@inproceedings{brian1986homotopy,
title={Homotopy classes in Sobolev spaces and the existence of energy minimizing maps},
author={Brian White},
url={http://archive.ymsc.tsinghua.edu.cn/pacm_paperurl/20170108203309459410409},
booktitle={Acta Mathematica},
volume={160},
number={1},
pages={1-17},
year={1986},
}
Brian White. Homotopy classes in Sobolev spaces and the existence of energy minimizing maps. 1986. Vol. 160. In Acta Mathematica. pp.1-17. http://archive.ymsc.tsinghua.edu.cn/pacm_paperurl/20170108203309459410409.