@article{CMH_1949__23_138990,
     author = {Ernst Specker},
     title = {Die erste {Cohomologiegruppe} von {\"Uberlagerungen} und {Homotopie-Eigenschaften} dreidimensionaler {Mannigfaltigkeiten.}},
     journal = {Commentarii mathematici Helvetici},
     pages = {303--333},
     publisher = {mathdoc},
     volume = {23},
     year = {1949},
     zbl = {0036.12803},
     url = {http://geodesic.mathdoc.fr/item/CMH_1949__23_138990/}
}