“Non-integrability of a Hamiltonian system, based on a problem of nonlinear vibration of an elastic string” (2009) Annual of Sofia University St. Kliment Ohridski. Faculty of Mathematics and Informatics, 99, pp. 137–153. Available at: https://annual.uni-sofia.bg/index.php/fmi/article/view/118 (Accessed: 16 December 2025).