Geometric Optics for Quasilinear Hyperbolic Boundary Value Problems

This paper presents the work in Kilque (2022) which investigate the existence of the leading profile of a WKB expansion for quasilinear initial boundary value problems with a highly oscillating forcing boundary term. The framework is weakly nonlinear, as the boundary term is of order \( O(\epsilon ) \) where the frequencies are of order \( O(1/\epsilon ) \) . We consider here multiple phases on the boundary, generating a countable infinite number of phases inside the domain, and we therefore use an almost periodic functional framework. The major difficulties of this work are the lack of symmetry in the leading profile equation and the occurrence of infinitely many resonances (opposite to the simple phase case studied earlier). The leading profile is constructed as the solution of a quasilinear problem, which is solved using a priori estimates without loss of derivatives.