BALANOV, Zalman, CHEN, Fulai, GUO, Jing and KRAWCEWICZ, Wiesław. Periodic solutions to reversible second order autonomous systems with commensurate delays. Topological Methods in Nonlinear Analysis. Online. 7 June 2021. Vol. 59, no. 2A, pp. 475 - 498. [Accessed 7 March 2026]. DOI 10.12775/TMNA.2020.039.