@article{CRMATH_2009__347_19-20_1159_0,
     author = {Majumdar, Apala and Robbins, J.M. and Zyskin, Maxim},
     title = {Tangent unit-vector fields: {Nonabelian} homotopy invariants and the {Dirichlet} energy},
     journal = {Comptes Rendus. Math\'ematique},
     pages = {1159--1164},
     publisher = {Elsevier},
     volume = {347},
     number = {19-20},
     year = {2009},
     doi = {10.1016/j.crma.2009.09.002},
     language = {en},
     url = {http://archive.numdam.org/articles/10.1016/j.crma.2009.09.002/}
}