@article{NSJOM_1988_18_1a_5,
     author = {Petri\'c Milenko},
     title = {On some operations on normal digraphs},
     journal = {Novi Sad Journal of Mathematics},
     pages = {57-67},
     publisher = {mathdoc},
     volume = {18},
     number = {1},
     year = {1988},
     url = {http://geodesic.mathdoc.fr/item/NSJOM_1988_18_1a_5/}
}