1. The essential spectrum of a class of ordinary differential operators;Balslev;Pacific J. Math.,1964
2. Asymptotic Behavior and Stability Problems in Ordinary Differential Equations;Cesari,1959
3. Unbounded Linear Operators;Goldberg,1966
4. Unrestricted solution fields of almost separable differential equations;Hartman;Trans. Amer. Math. Soc.,1948
5. On dichotomies for solutions of nth order linear differential equations;Hartman;Math. Ann.,1963