1. L.D. Landau and E.M. Lifschitz, Quantenmechanik 8. Auflage, Akademie-Verlag Berlin, 1988, Ch. 11.
2. The Feynman lectures, Vol. 3. Quantum mechanics, R.P. Feynman, R.B. Leighton and M. Sands, Addision-Wesley Publishing Co., London, 1970, Chs. 9–11.
3. Diabolical points in the spectra of triangles