@article{10_4153_CJM_1991_033_5,
     author = {Nelis, Peter},
     title = {Schur and {Projective} {Schur} {Groups} of {Number} {Rings}},
     journal = {Canadian journal of mathematics},
     pages = {540--558},
     year = {1991},
     volume = {43},
     number = {3},
     doi = {10.4153/CJM-1991-033-5},
     url = {http://geodesic.mathdoc.fr/articles/10.4153/CJM-1991-033-5/}
}