@article{MZM_1997_61_6_a15,
     author = {S. D. Kozlov and S. V. Dronov},
     title = {Generators of $\sigma$- and $\pi$-classes},
     journal = {Matemati\v{c}eskie zametki},
     pages = {943--945},
     publisher = {mathdoc},
     volume = {61},
     number = {6},
     year = {1997},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/MZM_1997_61_6_a15/}
}