1. For a simple derivation of this equation by analytic continuation and references to earlier papers see Barut, A.O., Phys. Rev. D10, 3335 (1974).
2. Wessel, W., Fortschritte der Physik 12, 409–440 (1964).
3. Barut, A.O. and Bracken, A.J., The Zitterbewegung and the Internal Geometry of the Electron, Phys. Rev. D (1981)
4. Proc. Group Theory Conference, Lecture Notes in Physics, Vol. 135, p. 206-211 (Springer, 1980).
5. Schrödinger, E., Sitzungsb. Preuss. Akad. Wiss. Phys.-Math. Kl. 24, 418 (1930).