@article{PM_2009_2009_1_a3,
     author = {Portilla, A. and  Tour{\'\i}s, E. },
     title = {A characterization of {Gromov} hyperbolicity of surfaces with variable negative curvature.},
     journal = {Publicacions matem\`atiques},
     pages = {83-110},
     publisher = {mathdoc},
     volume = {53},
     number = {1},
     year = {2009},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/PM_2009_2009_1_a3/}
}