@article{JMPA_1928_9_7__345_0,
     author = {Birkhoff, George D. and Smith, Paul A.},
     title = {Structure {Analysis} of {Surface} {Transformations}},
     journal = {Journal de Math\'ematiques Pures et Appliqu\'ees},
     pages = {345--379},
     publisher = {Gauthier-Villars},
     volume = {Ser. 9, 7},
     year = {1928},
     language = {en},
     url = {http://www.numdam.org/item/JMPA_1928_9_7__345_0/}
}