Belova M.M., “Ogranichennye resheniya sistem nelineinykh obyknovennykh differentsialnykh uravnenii vtorogo poryadka”, Doklady AN SSSR, 9 (1968), 603–606, translation from Dokl. Akad. Nauk SSSR 180, 266-268 (1968).Bounded solutions of systems of second order ordinary differential equations. (Russian)
M. M. Belova, “On bounded solutions of non-linear second-order differential equations”, Mat. Sb. (N.S.), 56(98):4 (1962), 469–503