1. Introduction to non-linear mechanics;Krylov,1937
2. Asymptotic methods in the theory of non-linear oscillations;Bogolyubov,1963
3. The averaging method in the theory of non-linear oscillatory systems;Volosov,1971
4. Analysis of non-linear systems;Blaker,1969
5. The method of pointwise mappings in the theory of non-linear oscillations;Neimark,1972