1. M.A. Naimark, Investigation of the spectrum and the expansion in eigenfunctions of a non-selfadjoint operator of the second order on a semi-axis. Am. Math. Soc. Trans. Ser. 2 16, 103–193 (1960)
2. R.R.D. Kemp, A singular boundary value problem of a non-selfadjoint differential operator. Can. J. Math. 10, 447–462 (1958)
3. B.S. Pavlov, The non-selfadjoint Schrödinger operator. Top. Math. Phys. 1, 87–110 (1967)
4. B.S. Pavlov, On separation conditions for the spectral componess of a dissipative operators. Math. USSR Izvest. 9, 113–137 (1975)
5. E.C. Titchmarsh, Eigenfunction Expansions Associated with Second Order Differential Equations (Oxford University Press, London, 1962)