1. J. C. Agapito, On Relatively Compact Perturbations of Special Expressions in L
2
w( I). Ph. D. Thesis, Dept. of Mathematics, University of the Philippines, Diliman, Manila, 1993.
2. E. Balslev and T. W. Gamelin, The essential spectrum of a class of ordinary differential operators. Pacific J. Math.
14(1964), 755–776.
3. N. Dunford and J. T. Schwartz, Linear Operators (Part I: General Theory, Part II: Spectral Theory, Part III: Spectral Operators). Wiley Interscience, New York, 1957, 1963, 1971.
4. S. Goldberg, Unbounded Linear Operators. McGraw-Hill, New York, 1966.
5. B. Schultze, Spectral properties of not necessarily self-adjoint linear differential operators. Adv. Math.
83(1990), 75–95.