@article{MZM_1980_28_6_a8,
     author = {E. Z. Dyment},
     title = {Exact bounds of denumerable collections of degrees of difficulty},
     journal = {Matemati\v{c}eskie zametki},
     pages = {899--910},
     publisher = {mathdoc},
     volume = {28},
     number = {6},
     year = {1980},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/MZM_1980_28_6_a8/}
}