@article{DAN_1965_165_2_a7,
     author = {E. A. Livshitz},
     title = {On duality principles for the problem of periodic solutions of higher order differential equations},
     journal = {Doklady Akademii Nauk},
     pages = {277--280},
     publisher = {mathdoc},
     volume = {165},
     number = {2},
     year = {1965},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DAN_1965_165_2_a7/}
}