@article{MM2_1990__69_1_155559,
     author = {Robert Hardt and Fang-Hua Lin},
     title = {The {Singular} {Set} of an energy minimzing map from {B4} to {S2.}},
     journal = {Manuscripta mathematica},
     pages = {275--290},
     publisher = {mathdoc},
     volume = {69},
     number = {1},
     year = {1990},
     zbl = {0713.58006},
     url = {http://geodesic.mathdoc.fr/item/MM2_1990__69_1_155559/}
}