@article{DAN_1987_296_2_a2,
     author = {P. I. Lizorkin and S. M. Nikol'skii},
     title = {Symmetric averaged differences on a sphere},
     journal = {Doklady Akademii Nauk},
     pages = {271--274},
     year = {1987},
     volume = {296},
     number = {2},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DAN_1987_296_2_a2/}
}