Energy estimates for the WR class
In this paper, we discuss energy estimates for a particular class of linear hyperbolic boundary value problems known as weakly regular of real type. Such class, also called WR in the literature, is relevant in many physical situations like the formation of shock waves in isentropic gas dynamics. In this and other WR examples, the failure of the uniform Lopatinskii condition plays a major role since it is associated with a loss of regularity in the scale of Sobolev spaces, eventually leading to energy inequalities that are ill-suited for nonlinear problems when solved by iteration. In the course of this work, we derive a priori estimates for the model case that are comparable to existing results, but using a more robust approach that we can extend to scalar or system problems with variable coefficients. The result is a significant step towards the ultimate goal of having estimates that can be combined with Piccard's or Newton's iterative scheme to analyze nonlinear situations.