@article{SM_2007_198_4_a3,
     author = {R. V. Mikhailov},
     title = {Asphericity and approximation properties of crossed modules},
     journal = {Sbornik. Mathematics},
     pages = {521--535},
     publisher = {mathdoc},
     volume = {198},
     number = {4},
     year = {2007},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/SM_2007_198_4_a3/}
}