TY  - JOUR
AU  - Pressman, Irwin S.
TI  - Obstructions to Liftings in Commutative Squares
JO  - Canadian journal of mathematics
PY  - 1971
SP  - 977
EP  - 982
VL  - 23
IS  - 6
UR  - http://geodesic.mathdoc.fr/articles/10.4153/CJM-1971-105-2/
DO  - 10.4153/CJM-1971-105-2
ID  - 10_4153_CJM_1971_105_2
ER  -