@article{IM_1987__90_143501,
     author = {C. Bardos and G. Lebeau and J. Rauch},
     title = {Scattering frequencies and {Gevrey} 3 singularities.},
     journal = {Inventiones mathematicae},
     pages = {77--114},
     year = {1987},
     volume = {90},
     zbl = {0723.35058},
     url = {http://geodesic.mathdoc.fr/item/IM_1987__90_143501/}
}