@article{AUPO_1993__32_1_a4,
     author = {D{\l}otko, Tadeusz},
     title = {Periodic solutions of some second order differential systems},
     journal = {Acta Universitatis Palackianae Olomucensis. Facultas Rerum Naturalium. Mathematica},
     pages = {35--41},
     publisher = {mathdoc},
     volume = {32},
     number = {1},
     year = {1993},
     mrnumber = {1273167},
     zbl = {0795.34030},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/AUPO_1993__32_1_a4/}
}