1. L. Brillouin,Wave Propagation in Periodic Structures(McGraw-Hill Book Company, Inc., New York, 1946), pp. 5, 17 and 140.
2. The condition F0 = 0 means that we do not want to reintroduce any component of the dominant wave. It was implicitly stated in Eq. (37).
3. See reference 1, Chap. I, II, IX, and V.
4. Comparing (68) and (70) it will be noted that this discussion corresponds to the case ω1<ω3; hence l5. See reference 1, pp. 11 and 12, Figs. 3.2, 3.3 and 3.4.