“…Although Ẽx,s and Bx,s couple in Eqs. (20)(21), if they do not have the α-terms, we can solve Eqs. (20)(21) exactly as shown in appendix H. There is another way to find the expressions of Ẽx,s and Bx,s , because the electromagnetic field, Ẽ and B, has only two degrees of freedom.…”