1. Non-linear Oscillations;Nayfeh,1979
2. Asymptotic Methods in the Theory of Non-linear Oscillations;Bogoliubov,1961
3. Oscillation in Non-linear Systems;Hale,1963
4. Coupling in certain classes of weakly non-linear vibrating systems;Sethna,1963
5. Resonant motion of a spherical pendulum;Miles;Physica D,1984