@article{DE_1992_28_5_a13,
     author = {S. V. Zhestkov},
     title = {Coefficient conditions for the existence of $\omega$-periodic solutions of quasilinear wave systems},
     journal = {Differencialʹnye uravneni\^a},
     pages = {843--847},
     year = {1992},
     volume = {28},
     number = {5},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DE_1992_28_5_a13/}
}