1. I. V. Tyutin and B. L. Voronov, Phys. Scripta, 87, 038119 (2013); arXiv:1211.6331v1 [math-ph] (2012).
2. Progr. Math. Phys.;D M Gitman,2012
3. N. I. Akhiezer and I. M. Glazman, Theory of Linear Operators in Hilbert Space [in Russian], Nauka, Moscow (1966); English transl. (Monogr. Stud. Math., Vol. 9), Pitman, Boston, Mass. (1981).
4. M. A. Naimark, Linear Differential Operators [in Russian], Nauka, Moscow (1969).
5. J. von Neumann, Functional Operators, Vol. 2, The Geometry of Orthogonal Spaces, Princeton Univ. Press, Princeton, N. J. (1950).