1. R. L. De Kronig and W. G. Penney, “Quantum mechanics of electrons in crystal lattices,” Proc. R. Soc. Lond. A 130(814), 499–513 (1931).
2. E. Fermi, “Sul moto dei neutroni nelle sostanze idrogenante,” Ric. Sci. Progr. Tecn. Econom. Naz. 2, 13–52 (1936).
3. F. A. Berezin and L. D. Faddeev, “A remark on the Schr ödinger equation with a singular potential,” [J] Sov. Math., Dokl. 2, 372–375 (1961 Dokl. Akad. Nauk SSSR [Soviet Math. Dokl.] 137(5), 1011–1014 (1961) [SovietMath. Dokl. 2, 372–375 (1961)].
4. S. Albeverio, R. Høegh-Krohn, F. Gesztesy, and H. Holden, “Some exactly solvable models in quantum mechanics and the low energy expansions,” in Proceedings of the Second International Conference on Operator Algebras, Ideals, and Their Applications in Theoretical Physics, Teubner-Texte Math., Leipzig, 1983 (Teubner, Leipzig, 1984), Vol. 67.
5. A. M. Savchuk and A. A. Shkalikov, “Sturm-Liouville operators with distribution potentials,” Trudy Moskov. Mat. Obshch. 64, 159–212 (2003) [Trans. Moscow Math. Soc. 2003, 143–192 (2003)].