1. Monographs and Studies in Mathematics;NI Akhiezer,1981
2. Albeverio, S., Brasche, J.F., Malamud, M.M., Neidhardt, H.: Inverse spectral theory for symmetric operators with several gaps: scalar-type Weyl functions. J. Funct. Anal. 228, 144–188 (2005)
3. Albeverio, S., Gesztesy, F., Hoegh-Krohn, R., Holden, H.: Solvable Models in Quantum Mechanics, 2nd edn. American Mathematical Society Chelsea Publications, Providence, RI (2005). (With an Appendix by P. Exner)
4. Albeverio, S., Kostenko, A.S., Malamud, M.M., Neidhardt, H.: Spherical Schrödinger operators with $$\delta $$-type interactions. J. Math. Phys. 54(5), 052103 (2013)
5. Aronszajn, N.: On a problem of Weyl in the theory of singular Sturm–Liouville equations. Ar. J. Math. 79, 597–610 (1957)