@article{JEMS_2006_8_2_a13,
     author = {YanYan Li and Louis Nirenberg},
     title = {A geometric problem and the {Hopf} {Lemma.} {I}},
     journal = {Journal of the European Mathematical Society},
     pages = {317--339},
     publisher = {mathdoc},
     volume = {8},
     number = {2},
     year = {2006},
     doi = {10.4171/jems/55},
     url = {http://geodesic.mathdoc.fr/articles/10.4171/jems/55/}
}