1. Asymptotic solutions of non-linear second-order differential equations with variable coefficients;Kuzmak;Prikl. matem. mekhan.,1959
2. Analysis of the longitudinal stability of an aeroplane;Krylov,1932
3. Perturbations methods in applied mathematics;Cole,1972
4. Perturbations methods;Knife,1976
5. Linear and non-linear waves;Whitham,1977