@article{MOMA_1998__125_4_178819,
     author = {Klaudiusz W\'ojcik},
     title = {On {Existence} of {Positive} {Periodic} {Solutions.}},
     journal = {Monatshefte f\"ur Mathematik},
     pages = {343--350},
     publisher = {mathdoc},
     volume = {125},
     number = {4},
     year = {1998},
     zbl = {0899.34028},
     url = {http://geodesic.mathdoc.fr/item/MOMA_1998__125_4_178819/}
}