“…Early descriptions of many of them date back to at least 1892, when the book by Greenhill [5] appeared, presenting a variety of such problems: a simple pendulum, catenaries, Watt's governor, Lintearia, a uniform chain that rotates steadily with a constant angular velocity about an axis to which the chain is fixed at two points, etc. Later applications include nonlinear plasma oscillations [6], Duffing oscillators [7], rigid plates satisfying the Johansen yield criterion [8], nonlinear transverse vibrations of a plate carrying a concentrated mass [9], a beam supported at an axially oscillating mount [10], doubly periodic cracks subjected to concentrated forces [11], surface waves in a plasma column [12], coupled modes of nonlinear flexural vibrations of a circular ring [13], dual-spin spacecrafts [14], spacecraft motion about slowly rotating asteroids [15], nonlinear vibration of buckled beams [16], a nonlinear wave equation [17], deep-water waves with two-dimensional surface patterns [18], oscillations of a body with an orbital tethered system [19], etc.…”