@article{MM2_1991__72_1_155640,
     author = {Roberto Pignoni},
     title = {On surfaces and their contours.},
     journal = {Manuscripta mathematica},
     pages = {223--250},
     publisher = {mathdoc},
     volume = {72},
     number = {1},
     year = {1991},
     zbl = {0736.53007},
     url = {http://geodesic.mathdoc.fr/item/MM2_1991__72_1_155640/}
}