@article{ZNSL_2002_284_a12,
     author = {M. N. Yakovlev},
     title = {Existence of~$2^n$ periodic solutions of a system of~$n$ differential equations of first order},
     journal = {Zapiski Nauchnykh Seminarov POMI},
     pages = {247--262},
     year = {2002},
     volume = {284},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/ZNSL_2002_284_a12/}
}