1. For a discussion of this and other approximation methods together with further references see H. Grad, in Hb. d. Phys. XII, S. Flügge, ed. ( Springer, Berlin, 1958 ).
2. W. Israel, J. Math. Phys., 4, 1163 (1963).
3. N. A. Chernikov, Acta Phys. Polonica, 27, 465 (1964).
4. C. Marie, Ann. Inst. Henri Poincaré, X, 67 (1969).
5. A. G. Walker, Proc. Edinburgh Math. Soc., 4, 238 (1936).