Dispersion for the wave equation inside strictly convex domains I: the   Friedlander model case
  We consider a model case for a strictly convex domain of dimension $d\geq 2$ with smooth boundary and we describe dispersion for the wave equation with Dirichlet boundary conditions. More specifically, we obtain the optimal fixed time decay rate for the smoothed out Green function: a $t^{1/4}$ loss occurs with respect to the boundary less case, due to repeated occurrences of swallowtail type singularities in the wave front set. 
Oana Ivanovici, Gilles Lebeau and Fabrice Planchon
math.AP