@article{DAN_1972_205_5_a1,
     author = {M. S. Burgin},
     title = {Central extensions in $\gamma$-categories},
     journal = {Doklady Akademii Nauk},
     pages = {1019--1021},
     year = {1972},
     volume = {205},
     number = {5},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DAN_1972_205_5_a1/}
}