1. Abramowitz M. and Stegun I.A.: 1972. Handbook of Mathematical Functions, New York, Dover
2. Courant E.D. and Snyder H.S.: 1958. Ann. Phys., 3, 1
3. Einstein A.: 1911. Inst. Intern. Phys. Solway, Rapports et discussions, 1, 450
4. Feller W.: 1971. An Introduction to Probability Theory and its Applications, Vol. II, New York, Wiley
5. Fröman N.: 1966. Arkiv för Physik, 32, 541