@misc{Hetzer_Georg_A_1979,
 author={Hetzer, Georg},
 volume={27},
 editor={Polska Akademia Nauk.},
 editor={Państwowe Wydawnictwo Naukowe (1951–1992)},
 number={1},
 copyright={Rights Reserved - Restricted Access},
 address={Varsovie},
 howpublished={online},
 year={1979},
 publisher={Państwowe Wydawnictwo Naukowe},
 language={eng},
 title={A note on periodic solutions of second order systems of nonlinear autonomous differential equations},
 type={Text},
}