@article{CMH_1990__65_3_140188,
     author = {Christopher B. Croke},
     title = {Rigidity for surfaces of non-positive curvature.},
     journal = {Commentarii mathematici Helvetici},
     pages = {150--170},
     publisher = {mathdoc},
     volume = {65},
     number = {3},
     year = {1990},
     zbl = {0704.53035},
     url = {http://geodesic.mathdoc.fr/item/CMH_1990__65_3_140188/}
}