@article{IM_1987__90_143503,
     author = {L Luengo},
     title = {The ?-constant stratum is not smooth.},
     journal = {Inventiones mathematicae},
     pages = {139--152},
     publisher = {mathdoc},
     volume = {90},
     year = {1987},
     zbl = {0627.32018},
     url = {http://geodesic.mathdoc.fr/item/IM_1987__90_143503/}
}