___

• [1] M.Sh. Birman and M.Z. Solomyak, Estimates of singular numbers of integral operators, Russian Math. Survey 32 (1), 15-89, 1977 (Translated from Uspekhi Mat. Nauk 32 (1), 17-84, 1977).
• [2] I. Chalendar, E. Fricain, M. Gürdal and M. T. Karaev, Compactness and Berezin symbols, Acta Sci. Math. (Szeged) 78 (1), 315-329, 2012.
• [3] F. Cobos, D.D. Haroske, T. Kühn and T. Ullrich, Mini-workshop: modern applications of s-numbers and operator ideals, Mathematisches Forschungs Institute Oberwolfach, Germany, 369-397, 8-14 February 2015.
• [4] N. Dunford and J.T. Schwartz, Linear Operators I, Interscience Publishers, 1958.
• [5] I.C. Gohberg and M.G. Krein, Introduction to the Theory of Linear Non-Selfadjoint Operators in Hilbert Space, American Mathematical Society, 1969.
• [6] Z.I. Ismailov, Compact inverses of first-order normal differential operators, J. Math. Anal. Appl. 320, 266-278, 2006.
• [7] Z.I. Ismailov, Multipoint normal differential operators for first order, Opuscula Math. 29, 399-414, 2009.
• [8] Z.I. Ismailov, E. Otkun Çevik and E. Unluyol, Compact inverses of multipoint normal differential operators for first order, Electron. J. Differential Equations 89, 1-11, 2011.
• [9] Z.I. Ismailov and E. Unluyol, Hyponormal differential operators with discrete spectrum, Opuscula Math. 30, 79-94, 2010.
• [10] M.T. Karaev, M. Gürdal and U. Yamancı, Special operator classes and their properties, Banach J. Math. Anal. 7 (2), 74-88, 2013.
• [11] M.A. Naimark and S.V. Fomin, Continuous direct sums of Hilbert spaces and some of their applications, Uspehi Mat. Nauk 10, 111-142, 1955, (in Russian).
• [12] E. Otkun Çevik and Z.I. Ismailov, Spectrum of the direct sum of operators, Electron. J. Differential Equations 210, 1-8, 2012.
• [13] A. Pietsch, Operators Ideals, North-Holland Publishing Company, 1980.
• [14] A. Pietsch, Eigenvalues and s-Numbers, Cambridge University Press, 1987.
• [15] R. Schatten and J. von Neumann, The cross-space of linear transformations, Ann. of Math. 47, 608-630, 1946.
• [16] E. Schmidt, Zur theorie der linearen und nichtlinearen integralgleichungen, Math. Ann. 64, 433-476, 1907.
• [17] H. Triebel, Über die verteilung der approximationszahlen kompakter operatoren in Sobolev-Besov-Raumen, Invent. Math. 4, 275-293, 1967.