@article{AIHPC_1990__7_4_269_0,
     author = {Bethuel, F.},
     title = {A characterization of maps in $H^1 (B^3, S^2)$ which can be approximated by smooth maps},
     journal = {Annales de l'I.H.P. Analyse non lin\'eaire},
     pages = {269--286},
     publisher = {Gauthier-Villars},
     volume = {7},
     number = {4},
     year = {1990},
     mrnumber = {1067776},
     zbl = {0708.58004},
     language = {en},
     url = {http://archive.numdam.org/item/AIHPC_1990__7_4_269_0/}
}