1. N. I. Akhiezer and I. M. Glazman,Theory of Linear Operators in Hilbert Space [in Russian], Kharkov (1978).
2. Sh. A. Alimov, V. A. Il'in, and E. M. Nikishin, “Questions of convergence of multiple trigonometric series and spectral decompositions, I,”Usp. Mat. Nauk,31, No. 6 (192). 28–83 (1976).
3. O. A. Amvrosova, “Eigenvalues asymptotics and equiconvergence theorems for operators with power singularities in boundary conditions,” In:Functional Analysis [in Russian], No. 21, Ulyanosk Gos. Ped. Inst. Press, Ulyanovsk (1983), pp. 3–11.
4. O. A. Amvrosova. “On one boundary-value problem with a power singularity in the boundary condition,” In:Investigations on Modern Problems of Mathematics [in Russian], Saratov Univ. Press, Saratov (1984), pp. 31–37.
5. F. V. Atkinson,Discrete and Continuous Boundary Problems [Russian translation], Mir, Moscow (1968).