@article{701410,
     title = {Singularities in the geometry of an obstacle},
     booktitle = {Proceedings of the Winter School "Geometry and Physics"},
     series = {GDML\_Books},
     publisher = {Circolo Matematico di Palermo},
     address = {Palermo},
     year = {1987},
     pages = {[71]-84},
     mrnumber = {MR0946713},
     zbl = {0642.53087},
     url = {http://dml.mathdoc.fr/item/701410}
}

Janeczko, Stanisƚaw. Singularities in the geometry of an obstacle, dans Proceedings of the Winter School "Geometry and Physics", GDML_Books,  (1987), pp. [71]-84. http://gdmltest.u-ga.fr/item/701410/