Figure 4: Plot of . The first few dispersion curves are shown. Solid curves for the nonlinear case (solutions of (7.14)); dashed curve for the linear case (solutions of (8.5)). The following parameters are used for both cases: , , , and for the nonlinear case and . Dashed lines are described by formulas: (thickness of the layer), (lower bound for ) and (upper bound for in the case of linear medium in the layer).