1. Fulton, C.T., Trans.Amer.Math.Soc. 229 (1977), 51–63.
2. Rellich, F., Math.Z. 49 (1943/44), 702–723.
3. Bulla, W.; Gesztesy, F., J.Math.Phys. 26 (1985), 2520–2528.
4. Burnap, C.; Greenberg, W.; Zweifel, P.F., Nuovo Cim. A50 (1979), 457–465.
5. Olver, F.W.J., Introduction to asymptotics and special functions, Academic Press, New York, 1974.