1. Theory of Ordinary Differential Equations;Coddington,1955
2. Asymptotic Methods in the Theory of Non-linear Oscillations;Bogoliubov,1961
3. Perturbation theorems for periodic surfaces;Diliberto;Rend. Circ. Mat. Palermo,1960
4. Perturbation theorems for periodic surfaces;Diliberto;Rend. Circ. Mat. Palermo,1961
5. On the method of Krylov-Bogoliubov-Mitropolski for the existence of integral manifolds of perturbed differential systems;Hale,1961