@article{AUPO_1991__30_1_a13,
     author = {Andres, J\'an and Vl\v{c}ek, Vladim{\'\i}r},
     title = {Note to the existence of periodic solutions for higher-order differential equations with nonlinear restoring term and time-variable coefficients},
     journal = {Acta Universitatis Palackianae Olomucensis. Facultas Rerum Naturalium. Mathematica},
     pages = {149--157},
     publisher = {mathdoc},
     volume = {30},
     number = {1},
     year = {1991},
     mrnumber = {1166434},
     zbl = {0757.34031},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/AUPO_1991__30_1_a13/}
}