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2. The oscillations about a position of equilibrium where a simple linear relation exists between the frequencies of the principal vibrations;Beth;Phil. Mag. Ser. 6,1913
3. Oscillations of an elastic pendulum as an example of two parametrically coupled linear systems;Vitt;Zh Tekh Fiz,1933
4. Nonlinear resonances affecting gravity gradient stability;Breakwell,1966
5. The Motion of a Satellite about a Centre of Mass in a Gravitational Field;Beletskii,1975