1. F. A. Berezin and L. D. Faddeev, Dokl. Akad. Nauk SSSR 137 (5), 1011 (1961) [Soviet Math. Dokl. 2, 372 (1961)].
2. S. Albeverio, F. Gesztesy, R. Hoegh-Krohn, and H. Holden, Solvable Models in Quantum Mechanics (AMS Chelsea Publ., Providence, RI, 2005).
3. A. M. Savchuk, UspekhiMat. Nauk 55 (6 (336)), 155 (2000) [RussianMath. Surveys 55 (6), 1168 (2000)].
4. A. M. Savchuk and A. A. Shkalikov, in Trudy Moskov. Mat. Obshch. (Moscow, 2003), Vol. 64, No. 1, pp. 159–212, [Trans. MoscowMath. Soc. 2003, 143 (2003)]
5. A. S. Kostenko and M. M. Malamud, Funktsional. Anal. Prilozhen. 44 (2), 87 (2010) [Functional Anal. Appl. 44 (2), 151 (2010)].