@article{JRAM_1974__270_151499,
     author = {K.W. Johnson},
     title = {Varietal generalisations of {Schur} multipliers, stem extensions and stem covers.},
     journal = {Journal f\"ur die reine und angewandte Mathematik},
     pages = {169--183},
     publisher = {mathdoc},
     volume = {270},
     year = {1974},
     zbl = {0291.18019},
     url = {http://geodesic.mathdoc.fr/item/JRAM_1974__270_151499/}
}