Vector Fields on the n-Sphere.

@article{Steenrod1951VectorFO,
  title={Vector Fields on the n-Sphere.},
  author={N. E. Steenrod and J H Whitehead},
  journal={Proceedings of the National Academy of Sciences of the United States of America},
  year={1951},
  volume={37 1},
  pages={58-63}
}