@article{SM_1989_64_2_a16,
     author = {P. I. Plotnikov},
     title = {Existence of a~countable set of periodic solutions of the problem of forced oscillations for a~weakly nonlinear wave equation},
     journal = {Sbornik. Mathematics},
     pages = {543--556},
     publisher = {mathdoc},
     volume = {64},
     number = {2},
     year = {1989},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/SM_1989_64_2_a16/}
}