1. The essential spectrum of a class of ordinary differential operators;Balslev;Pacific J. Math.,1964
2. Limit-point criteria for not necessarily symmetric quasi-differential expressions;Frentzen,1987
3. Unbounded Linear Operators;Goldberg,1966
4. On the limit-n classification of ordinary differential operators with positive coefficients;Kauffman;Proc. London Math. Soc.,1977
5. The deficiency index problem for powers of ordinary differential expressions;Kauffman,1977