@article{10_1090_S0894_0347_99_00301_X,
     author = {Hoffmann, Detlev},
     title = {Pythagoras numbers of fields},
     journal = {Journal of the American Mathematical Society},
     pages = {839--848},
     year = {1999},
     volume = {12},
     number = {3},
     doi = {10.1090/S0894-0347-99-00301-X},
     url = {http://geodesic.mathdoc.fr/articles/10.1090/S0894-0347-99-00301-X/}
}