“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, pp. 137–153, Dec. 2009, Accessed: Dec. 16, 2025. [Online]. Available: https://annual.uni-sofia.bg/index.php/fmi/article/view/118