1. N. I. Akhiezer and I. M. Glazman, The Theory of Linear Operators in Hilbert Space (Nauka, Moscow, 1966
2. Pitman, Boston, Mass., 1981), Vol. 2.
3. A. Alonso and B. Simon, J. Operator Theory 4, 251–270 (1980).
4. M. S. Ashbaugh, F. Gesztesy, M. Mitrea, and G. Teschl, Adv. Math. 223, 1372–1467 (2010).
5. M. Sh. Birman, Mat. Sb. 38 (4), 431–450 (1956).