@article{MASLO_1980__30_3_a7,
     author = {D\"orfler, Willibald and Harary, Frank and Malle, G\"unther},
     title = {Covers of digraphs},
     journal = {Mathematica slovaca},
     pages = {269--280},
     publisher = {mathdoc},
     volume = {30},
     number = {3},
     year = {1980},
     mrnumber = {587254},
     zbl = {0442.05056},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/MASLO_1980__30_3_a7/}
}