1. F. Hertweck andA. Schlüter:Zeits. Naturf.,12a, 844 (1957).
2. S. Chandrasekhar:Adiabatic invariants in the motions of charged particles, inPlasma in a Magnetic Field,R. K. M. Landshoff, ed., (Stanford, Cal., 1958), p. 3.
3. G. Backus, A. Lesaed andE. Kulsbud:Zeits. Naturj.,15 a, 1007 (1960).