@article{MZ_1991__206_1_174212,
     author = {J. Sj\"ostrand and A.G. Ramm},
     title = {An inverse problem of the wave equation.},
     journal = {Mathematische Zeitschrift},
     pages = {119--130},
     publisher = {mathdoc},
     volume = {206},
     number = {1},
     year = {1991},
     zbl = {0697.35164},
     url = {http://geodesic.mathdoc.fr/item/MZ_1991__206_1_174212/}
}