1. E. Fermi, “Sopra lo sostamento per pressione delle righe elevante delle serie spettrali,” Nouvo Cimento,11, 157–166 (1934).
2. F. A. Berezin and L. D. Faddeev, “A remark on Schrödinger's equation with a singular potential,” Dokl. Akad. Nauk SSSR,137, No. 5, 1011–1014 (1961).
3. Yu. N. Demkov and V. N. Ostrovskii, The Potential Method of Zero Radius in Atomic Physics [in Russian], Leningrad State Univ. (1975).
4. B. S. Pavlov, “A model of zero-radius potential with internal structure,” Teor. Mat. Fiz.,59, No. 3, 345–353 (1984).
5. M. G. Krein, “On the resolvents of a Hermitian operator with defect index (m, m),” Dokl. Akad. Nauk SSSR,52, No. 8, 657–660 (1946).