Christov, Ognyan. 2001. “Remark on the Non-Integrability of the Perturbed Motion of the Particle in a Central Field in Constant Curvature Spaces”. Ann. Sofia Univ. Fac. Math. And Inf. 93 (December):25-30. https://stipendii.uni-sofia.bg/index.php/fmi/article/view/199.