@article{ARM_1978__14_4_a6,
     author = {Nieminen, Juhani},
     title = {Operations on graphs determining congruences on graphs},
     journal = {Archivum mathematicum},
     pages = {215--218},
     publisher = {mathdoc},
     volume = {14},
     number = {4},
     year = {1978},
     mrnumber = {512764},
     zbl = {0402.05047},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/ARM_1978__14_4_a6/}
}