24. John Locker, Functional analysis and two-point differential operators, Pitman Research Notes
in Mathematics, vol. 144, Longmans, Harlow, Essex, 1986.
25. , The nonspectral Birkhoff-regular differential operators determined by — D2, Journal
of Mathematical Analysis and Applications 154 (1991), 243-254.
26. , The spectral theory of second order two-point differential operators: I. A priori
estimates for the eigenvalues and completeness, Proceedings of the Royal Society of Edinburgh
121A (1992), 279-301.
27. , The spectral theory of second order two-point differential operators: II. Asymptotic
expansions and the characteristic determinant, Journal of Differential Equations 114 (1994),
28. , The spectral theory of second order two-point differential operators: III. The eigen-
values and their asymptotic formulas, Rocky Mountain Journal of Mathematics 26 (1996),
29. , The spectral theory of second order two-point differential operators: IV. The associ-
ated projections and the subspace Soo(L), Rocky Mountain Journal of Mathematics 26 (1996),
30. John Locker and Patrick Lang, Eigenfunction expansions for the nonspectral differential op-
erators determined by —D2, Journal of Differential Equations 96 (1992), 318-339.
31. M. A. Naimark, Linear differential operators, I, GITTL, Moscow, 1954, English transl., Ungar,
New York, 1967.
32. Bruce P. Palka, An introduction to complex function theory, Springer-Verlag, New York, 1991.
33. Walter Rudin, Real and complex analysis, third ed., McGraw-Hill, New York, 1987.
34. , Functional analysis, second ed., McGraw-Hill, New York, 1991.
35. Martin Schechter, Principles of functional analysis, Academic Press, New York, 1971.
36. Bernd Schultze, Strongly irregular boundary value problems, Proceedings of the Royal Society
of Edinburgh 82A (1979), 291-303.
37. J. Schwartz, Perturbations of spectral operators, and applications. I. Bounded perturbations,
Pacific Journal of Mathematics 4 (1954), 415-458.
38. M. H. Stone, A comparison of the series of Fourier and Birkhoff, Transactions of the American
Mathematical Society 28 (1926), 695-761.
39. , Irregular differential systems of order two and the related expansion problems, Trans-
actions of the American Mathematical Society 29 (1927), 23-53.
40. , Linear transformations in Hilbert space and their applications to analysis, American
Mathematical Society Colloquium Publications, vol. XV, American Mathematical Society,
New York, 1932.
41. J. Tamarkin, Some general problems of the theory of ordinary linear differential equations
and expansion of an arbitrary function in series of fundamental functions, Mathematische
Zeitschrift 27 (1927), 1-54.
42. Angus E. Taylor and David C. Lay, Introduction to functional analysis, second ed., Krieger
Publishing Company, Malabar, Florida, 1980.
43. E. C. Titchmarsh, The theory of functions, second ed., Oxford University Press, Oxford, 1939.
44. Philip W. Walker, A nonspectral Birkhoff-regular differential operator, Proceedings of the
American Mathematical Society 66 (1977), 187-188.
45. S. Yakubov, Completeness of root functions of regular differential operators, Pitman Mono-
graphs and Surveys in Pure and Applied Mathematics, vol. 71, Longman Scientific & Techni-
cal, Harlow, Essex, 1994.