The purpose of this text is pedagogical, aimed at undergraduates in
classical mechanics. A massive point in the middle of an elastic
horizontal string is assigned the nonlinear ODE and its 1D motion is
discussed.

MR2511516 O'Shea, Michael J. Harmonic and anharmonic behaviour of a
simple oscillator. European J. Phys. 30 (2009), no. 3, 549--558.
81Q05