1. Biorthogonal systems and bases in Hilbert space;Bari;Moskov. Gos. Univ. Učenye Zapiski Matematika,1951
2. A survey of the theory of spectral operators;Dunford;Bull. Amer. Math. Soc.,1958
3. N. Dunford J. T. Schwartz Linear Operators. Part III. Spectral Operators (Wiley, New York, 1971).
4. Spectra of 1-D periodic Dirac operators and smoothness of potentials;Djakov;C. R. Math. Acad. Sci. Soc. R. Can.,2003
5. Instability zones of a periodic 1D Dirac operator and smoothness of its potential;Djakov;Comm. Math. Phys.,2005