@article{JRAM_1971__247_151102,
     author = {J\"urgen Eufinger},
     title = {Operations on directed graphs.},
     journal = {Journal f\"ur die reine und angewandte Mathematik},
     pages = {146--154},
     year = {1971},
     volume = {247},
     zbl = {0223.05107},
     url = {http://geodesic.mathdoc.fr/item/JRAM_1971__247_151102/}
}