1. A. A. Andronov, “Les cycles limites de Poincaré et la théorie des oscillations auto-entretenues,” C. R. Seances Acad. Sci. 189, 559–561 (1929).
2. L. S. Pontryagin, “On dynamical systems close to Hamiltonian systems,” Zh. Eksp. Teor. Fiz. 4 (9), 883–885 (1934).
3. A. D. Morozov, Resonances, Cycles and Chaos in Quasi-Conservative Systems (Regular and Chaotic Dynamics, Izhevsk, Moscow, 2005) [in Russian].
4. S. A. Korolev and A. D. Morozov, “On periodic perturbations of self-oscillating pendulum equations,” Russ. J. Nonlinear Dyn. 6 (1), 79–89 (2010).
5. M. Bonnin, “Existence, number, and stability of limit cycles in weakly dissipative, strongly nonlinear oscillators,” Nonlinear Dyn. 62 (1-2), 321–332 (2010).