Braynov, P., and O. Christov. “Non-Integrability of a Hamiltonian System, Based on a Problem of Nonlinear Vibration of an Elastic String”. Ann. Sofia Univ. Fac. Math. And Inf., vol. 99, Dec. 2009, pp. 137-53, https://annual.uni-sofia.bg/index.php/fmi/article/view/118.