@article{DAN_1969_188_4_a12,
     author = {S. S. Ivanov and D. Ya. Petrina},
     title = {Equations arising in the summation of perturbation-theory series for the scattering matrix},
     journal = {Doklady Akademii Nauk},
     pages = {776--779},
     year = {1969},
     volume = {188},
     number = {4},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DAN_1969_188_4_a12/}
}