Recently, many works have been devoted to the discussion of the propagation and interaction of singularities for solutions of nonlinear hyperbolic equations. Among them the study of the interaction of nonlinear progressing waves is noticeable. First in [ 11, J. M. Bony discussed the behavior of’the interaction of two progressing waves. He showed that for semilinear hyperbolic equations of second order, when two progressing waves intersect transversally, no anomalous singularities are caused by the interaction, and for semilinear hyperbolic equations of higher order anomalous progressing waves appear issuing from the intersection. However, the interaction is much more complicated when three progressing waves intersect transversally with each other at one point. J. Rauch and M. Reed constructed an example showing that in the triple-intersection case, besides the linear-predicted singularities, a new progressing wave caused by interaction issuing from the intersection point propagates along the characteristic cone. Recently, R. Melrose and N. Ritter [3], J. M. Bony 141, and M. Beals [5] discussed the general triple-intersection of progressing waves for semilinear wave equations with two space variables by different methods. They pointed out that in the generalized case the singularities have the same structure as that in RauchhReed’s example. On the other hand, M. Beals and G. Metivier considered reflection of progressing waves for boundary value problems of semilinear hyperbolic equations. On the basis of these works, we study in this paper the interaction of two progressing waves with reflection on the boundary for boundary value problems of semilinear wave equations in two space variables. Our result shows that, in this case, beside the linear-predicted singularities there appears an anomalous reflected wave, which issues from the intersection
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