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• [1] Arveson W., (1994) C^*-algebras and Numerical Linear Algebra, Journal of Functional Analysis 122, 333-360.
• [2] Davies E.B., (2002) Non-Self-Adjoint Differential Operators, Bulletin of the London Mathematical Society 34, 513-532.
• [3] Dencker N., Sjöstrand J. and Zworski M., (2004) Pseudospectra of Semiclassical (Pseudo-) Differential Operators., Communications on Pure and Applied Mathematics 57 (3), 384-415.
• [4] Bender C.M., Brody D.C and Jones H.F., (2002) Complex Extension of Quantum Mechanics, Physical Review Letters 89 (27), 1-4.
• [5] Hatano N. and Nelson D.R., (1996) Localization Transitions in Non-Hermitian Quantum Mechanics, Physical Review Letters 77, 570-573.
• [6] Trefethen L.N. and Chapman S.J., (2004) Wave Packet Pseudomodes of Twisted Toeplitz Matrices, Communications on Pure and Applied Mathematics 57 (9), 1233-1264.
• [7] Trefethen L.N and Embree M., (2005) Spectra and Pseudospectra: The Behavior of Nonnormal Matrices and Operators. Princeton University Press, Princeton, NJ, USA.
• [8] Hansen A.C., (2011) On the Solvability Complexity Index, the n-Pseudospectrum and Approximations of Spectra of Operators. Journal of the American Mathematical Society 24 (1), 81-124.
• [9] Brunner H., Iserles A. and Norsett S.P., (2010) The Spectral Problem for a Class of Highly Oscillatory Fredholm Integral Operators. IMA Journal of Numerical Analysis 30: 108-130. [10] Hansen A.C., (2010) Infinite-Dimensional Numerical Linear Algebra: Theory and Applications. Proceedings of the Royal Society London Series A Mathematical, Physical, Engineering Sciences 2010; 466 (2124): 3539-3559.
• [11] Dunford N. and Schwartz J.T., (1963) Linear Operators II. Interscience, New York, USA.
• [12] Ismailov Z.I., (2009) Multipoint Normal Differential Operators for First Order, Opuscula Mathematica 29 (4), 399-414.
• [13] Kochubei A.N., (1979) Symmetric operators and nonclassical spectral problems, Matematicheskie Zametki 25 (3), 425-434.
• [14] Timoshenko S., (1961) Theory of Elastic Stability. McGraw-Hill Book Co, New York, USA.
• [15] Zettl A. Sturm-Lioville Theory: Mathematical Surveys and Monographs. American Mathematical Society, Providence, RI, USA.
• [16] Gohberg I.C. and Krein M.G., (1969) Introduction to the Theory of Linear Non-Self-Adjoint Operators. Providence. American Mathematical Society, RI, USA.
• [17] Pietsch A., (1980) Operators Ideals. North-Holland Publishing Company, Amsterdam, Netherlands.
• [18] Pietsch A., (1987) Eigenvalues and s-Numbers. Cambridge University Press, Londan, England.
• [19] Çevik E.O and Ismailov Z.I., (2012) Spectrum of the Direct Sum of Operators, Electronic Journal of Differential Equations 210, 1-8.
• [20] Ismailov Z.I, Cona L. and Çevik E.O., (2015) Gelfand Numbers of Diagonal Matrices, Hacettepe Journal of Mathematics and Statistics 44 (1), 75-81.
• [21] Ismailov Z.I., Çevik E.O. and Unluyol E., (2011) Compact Inverses of Multipoint Normal Differential Operators for First Order, Electronic Journal of Differential Equations 89, 1-11.
• [22] Naimark N.A and Fomin S.V., (1955) Continuous Direct Sums of Hilbert Spaces and Some of Their Applications, Uspekhi Matematicheskikh Nauk 25 (64), 111-434 (in Russian).
• [23] Cui J., Li C.K. and Poon Y.T., (2014) Pseudospectra of Special Operators and Pseudospectrum Preservers. Journal of Mathematical Analysis and Applications 419: 1261-1273.
• [24] Hansen A.C. and Nevanlinna O., (2017) Complexity Issues in Computing Spectra, Pseudospectra and Resolvents. Banach Center Publications, Warsaw, Poland.
• [25] Seidel M, (2012) On (N,ϵ)-Pseudospectra of Operators on Banach Spaces, Journal of Functional Analysis 262, 4916-4927.