@article{DAN_1988_299_3_a3,
     author = {A. I. Vagabov},
     title = {$n$-fold summability of series in principal functions of pencils},
     journal = {Doklady Akademii Nauk},
     pages = {534--538},
     year = {1988},
     volume = {299},
     number = {3},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DAN_1988_299_3_a3/}
}