1. Lyapunov, A.M., Issledovanie odnogo iz osobennykh sluchaev zadachi ob ustoichivosti dvizheniya (Study of a Singular Case of the Motion Stability Problem), Collection of Papers, Vol. 2, Moscow–Leningrad, 1956.
2. Bibikov, Yu.N. and Savel’eva, A.G., Periodic perturbations of a nonlinear oscillator, Differ. Equations, 2016, vol. 52, no. 4, pp. 405–412.
3. Bibikov, Yu.N. and Pliss, V.A., Bifurcation of the state of equilibrium of an oscillator with nonlinear restoring force of third order, Vestn. St. Petersburg Univ. Math., 2015, vol. 48, no. 2, pp. 57–60.
4. Basov, V.V. and Bibikov, Yu.N., On stability of the equilibrium in a case of periodic perturbation of the center, Differ. Equations, 1997, vol. 33, no. 5, pp. 587–590.
5. Hale, J.K., Integral manifolds of perturbed differential systems, Ann. of Math., 1961, vol. 73, no. 3, pp. 496–531.