@article{IM_1984__77_143155,
     author = {P. Sarnak and R. Osserman},
     title = {A new curvature invariant and entropy of geodesic flow.},
     journal = {Inventiones mathematicae},
     pages = {455--462},
     publisher = {mathdoc},
     volume = {77},
     year = {1984},
     zbl = {0536.53048},
     url = {http://geodesic.mathdoc.fr/item/IM_1984__77_143155/}
}