@article{10_21136_CPM_1967_108397,
     author = {Zelinka, Bohdan},
     title = {Hasse's operator and directed graphs},
     journal = {\v{C}asopis pro p\v{e}stov\'an{\'\i} matematiky},
     pages = {313--317},
     publisher = {mathdoc},
     volume = {092},
     number = {3},
     year = {1967},
     doi = {10.21136/CPM.1967.108397},
     mrnumber = {0223274},
     zbl = {0175.21003},
     language = {en},
     url = {http://geodesic.mathdoc.fr/articles/10.21136/CPM.1967.108397/}
}