Finite speed of propagation for mixed problems in the WR class.

In this article we are interested in the propagation speed for solution of hyperbolic boundary value problem in the WR class. Using the Holm-gren principle, we show that this speed is finite and we are able to give an explicit expression for the maximal speed. Due to propagation phe-nomenon along the boundary specific to the WR class, the maximal speed can be larger than the propagation speed for the Cauchy problem. This is consistent with examples of the litterature