1. F. A. Berezin and M. A. Shubin, The Schrödinger Equation [in Russian], Moscow State University Press, Moscow (1983).
2. I. M. Glazman, Direct Methods of Qualitative Spectral Decomposition of Singular Differential Operators [in Russian], Fizmatgiz, Moscow (1963).
3. M. Reed and B. Simon, Methods of Modern Mathematical Physics, Vol. 4. Analysis of Operators, Academic Press, New York (1978).
4. F. W. J. Olver, Introduction to Asymptotics and Special Functions, Academic Press, New York?London (1974).
5. A. F. Vakulenko, ?Treves inequality and the nonexistence of positive eigenvalues of the Schrödinger operator with complex potential,? Zap. Nauchn. Sem. LOMI,147, 13?17 (1985).