TY  - JOUR
AU  - Christopher B. Croke
TI  - Rigidity for surfaces of non-positive curvature.
JO  - Commentarii mathematici Helvetici
PY  - 1990
SP  - 150
EP  - 170
VL  - 65
IS  - 3
PB  - mathdoc
UR  - http://geodesic.mathdoc.fr/item/CMH_1990__65_3_140188/
ID  - CMH_1990__65_3_140188
ER  -