@article{ZNSL_1981_112_a9,
     author = {F. B. Koval'chik},
     title = {Certain analogues of the {Hardy--Litlewood} problem and density methods},
     journal = {Zapiski Nauchnykh Seminarov POMI},
     pages = {121--142},
     publisher = {mathdoc},
     volume = {112},
     year = {1981},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/ZNSL_1981_112_a9/}
}