@article{DAN_1965_165_1_a10,
     author = {S. L. Sobolev},
     title = {Representation of periodic analytic functions by a sum of squares},
     journal = {Doklady Akademii Nauk},
     pages = {40--43},
     year = {1965},
     volume = {165},
     number = {1},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DAN_1965_165_1_a10/}
}