Geodesic
On the absence of periodic solutions taking the value $x=0$ in a class of differential equations of the form $\ddot x+x=F(x,\,\dot x)$
Izvestiâ vysših učebnyh zavedenij. Matematika, no. 3 (1964), pp. 120-122 
G. E. Popov. On the absence of periodic solutions taking the value $x=0$ in a class of differential equations of the form $\ddot x+x=F(x,\,\dot x)$. Izvestiâ vysših učebnyh zavedenij. Matematika, no. 3 (1964), pp. 120-122. http://geodesic.mathdoc.fr/item/IVM_1964_3_a17/
@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},
     year = {1964},
     number = {3},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/IVM_1964_3_a17/}
}
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%D 1964
%P 120-122
%N 3
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%G ru
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