1. Advanced ordinary differential equations;Friedrichs,1949
2. Note to Poincaré's perturbation method;Cronin;Duke Math. J.,1959
3. Poincaré's perturbation method and topological degree;Cronin,1960
4. Stability of periodic solutions of the perturbation problem;Cronin;J. Math. and Mech.,1961
5. Families of solutions of a perturbation problem;Cronin,1961