@article{10_4153_CJM_1971_105_2,
     author = {Pressman, Irwin S.},
     title = {Obstructions to {Liftings} in {Commutative} {Squares}},
     journal = {Canadian journal of mathematics},
     pages = {977--982},
     year = {1971},
     volume = {23},
     number = {6},
     doi = {10.4153/CJM-1971-105-2},
     url = {http://geodesic.mathdoc.fr/articles/10.4153/CJM-1971-105-2/}
}