@article{BSMF_1990__118_2_147_0,
     author = {Xu, Chao-Jiang},
     title = {Subelliptic variational problems},
     journal = {Bulletin de la Soci\'et\'e Math\'ematique de France},
     pages = {147--169},
     publisher = {Soci\'et\'e math\'ematique de France},
     volume = {118},
     number = {2},
     year = {1990},
     doi = {10.24033/bsmf.2141},
     mrnumber = {92b:49008},
     zbl = {0717.49004},
     language = {en},
     url = {http://www.numdam.org/articles/10.24033/bsmf.2141/}
}