@article{10_4153_CJM_2002_036_1,
     author = {Cegarra, A. M. and Gar\'cia-Calcines, J. M. and Ortega, J. A.},
     title = {On {Graded} {Categorical} {Groups} and {Equivariant} {Group} {Extensions}},
     journal = {Canadian journal of mathematics},
     pages = {970--997},
     year = {2002},
     volume = {54},
     number = {5},
     doi = {10.4153/CJM-2002-036-1},
     url = {http://geodesic.mathdoc.fr/articles/10.4153/CJM-2002-036-1/}
}