E(x, t)is a function which varies slowly as compared to u', "(z -X,) in the neighborhood of X,. Also Q is again either the identity operator or the grad operator r7. If the vector q&~"is replaced by its average value q = &u p, in Eqs. (A25), then Eqs. (2. 3a) -(2. 3c) follow directly from Eqs. (A25) and the definitions (2. 2) when the electromagnetic field propagates in the z direction.
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