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2. Sur les vibrations tautochrones dans les systèmes conservatifs et non conservatifs;Koukles;C. R. Acad. Sci. URRS,1937
3. Mechanics;Landau,1960
4. Nonlinear oscillations of fixed period;Levin;J. Math. Anal. Appl.,1963
5. Hill's Equation;Magnus,1966