@article{KJM_2007_30_1_a6,
     author = {Dragan Dimitrovski and Vladimir Rajovi\'c},
     title = {A contribution to the problem of coexistence of two periodical solutions of the {Hill's} equation},
     journal = {Kragujevac Journal of Mathematics},
     pages = {99 - 117},
     publisher = {mathdoc},
     volume = {30},
     number = {1},
     year = {2007},
     zbl = {1224.34119},
     url = {http://geodesic.mathdoc.fr/item/KJM_2007_30_1_a6/}
}