@article{IM_1987__88_143457,
     author = {M. Ratner},
     title = {Rigid reparametrizations and cohomology for horocycle flows.},
     journal = {Inventiones mathematicae},
     pages = {341--374},
     publisher = {mathdoc},
     volume = {88},
     year = {1987},
     zbl = {0697.58046},
     url = {http://geodesic.mathdoc.fr/item/IM_1987__88_143457/}
}