@article{MZM_1974_16_3_a12,
     author = {S. S. Ryshkov},
     title = {Density of an $(r,R)$-system},
     journal = {Matemati\v{c}eskie zametki},
     pages = {447--454},
     publisher = {mathdoc},
     volume = {16},
     number = {3},
     year = {1974},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/MZM_1974_16_3_a12/}
}