1. V. P. Mikhailov, ‘‘On the bases in $$L_{2}(0,1)$$,’’ Dokl. Akad. Nauk SSSR 144, 981–984 (1962).
2. G. M. Kesel’man, ‘‘On the unconditional convergence of expansions in the eigenfunctions of some differential operators,’’ Izv. Vyssh. Uchebn. Zaved., Mat. 2, 82–93 (1964).
3. N. Dunford and J. T. Schwartz, Linear Operators, Part 3: Spectral Operators (Wiley, New York, 1970).
4. M. A. Naimark, Linear Differential Operators, Part I (Frederick Ungar, New York, 1967).
5. P. W. Walker, ‘‘A nonspectral Birkhoff-regular differential operators,’’ Proc. Am. Math. Soc. 66, 187–188 (1977).