@article{MAN_1989__285_1_164590,
     author = {Fr\'ed\'eric H\'elein},
     title = {Approximations of {Sobolev} maps between an open set and an {Euclidean} sphere, boundary data, and singularities.},
     journal = {Mathematische Annalen},
     pages = {125--140},
     publisher = {mathdoc},
     volume = {285},
     number = {1},
     year = {1989},
     zbl = {0659.35002},
     url = {http://geodesic.mathdoc.fr/item/MAN_1989__285_1_164590/}
}