@article{CMUC_1984_25_4_a2,
     author = {Poljak, Svatopluk and Turz{\'\i}k, Daniel},
     title = {On systems, periods and semipositive mappings},
     journal = {Commentationes Mathematicae Universitatis Carolinae},
     pages = {597--614},
     year = {1984},
     volume = {25},
     number = {4},
     mrnumber = {782011},
     zbl = {0576.05059},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/CMUC_1984_25_4_a2/}
}