@article{IVM_1964_3_a17,
     author = {G. E. Popov},
     title = {On the absence of periodic solutions taking the value $x=0$ in a class of differential equations of the form $\ddot x+x=F(x,\,\dot x)$},
     journal = {Izvesti\^a vys\v{s}ih u\v{c}ebnyh zavedenij. Matematika},
     pages = {120--122},
     publisher = {mathdoc},
     number = {3},
     year = {1964},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/IVM_1964_3_a17/}
}