1. Non-linear oscillations of fixed period;Levin;J. Math. Anal. Appl.,1963
2. Periodic solutions of x″ + cx′ + g(x) = ϵf(t);Loud;Mem. Amer. Math. Soc.,1959
3. Analytical Theory of non-linear oscillations I;Obi,1974
4. Potential forces with yield periodic motions of fixed period;Urabe;J. Math. Mech.,1961