@article{ARM_1980__16_4_a2,
     author = {Chajda, Ivan},
     title = {Relational classes and their characterizations},
     journal = {Archivum mathematicum},
     pages = {199--203},
     publisher = {mathdoc},
     volume = {16},
     number = {4},
     year = {1980},
     mrnumber = {594466},
     zbl = {0449.08002},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/ARM_1980__16_4_a2/}
}