@article{CMH_1933__5_138598,
     author = {J. Hadamard},
     title = {La propagation des ondes et les caustiques.},
     journal = {Commentarii mathematici Helvetici},
     pages = {137--174},
     year = {1933},
     volume = {5},
     zbl = {59.0492.02},
     url = {http://geodesic.mathdoc.fr/item/CMH_1933__5_138598/}
}