Geodesic
The uniqueness of the limit cycle of the system $\dot y=-g(x)$, $\dot x=-f(x)$
Differencialʹnye uravneniâ, Tome 5 (1969) no. 3, pp. 563-564 
G. S. Rychkov. The uniqueness of the limit cycle of the system $\dot y=-g(x)$, $\dot x=-f(x)$. Differencialʹnye uravneniâ, Tome 5 (1969) no. 3, pp. 563-564. http://geodesic.mathdoc.fr/item/DE_1969_5_3_a17/
@article{DE_1969_5_3_a17,
     author = {G. S. Rychkov},
     title = {The uniqueness of the limit cycle of the system $\dot y=-g(x)$, $\dot x=-f(x)$},
     journal = {Differencialʹnye uravneni\^a},
     pages = {563--564},
     year = {1969},
     volume = {5},
     number = {3},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DE_1969_5_3_a17/}
}
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