Marcus, I. L. Caldas and R. L. Viana, "Energy Distribution in Intrinsically Coupled Systems: The Spring Pendulum Paradigm," Physica A: Statistical Mechanics and Its Applications, 509(1), 2018 pp. 1110–1119. doi:10.1016/j.physa.2018.06.089.