@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/