@article{GDZPPN002067528,
title = {Rigidity for surfaces of non-positive curvature.},
journal = {Commentarii mathematici Helvetici},
volume = {65},
year = {1990},
pages = {150-170},
zbl = {0704.53035},
url = {http://dml.mathdoc.fr/item/GDZPPN002067528}
}
Croke, Christopher B. Rigidity for surfaces of non-positive curvature.. Commentarii mathematici Helvetici, Tome 65 (1990) pp. 150-170. http://gdmltest.u-ga.fr/item/GDZPPN002067528/