@article{DAN_1969_184_2_a9,
     author = {E. L. Tonkov},
     title = {The second order periodic equation},
     journal = {Doklady Akademii Nauk},
     pages = {296--299},
     publisher = {mathdoc},
     volume = {184},
     number = {2},
     year = {1969},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DAN_1969_184_2_a9/}
}