1. Larsen, S. Y., Mascheroni, P. L.: Phys. Rev.A2, 1018 (1970)
2. Lecture Notes in Physics;S. Y. Larsen,1978
3. Sommerfeld, A.: Partial Differential Equations. New York: Academic Press 1967; Erdélyi, A., Magnus, W., Oberhettinger, F., Tricomi, F.: Higher Transcendental Functions (Bateman Manuscript Project), vol. II, chapt. XI. New York: McGraw-Hill 1953
4. See for example: Fabre de la Ripelle, M.: Ann. Phys. (NY)147, 281 (1983).
5. Unsöld, A.: Ann. d. Phys.82, 355 (1927)