Christov, O. “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., vol. 93, Dec. 2001, pp. 25-30, https://ftl5.uni-sofia.bg/index.php/fmi/article/view/199.