Braynov, Petya, 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. 99 (December 12, 2009): 137–153. Accessed December 4, 2024. https://stipendii.uni-sofia.bg/index.php/fmi/article/view/118.