@article{AFM_1973_521-560_a4,
     author = {Timo Erkama},
     title = {Group actions and extension problems for maps of balls},
     journal = {Annales Fennici Mathematici},
     publisher = {mathdoc},
     volume = {521-560},
     year = {1973},
     doi = {10.5186/aasfm.1973.556},
     language = {en},
     url = {http://geodesic.mathdoc.fr/articles/10.5186/aasfm.1973.556/}
}