1. Reed, M, Simon, B: Methods of Modern Mathematical Physics, vol. 3. Academic Press, London (1970)
2. Rosenblum, G, Solomyak, M, Shubin, M: Spectral Theory of Differential Operators. VINITI, Moscow (1989)
3. Gilbarg, D, Trudinger, NS: Elliptic Partial Differential Equations of Second Order. Springer, Berlin (1977)
4. Levin, B, Kheyfits, A: Asymptotic behavior of subfunctions of time-independent Schrödinger operator. In: Some Topics on Value Distribution and Differentiability in Complex and P-Adic Analysis, Chap. 11, pp. 323-397. Science Press, Beijing (2008)
5. Simon, B: Schrödinger semigroups. Bull. Am. Math. Soc. 7, 447-526 (1982)