1. Theory of Ordinary Differential Equations;Coddington,1955
2. Asymptotic Behavior and Stability Problems in Ordinary Differential Equations;Cesari,1963
3. Asymptotic Expansions;Erdélyi,1956
4. Perturbations of special operators, and applications I. Bounded perturbations;Schwaztr;Pacific J. Math.,1954
5. On Hill's problems with complex parameters and a real periodic function;Strutt,1948