@article{CMH_1967__42_139339,
     author = {B. Steer and J.M. Boardman},
     title = {On {Hopf} {Invariants.}},
     journal = {Commentarii mathematici Helvetici},
     pages = {180--221},
     publisher = {mathdoc},
     volume = {42},
     year = {1967},
     zbl = {0166.19201},
     url = {http://geodesic.mathdoc.fr/item/CMH_1967__42_139339/}
}